Do-it-yourself time relay: review of 3 homemade options

How does the 555 chip work?

Before moving on to the example of a relay device, let's consider the structure of the microcircuit.
All further descriptions will be made for the NE555 series chip manufactured by Texas Instruments. As can be seen from the figure, the basis is an RS flip-flop with an inverse output, controlled by outputs from comparators. The positive input of the upper comparator is called THRESHOLD, the negative input of the lower one is called TRIGGER. Other comparator inputs are connected to a supply voltage divider consisting of three 5 kOhm resistors.

As you most likely know, an RS flip-flop can be in a steady state (it has a memory effect of 1 bit) either in a logical “0” or in a logical “1”. How it works:

• The arrival of a positive pulse at input R (RESET) sets the output to logical “1” (namely “1”, not “0”, since the trigger is inverse - this is indicated by the circle at the trigger output);
• The arrival of a positive pulse at input S (SET) sets the output to logical “0”.

Three 5 kOhm resistors divide the supply voltage by 3, which leads to the fact that the reference voltage of the upper comparator (the “–” input of the comparator, also known as the CONTROL VOLTAGE input of the microcircuit) is 2/3 Vcc. The reference voltage of the lower one is 1/3 Vcc.

With this in mind, it is possible to create tables of states of the microcircuit regarding the TRIGGER, THRESHOLD inputs and OUT output

Note that the OUT output is the inverted signal from the RS flip-flop

In our case, to create a time relay, the following trick is used: the TRIGGER and THRESHOLD inputs are combined together and a signal is supplied to them from the RC chain. The state table in this case will look like this:

The NE555 connection diagram for this case is as follows:

After power is applied, the capacitor begins to charge, which leads to a gradual increase in the voltage across the capacitor from 0V onwards. In turn, the voltage at the TRIGGER and THRESHOLD inputs will, on the contrary, decrease, starting from Vcc+. As can be seen from the state table, there is a logical “0” at the OUT output after Vcc+ is applied, and the OUT output switches to a logical “1” when the voltage at the indicated TRIGGER and THRESHOLD inputs drops below 1/3 Vcc.

The important fact is that the relay delay time, that is, the time interval between applying power and charging the capacitor until the OUT output switches to logic “1,” can be calculated using a very simple formula:

T = 1.1 * R * C

Next, we present a drawing of a variant of the microcircuit in a DIP package and show the location of the chip pins:

It is also worth mentioning that in addition to the 555 series, the 556 series is produced in a package with 14 pins. The 556 series contains two 555 timers.

Hello friends!

Today we will take a detailed look at the circuit and design of a fairly useful device - a time relay with a load switch-off delay. Of course, the device can be used to both switch on a load and switch between two different loads. The operating voltage of the load can be up to 220V, the maximum switching current is up to 5 A. Using simple calculations, we find that the load power can be up to 1100 W.

Device diagram and principle of its operation

First of all, let's study the time delay relay circuit. An important point: I am not the developer of the scheme and do not claim copyright.

The presented scheme works as follows. When you press the clock button SW1, capacitor C1 is charged, transistor VT1 opens (transistor VT2 and transistor VT3 are in the closed state). Since the relay contacts (X3 and X4) are open, the load is disconnected. During the discharge of capacitor C1, transistor VT1 closes. At the same time, transistors VT2 and VT3 open, and current begins to flow through the relay coil, which leads to the closure of the relay contacts (X3 and X4) and the load being turned on.

You can guess that the main timing element is capacitor C1. The maximum on/off delay time directly depends on it. Also, the relay response time depends on the resistance of the variable resistor R1. Accordingly, to change the delay time, it is enough to change the values ​​of resistor R1 and capacitor C1.

The circuit is powered by a 12 V DC source. Current consumption does not exceed 100 mA.

As for the details. All transistors used in the circuit are of the same type - BC547. These transistors can be replaced with transistors with similar parameters. For example, instead of BC547, you can quite successfully use transistors of the KT3102 series with any letter indices.

Electromechanical relay – BS115C with operating voltage 9V. In principle, the relay can be any small-sized one with an operating voltage from 9 to 12V, for example, it can be a JQC-3F-1C-9VDC relay.

Time relay circuit board

The device is assembled on a printed circuit board made of foil fiberglass, measuring 41x35 mm. For ease of installation, I recommend putting a “diagram” of the arrangement of elements on the board. Drawing the arrangement of elements can be done using the same laser-iron method.

Printed circuit board drawing and arrangement of elements

This is how my printed circuit board turned out:

Switch-off delay relay design

The device can be assembled in absolutely any housing of suitable dimensions. Do not forget that in addition to the relay itself, the power supply must also fit in the case. In my case, a plastic case was used to assemble the power supply. I think that a similar case can be purchased without any problems in almost any radio store.

As you can see, both the board with the relay and the power supply fit into such a case just fine. By the way, you can use a cell phone charger as a power supply. In order to increase the output voltage of such charging, it is enough to replace the zener diode in it with a higher voltage. You can find information on how to do this correctly on YouTube.

For maximum convenience of using the relay, I made a front panel with inscriptions and response time marks. Making such a panel is very simple - you only need skills in working with a graphic editor and a little patience.

That's all, actually. Finally, to complete the material, I suggest you watch a video about this time relay and its assembly. Let's look:

Author of the article and video Anton Pisarev

Scope of application of time relay

Man has always sought to make his life easier by introducing various devices into everyday life. With the advent of electric motor-based equipment, the question arose about equipping it with a timer that would control this equipment automatically.

Turn it on for a specified time - and you can go do other things. The unit will switch itself off after the set period. For such automation, a relay with an auto-timer function was required.

A classic example of the device in question is in a relay in an old Soviet-style washing machine. On its body there was a handle with several divisions. I set the desired mode, and the drum spins for 5–10 minutes until the clock inside reaches zero.

The electromagnetic time relay is small in size, consumes little electricity, has no breakable moving parts and is durable

Today, time relays are installed in various equipment:

• microwaves, ovens and other household appliances;
• exhaust fans;
• automatic watering systems;
• automatic lighting control.

In most cases, the device is made on the basis of a microcontroller, which simultaneously controls all other operating modes of automated equipment. It's cheaper for the manufacturer. There is no need to spend money on several separate devices responsible for one thing.

Based on the type of element at the output, time relays are classified into three types:

• relay – the load is connected via a “dry contact”;
• triac;
• thyristor.

The first option is the most reliable and resistant to network surges. A device with a switching thyristor at the output should be used only if the connected load is insensitive to the shape of the supply voltage.

To make your own time relay, you can also use a microcontroller. However, homemade products are mainly made for simple things and work conditions. An expensive programmable controller in such a situation is a waste of money.

There are much simpler and cheaper circuits based on transistors and capacitors. Moreover, there are several options; there is plenty to choose from for your specific needs.

Time relay circuit | Electrician in the house

Time relay circuit

Time relay circuit

Let's consider the simplest circuit of a 220 volt time relay. This time relay circuit can be used for various needs. For example, with the indicated elements, for a photographic enlarger or for temporary lighting of a staircase or landing.

The diagram shows:

• D1-D4 - diode bridge KTs 405A or any diodes with a maximum permissible forward rectified current (Iv.max) not lower than 1A and a maximum permissible reverse voltage (Urev.max) not lower than 300 V.
• D5 - diode KD 105B or any diode with Iv.max not less than 0.3A and Uob.max not lower than 300V.
• VS1 - thyristor KU 202N or KU 202K(L,M), VT151, 2U202M(N).
• R1 - MLT resistor - 0.5, 4.3 mOhm.
• R2 - MLT resistor - 0.5, 220 Ohm.
• R3 - MLT resistor - 0.5, 1.5 kOhm.
• C1 - capacitor 0.5 µF, 400 V.
• L1 - incandescent lamp(s) with a power of no more than 200 W.
• S1 - switch or button.

Time relay circuit operation

When contacts S1 are closed, capacitor C1 begins to charge, “+” is applied to the control electrode of the thyristor, the thyristor opens, the circuit begins to consume a large current and lamp L1, connected in series with the circuit, lights up. The lamp also acts as a current limiter through the circuit, so the circuit will not work with energy-saving lamps. When capacitor C1 is fully charged, current stops flowing through it, the thyristor closes, and lamp L1 goes out. When contacts S1 open, the capacitor discharges through resistor R1 and the time relay returns to its original state.

Finalization of the time relay circuit

With the specified parameters of the circuit elements, the burning time of L1 will be 5-7 seconds. To change the relay response time, you need to replace capacitor C1 with a capacitor of a different capacity. Accordingly, as the capacity increases, the operating time of the time relay increases. You can put two or more capacitors in parallel and connect or disconnect them with switches, in this case you will get a stepwise adjustment of the time relay operation time. To smoothly adjust the time, you need to add a variable resistor R4. You can combine both adjustment methods to obtain a relay with almost any response duration.

Modified time relay circuit

Changes in the scheme:

• C2 is an additional capacitor, you can take the same one as C1.
• S2 - switch (toggle switch) connecting capacitor C2 (increasing the operating time of the time relay).
• R4 is a variable resistor, you can take SP-1, 1.0-1.5 kOhm, or a similar value.

During prototyping, with the component ratings indicated on the diagrams, the light bulb (60W) lit up for about 5 seconds. By adding in parallel a capacitor C2 with a capacity of 1 μF and a resistor R4 of 1.0 kOhm, it became possible to regulate the burning time of the light bulb from 10 to 20 seconds (using R4).

Another time relay circuit can be taken from the article “Automatic air freshener”; such a circuit can be used for almost any device.

Be careful when setting up and operating the device; circuit parts are under dangerous voltage.

PS Many thanks to Mr. Yakovlev V.M. for your help.

It will be interesting to read:

Useful devices, Electronic devices, DIY electrical circuits, electronics, electrical circuit

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Transistor time relay (option 2)

Another version of the time relay circuit on one transistor is shown in Fig. 3. In this relay, the dwell time is determined by the discharge time of capacitor C1 through resistors R1. R4 and the input circuit of transistor T1. By changing the value of variable resistor R4, you can smoothly change the exposure time.

Rice. 3. Second version of a time relay on a transistor, diagram.

In the initial state, the voltage on capacitor C1 is zero, and therefore there is no voltage at the base of transistor 77. The current in the collector circuit is so small that relay P1 does not operate.

When you press the Kn button, capacitor C1 is almost instantly charged to the voltage at the output of the rectifier. As soon as you release the button, the voltage on capacitor C1 will be applied negatively to the base of the transistor, and the collector current will increase sharply.

In this case, relay P1 will operate, close its normally open contacts 1-2, and power will be supplied to the executive circuit. The relay armature will be attracted until capacitor C1 is discharged.

As the capacitor discharges, the collector current will decrease. When it becomes less than the release current of the relay, the latter will open contacts 1-2 and the supply of voltage to the actuator circuit will stop.

The discharge time of capacitor C1 is mainly determined by variable resistor R4, the scale of which is graduated in seconds. Electromagnetic relay P1 has the same parameters as in the previous diagram.

Transformer Tr1 is made on an Ш16 core, the thickness of the set is 20 mm. Winding 1a contains 1900 turns, and winding 16-1400 turns of PEV-1 0.12 wire. Winding II contains 925 turns of PEV-0.15 wire. To obtain various rectified voltages from the 700, 775 and 850th turns, taps are made.

We create a time relay for 12 and 220 volts

Transistor and microcircuit timers operate at a voltage of 12 volts. For use with 220 volt loads, diode devices with a magnetic starter are installed.

To assemble a controller with a 220 volt output, stock up on:

• three resistances;
• four diodes (current more than 1 A and reverse voltage 400 V);
• capacitor with an indicator of 0.47 mF;
• thyristor;
• start button.

After pressing the button, the network closes and the capacitor begins to charge. The thyristor, which was open during charging, closes after charging the capacitor. As a result, the current supply stops and the equipment turns off.

The correction is carried out by choosing the resistance R3 and the power of the capacitor.

Manufacturing with diodes

To install a diode system, the necessary elements are:

• 3 resistors;
• 2 diodes rated for current 1 A;
• thyristor VT 151;
• starting device.

The switch and one contact of the diode bridge are connected to a 220 volt power supply. The second wire of the bridge is connected to the switch. The thyristor is connected to resistances of 200 and 1,500 Ohms and a diode. The second terminals of the diode and the 200th resistor are connected to the capacitor. A 4300 Ohm resistance is connected in parallel with the capacitor.

Using transistors

To assemble a circuit using transistors, you need to stock up on:

• capacitor;
• 2 transistors;
• three resistors (nominal 100 kOhm K1 and 2 models R2, R3);
• button.

After turning on the button, the capacitor is charged through resistors r2 and r3 and the emitter of the transistor. At the same time, the voltage across the resistance drops as the transistor opens. After the second transistor opens, the relay is activated.

As the capacitance is charged, the current drops, and with it the voltage across the resistance to the point at which the transistor closes and the relay is released. For a new start, the capacity must be completely discharged; this is done by pressing a button.

Chip-based creation

To create a system based on microcircuits you will need:

• 3 resistors;
• diode;
• TL431 chip;
• button;
• containers.

The relay contact is connected in parallel to the button to which the “+” of the power source is connected. The second relay contact is connected to a 100 Ohm resistor. The resistor is also connected to resistances.

The second and third pins of the microcircuit are connected to a 510 Ohm resistor and a diode, respectively. The last contact of the relay is also connected to the semiconductor, with the execution device. “–” of the power supply is connected to a 510 Ohm resistance.

Using ne555 timer

The simplest circuit to implement is the NE555 integrated timer, so this option is used in many electrical circuits. To install the time controller you will need:

• board 35x65;
• Sprint Layout program file;
• resistor;
• screw terminal blocks;
• spot soldering iron;
• transistor;
• diode.

The circuit is mounted on the board, the resistor is located on its surface or brought out by wires. The board has space for screw terminal blocks. After soldering the components, excess solder is removed and the contacts are checked. To protect the transistor, a diode is mounted in parallel with the relay. The device sets the response time. If you connect a relay to the output, you can adjust the load.

• the user presses a button;
• the circuit closes and voltage appears;
• The light comes on and the time countdown begins;
• After the set period has expired, the light goes out and the voltage becomes 0.

The user can adjust the operating interval of the clock mechanism within 0 - 4 minutes, with a capacitor - 10 minutes. The transistors used in the circuit are bipolar devices of low and medium power of the npn type.

The delay depends on the resistances and capacitor.

Multifunction devices

Multifunctional time controllers perform:

• counting down time in two versions simultaneously during one period;
• parallel counting of time periods continuously;
• countdown;
• stopwatch function;
• 2 autostart options (the first option after pressing the start button, the second – after applying current and expiration of the set period).

To operate the device, a memory block is installed in it, in which settings and subsequent changes are saved.

Schematic diagram

The schematic diagram of a time relay is shown in Figure 1. When a power source is connected to the circuit, capacitor C1 begins charging through resistors R2 and R3 and the emitter junction of transistor VT1. It opens and a voltage drop forms across resistor R3 as the emitter current flows through it.

Rice. 1. Scheme of a simple homemade time relay using two KT3102 transistors.

This voltage drop unlocks transistor VT2, and the electromagnetic relay K1 is activated. which, with its contacts K1.1, connects the HL1 LED to the power bus. Resistor R4 limits the LED current.

As charging proceeds, the voltage across the capacitor increases and the charging current decreases. Accordingly, the emitter current and the voltage drop across resistor R3 decrease. Finally, at a certain capacitor voltage, the charging current becomes so small. that transistor VT1 is locked, followed by transistor VT2.

As a result, the relay releases and the LED goes out. To start the time relay the next time, you must briefly press the SB1 button to completely discharge capacitor C1.

The required period of time during which relay K1 is in the activated state is set by selecting the capacitance of the capacitor and the resistance of resistors R2 and R3.

If the relay has another pair of contacts, they can be used to turn on or turn off other consumers. But then the second pair of contacts must be normally closed. The choice of relay type is made by the value of its operating voltage, which must be equal to the supply voltage of the device.

Scope of application

In the process of development of human civilization, people have always tried to make their lives easier and came up with various useful devices. After the popularization of electrical equipment among the population, the need arose to invent a timer that would turn off the device after a certain time. That is, you can turn on the unit and go about your business, after which the timer will automatically turn it off at the specified or programmed time. For these purposes, a time relay was created. The 12 V device is characterized by ease of manufacture, so making it yourself will not be difficult.

As an example, we can cite relays from an old washing machine, which were popular during the years of the Soviet Union. In the classic version, they had a mechanical round handle with divisions. After turning it in a certain direction, the countdown began, and the machine stopped when the timer inside the relay reached zero.

Time relays also exist in modern electrical engineering:

• microwave ovens or other similar equipment;
• automatic watering systems;
• fans for air injection or exhaust;
• automatic lighting control systems.

This is easier and more economical for the manufacturer, since there is no need to install two elements that perform the same function if one control unit can provide all tasks.

All models (both factory-made and home-made) according to the type of element located at the output are divided into:

• relay;
• triac;
• thyristor.

In the first option, the entire load is connected and passes through a “dry contact”. It is the most reliable among its analogues. For self-production, you can also use a microcontroller. But it is not advisable to do this, since ordinary homemade time relays are made for simple tasks. Therefore, using microcontrollers is a waste of money. In this case, it is better to use simple circuits using capacitors and transistors.

Where can I use it?

The analysis of time relays in our article was made using the example of household electrical appliances. But these devices are now installed in many operating and process designs. For example, in greenhouses, where it is necessary to control lighting by the hour.

Relays of this type are used when growing plants using artificial light.

To do this, a timer is installed in the 220V electrical lighting circuit, which is connected to an actuator that turns the lighting system on and off. The same device can be installed in a technological chain of several machines. It will be configured for a technology that takes into account the specific turn-on and turn-off times of each machine (electrical equipment) separately. That is, there are a large number of options for using time relays.

It should be noted that programming the timer is one of the most important categories for its correct operation. Currently, manufacturers offer time relays with a turn-off delay of 12-220V, with which you can program its operation for one day (daily), for a week, a month and a year. That is, the range of settings is practically unlimited. That for many technological processes (circuits) this is an important criterion for effective and correct operation.

The simplest 12V timer at home

The simplest solution is a 12 volt time relay. Such a relay can be powered from a standard 12v power supply, of which there are many sold in various stores.

The figure below shows a diagram of a device for turning on and automatically turning off a lighting network, assembled on one integrated counter of the K561IE16 type.

Drawing. A variant of a 12v relay circuit that turns on the load for 3 minutes when power is applied.

This circuit is interesting in that the flashing LED VD1 acts as a clock pulse generator. Its flicker frequency is 1.4 Hz. If you cannot find an LED of this particular brand, you can use a similar one.

Let's consider the initial state of operation, at the moment of supplying 12v power. At the initial moment of time, capacitor C1 is fully charged through resistor R2. Log.1 appears at pin No. 11, making this element zeroed.

The transistor connected to the output of the integrated meter opens and supplies 12V voltage to the relay coil, through the power contacts of which the load switching circuit is closed.

The further principle of operation of the circuit, operating at a voltage of 12V, is to read the pulses coming from the VD1 indicator with a frequency of 1.4 Hz to pin No. 10 of the DD1 counter. With each decrease in the level of the incoming signal, there is, so to speak, an increment in the value of the counting element.

When the 256th pulse arrives (this equals 183 seconds or 3 minutes), a log appears on pin No. 12. 1. This signal is a command to close transistor VT1 and interrupt the load connection circuit through the relay contact system.

At the same time, logic 1 from pin No. 12 is supplied through diode VD2 to clock leg C of element DD1. This signal blocks the possibility of receiving clock pulses in the future; the timer will no longer operate, until the 12V power supply is reset.

The initial parameters for the operation timer are set in different ways by connecting the transistor VT1 and the diode VD3 indicated in the diagram.

By slightly transforming such a device, you can make a circuit that has the opposite principle of operation. The KT814A transistor should be changed to another type - KT815A, the emitter should be connected to the common wire, the collector to the first contact of the relay. The second relay contact should be connected to a 12V supply voltage.

Drawing. A variant of a 12v relay circuit that turns on the load 3 minutes after power is applied.

Now, after power is applied, the relay will be turned off, and the control pulse that opens the relay in the form of logic 1 output 12 of element DD1 will open the transistor and supply 12V voltage to the coil. After which, the load will be connected to the electrical network through the power contacts.

This version of the timer, operating from a voltage of 12V, will keep the load disconnected for a period of 3 minutes, and then connect it.

When making the circuit, do not forget to place a capacitor with a capacity of 0.1 μF, designated C3 in the circuit and a voltage of 50V, as close as possible to the supply terminals of the microcircuit, otherwise the counter will often fail and the relay dwell time will sometimes be less than it should be.

In particular, this is programming the exposure time. By using, for example, a DIP switch as shown in the figure, you can connect some contacts of the switches to the outputs of the counter DD1, and combine the second contacts together and connect them to the connection point of the elements VD2 and R3.

Thus, using microswitches you can program the relay delay time.

Connecting the connection point of elements VD2 and R3 to different outputs of DD1 will change the dwell time as follows:

Universal single-channel cyclic timer

Another option: Universal single-channel cyclic timer.

Scheme:

Device capabilities: - timer cycle duration adjustable during firmware up to 4 billion seconds (4-byte variable). - two actions per cycle (turn on and off the load), set using three buttons. - ability to turn on/off the load bypassing the timer. - discrete counting 1 second. - average current consumption without load is 11 microamps (approximately 2 years of operation from CR2032). - stroke correction (rough). - 2.1 volt low power reset (BOD) is disabled, because consumes 120 µA.

Operating principle: the timer repeats the recorded actions (on/off) with a certain period (cycle) set by the user in the EEPROM memory when flashing the controller. Example task: you need to turn on the load at 21:00 and turn it off at 7:00 and do this every three days. Solution: flash the timer with a “3 day” cycle and start it. The first time we approach the timer at 21:00, hold down the PROG button and, without releasing it, press the ON button, the LED will light up for 0.5 seconds and the output will turn on. The second time we approach the timer at 7:00, hold down the PROG button and, without releasing it, press the OFF button, the LED will light up for 0.5 seconds and the output will turn off. That's it, the timer is programmed and will perform these actions every three days at the same time. If the load needs to be turned on or off bypassing the timer, you must press the ON or OFF button without the PROG button, the program will not fail and the next time the load will turn on/off at the previously set time. You can check the operation of the timer by pressing the PROG button, the LED will flash once a second .

Description of testing with different capacitors in the previous article.

To make device setup easier, a calculator (EEPROM code generator) was also written. It can be used to create a HEX file to replace part of the code in the firmware file.

Update 02/29/2016 Configurator 04/16/2016 Forum

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DIY time relay

Let's look at the simplest ways to make slowing systems with your own hands.

12 Volt

We will need a printed circuit board, a soldering iron, a small set of capacitors that act as relays, transistors, and emitters.

The circuit is designed in such a way that when the button is turned off, there is no voltage on the plates of the container. When the button is short-circuited, the capacitor quickly charges and then begins to discharge, supplying voltage through the transistors and emitters.

In this case, the switch will be closed or open until a few volts remain on the capacitor.

You can regulate the duration of discharge of the capacitor by its capacitance or the resistance value of the connected circuit.

Work order:

• the board is being prepared;
• the paths are treaded;
• transistors, diodes and relays are unsoldered.

220 volt

Fundamentally, this scheme is not very different from the previous one. Current passes through the diode bridge and charges the capacitor. At this time, the lamp is lit, which acts as a load. Then the process of discharging and triggering of the timer occurs. The assembly procedure and set of tools are the same as in the first option.

NE555 circuit

The 555 chip is also called an integrated timer. Its use guarantees the stability of maintaining the time interval; the device does not respond to voltage changes in the network.

When the button is turned off, one of the capacitors is discharged, and the system can remain in this state indefinitely. After pressing the button, the capacity begins to charge. After a certain time, it is discharged through the transistor of the circuit.

The discharge transistor opens and the system returns to its original state.

There are 3 operating modes:

• monostable. When an input signal is received, it turns on, a wave of a certain length comes out and turns off waiting for a new signal;
• cyclical. At specified intervals, the circuit goes into operating mode and turns off;
• bistable. Or a switch (pressed the button, it works, pressed it, it doesn’t work).

Timer with delay

After voltage is applied, the capacitance is charged, the transistor opens, while the other two are closed. Therefore, there is no load at the output. During the discharge of the capacitor, the first transistor closes, the other two open. Power begins to flow to the relay, the output contacts close.

The period depends on the capacitance of the capacitor and variable resistor.

Cyclic device

The most commonly used counters are generators. The first of which produces a signal at specified intervals, and the second receives them, setting a logical zero or one after a certain number of them.

All this is created using a controller; you can find many circuits, but they will require some knowledge of radio engineering.

Another option is to completely discharge or charge the capacity using a microcircuit and send a signal to the control transistor, which operates in switch mode.

Time relay for automatic load shedding

Sometimes it is necessary to turn off the receiver or backlight after a certain period of time. This problem can be solved by the circuit shown in Fig. 1.

Rice. 1. Timer circuit for automatic load shutdown.

With the ratings of the timing elements indicated in the diagram, the shutdown delay will be about 40 minutes (for micropower timers, this time can be significantly increased, since they allow R2 to be set with a higher rating).

In standby mode, the device does not consume power, since transistors VT1 and VT2 are locked. Switching on is done by button SB1 - when pressed, transistor VT2 opens and supplies power to the microcircuit. At the output of timer 3, a voltage appears, which opens the transistor switch VT1 and supplies voltage to the load, for example, to the BL1 lamp.

The button is blocked, and the circuit will remain in this state while capacitor C2 is charging, after which it will turn off the load. Resistor R3 limits the discharge current of the timing capacitor, which increases the reliability of the device. To obtain large delay intervals, capacitor C2 must be used with a low leakage current, for example tantalum from the K52-18 series.

Time relay on a field-effect transistor

A simple time relay (or a simple time relay for beginners 2) on a bipolar transistor is not difficult to manufacture, but long delays cannot be obtained with such a relay. The duration of the delay is determined by an RC circuit consisting (for a time relay and a bipolar transistor) of a capacitor, a resistor in the base circuit and a base-emitter junction of the transistor. The larger the capacitor, the greater the delay. The greater the total resistance of the resistor in the base circuit and the base-emitter junction, the greater the delay. It is impossible to increase the resistance of the base-emitter junction to obtain a large delay because this is a constant parameter of the transistor used. The resistance of the resistor in the base circuit cannot be increased indefinitely because To open, the transistor requires a current of at least h31e less than the current required to turn on the relay. If, for example, 100 mA is required to turn on the relay, h31e = 100, then to open the transistor, the base current Ib = 1 mA is required. To open a field-effect transistor with an insulated gate, a large current is not required; in this case, you can even neglect this current and assume that no current is required to open such a transistor. The insulated gate field effect transistor is voltage controlled so you can use an RC circuit with any resistance and therefore make any delays. Let's look at the diagram:

Figure 1 — Time relay on a field-effect transistor

This circuit is similar to the bipolar transistor circuit from the previous article, only here instead of an n-MOSFET bipolar transistor (n-channel field-effect transistor with an insulated gate (and induced channel)) and a resistor (R1) is added to discharge the capacitor C1. Resistor R3 is optional:

Figure 2 - Time relay on a field-effect transistor without R3

Field-effect transistors with an insulated gate can be damaged by static electricity, so they must be handled carefully: try not to touch the gate terminal with your hands or charged objects, if possible, ground the gate terminal, etc.

The process of checking the transistor and the finished device is shown in the video:

Because Since the parameters of the RC circuit are negligibly affected by the parameters of the transistor, calculating the delay duration is quite simple. In this circuit, the duration of the delay is still affected by the duration of holding the button and the lower the resistance of resistor R2, the weaker this influence, but do not forget that this resistor is needed to limit the current at the moment the button contacts are closed, if its resistance is made too low or replaced jumper, then when you press the button, the power supply may fail or its short-circuit protection may trip. (if there is one), the button contacts may melt to each other, in addition, this resistor limits the current when resistor R1 is set to the minimum resistance. Resistor R2 also lowers the voltage (UCmax) to which capacitor C1 is charged when the SB1 button is pressed, which leads to a decrease in the delay duration. If the resistance of resistor R2 is low, then it has little effect on the delay duration. The duration of the delay is affected by the voltage at the gate relative to the source at which the transistor closes (hereinafter referred to as the closing voltage). To calculate the duration of the delay, you can use the program:

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Time relay on a composite transistor

A time relay assembled according to the diagram in Fig. 2, is characterized by the use of a composite transistor (T1, T2), due to which it has higher sensitivity.

The composite transistor has a current gain equal to the product of the current gains of the individual transistors, and therefore, with the same control current, the collector current is much greater than in the previous circuit.

This made it possible to abandon the use of an expensive relay and replace it with a conventional electromagnetic one.

Rice. 2. Time relay based on a composite transistor.

The time delay is changed smoothly by resistor R2 and abruptly by switch B2. When testing this circuit using a relay of type RSM-2 (passport 10.171.81.21), for which, due to unloading of the armature, it was possible to obtain actuation and release currents of 10 and 4 mA, the holding time turned out to be equal: at the first limit 1-6 sec, at the second - 6 - 24 and at the third limit 24 - 125 seconds.

Each of the capacitors C2, C3 is composed of several capacitors with a minimum leakage current and an operating voltage of at least 10 V. It should be noted that the time delay limits depend on the actual capacitance of capacitors C1-C3 and the amount of leakage, so they are specified during the setup process.

Cyclic on-off timer. DIY cyclic time relay

circuit for 12 and 220 volts

Modern equipment often requires a timer, i.e. a device that does not work immediately, but after a period of time, which is why it is also called a delay relay. The device creates temporary delays in turning on or off other devices. It is not necessary to purchase it in a store, because a well-designed homemade time relay will effectively perform its functions.

Scope of application of time relay

Areas of use of the timer:

• regulators;
• sensors;
• automation;
• various mechanisms.

All these devices are divided into 2 classes:

1. Cyclic.
2. Intermediate.

The first is considered an independent device. It sends a signal after a specified time interval. In automatic systems, a cyclic device turns on and off the necessary mechanisms. It is used to control lighting:

• on the street;
• in aquarium;
• in the greenhouse.

The cyclic timer is an integral device in the Smart Home system. It is used to perform the following tasks:

1. Turning the heating on and off.
2. Event reminder.
3. At a strictly specified time, turns on the necessary devices: washing machine, kettle, light, etc.

In addition to the above, there are also industries in which cyclic delay relays are used:

• the science;
• medicine;
• robotics.

An intermediate relay is used for discrete circuits and serves as an auxiliary device. It automatically interrupts the electrical circuit. The scope of application of the intermediate timer relay begins where signal amplification and galvanic isolation of the electrical circuit are required. Intermediate timers are divided into types depending on their design:

1. Pneumatic. The relay does not operate instantly after receiving a signal; the maximum response time is up to one minute. Used in control circuits of metal-cutting machines. The timer controls the drives for step adjustment.
2. Motor. The time delay setting range starts from a couple of seconds and ends in tens of hours. Delay relays are part of overhead power line protection circuits.
3. Electromagnetic. Designed for DC circuits. With their help, acceleration and braking of the electric drive occurs.
4. With clock mechanism. The main element is a charged spring. Regulation time is from 0.1 to 20 seconds. Used in relay protection of overhead power lines.
5. Electronic. The operating principle is based on physical processes (periodic pulses, charge, discharge of capacitance).

Schemes of various time relays

There are different versions of time relays, each type of circuit has its own characteristics. You can make timers yourself. Before you make a time relay with your own hands, you need to study its structure. Schemes of simple time relays:

• on transistors;
• on microcircuits;
• for output power 220 V.

Let's describe each of them in more detail.

Transistor circuit

Required radio components:

1. Transistor KT 3102 (or KT 315) - 2 pcs.
2. Capacitor.
3. Resistor rated 100 kOhm (R1). You will also need 2 more resistors (R2 and R3), the resistance of which will be selected along with the capacitance depending on the timing of the timer.
4. Button.

When the circuit is connected to a power source, the capacitor will begin to charge through resistors R2 and R3 and the emitter of the transistor. The latter will open, so the voltage will drop across the resistance. As a result, the second transistor will open, which will trigger the electromagnetic relay.

As the capacitance is charged, the current will decrease. This will cause the emitter current to decrease and the voltage drop across the resistance to a level that will cause the transistors to close and the relay to release. To restart the timer, you will need to briefly press the button, which will cause the tank to completely discharge.

To increase the time delay, a field-effect transistor circuit with an insulated gate is used.

Chip-based

The use of microcircuits will eliminate the need to discharge the capacitor and select the ratings of radio components to set the required response time.

Required electronic components for a 12 volt time relay:

• resistors with a nominal value of 100 Ohm, 100 kOhm, 510 kOhm;
• diode 1N4148;
• capacity 4700 uF and 16 V;
• button;
• TL 431 chip.

The positive pole of the power supply must be connected to the button, to which one relay contact is connected in parallel. The latter is also connected to a 100 Ohm resistor. On the other hand it hurts

Schemes of various time relays

There are different versions of time relays, each type of circuit has its own characteristics. You can make timers yourself. Before you make a time relay with your own hands, you need to study its structure. Schemes of simple time relays:

• on transistors;
• on microcircuits;
• for output power 220 V.

Let's describe each of them in more detail.

Transistor circuit

Required radio components:

1. Transistor KT 3102 (or KT 315) - 2 pcs.
2. Capacitor.
3. Resistor rated 100 kOhm (R1). You will also need 2 more resistors (R2 and R3), the resistance of which will be selected along with the capacitance depending on the timing of the timer.
4. Button.

When the circuit is connected to a power source, the capacitor will begin to charge through resistors R2 and R3 and the emitter of the transistor. The latter will open, so the voltage will drop across the resistance. As a result, the second transistor will open, which will trigger the electromagnetic relay.

As the capacitance is charged, the current will decrease. This will cause the emitter current to decrease and the voltage drop across the resistance to a level that will cause the transistors to close and the relay to release. To restart the timer, you will need to briefly press the button, which will cause the tank to completely discharge.

To increase the time delay, a field-effect transistor circuit with an insulated gate is used.

Chip-based

The use of microcircuits will eliminate the need to discharge the capacitor and select the ratings of radio components to set the required response time.

Required electronic components for a 12 volt time relay:

• resistors with a nominal value of 100 Ohm, 100 kOhm, 510 kOhm;
• diode 1N4148;
• capacity 4700 uF and 16 V;
• button;
• TL 431 chip.

The positive pole of the power supply must be connected to the button, to which one relay contact is connected in parallel. The latter is also connected to a 100 Ohm resistor. On the other side, the resistor is connected to resistances of 510 and 100 kOhm. One of the terminals of the latter goes to the microcircuit. The second pin of the microcircuit is connected to a 510 kOhm resistor, and the third to a diode. A second relay contact is connected to the semiconductor device, which is connected to the actuator. The negative terminal of the power supply is connected to a 510 kΩ resistor.