time delay relay circuit schematic

555 Time Delay Relay Circuit

This time delay relay circuit is built with IC NE/SE555, produced by Intersil which contains a precision timer. Stability to temperature variations is 0.005 %/oC. In the circuit diagram, the IC works as a monostable multivibrator. It is set by operating the key Dr1. With Dr2 you can reset the assembly any time you desire.

Time delay relay operation

Pin 3 provides (after setting) a positive rectangular pulse, whose amplitude is approximately equal to the 12V power supply voltage. This output should not be charged with more than 200 mA over a period of time.

Pulse width can be calculated with the formula:

t = (R2 + Rp1)*Cs1

In this formula t is in seconds if the R2 and Rp1 values are set in ohms and Cs1 is in Farads. Cs1 is the switched capacitor to potentiometer P1 by the S1 switch. Lighting time is set roughly in the decade with S1, while P1 serves to fine tune.

Time delay:
S1a = 0.1 … 1 s
S2b = 1 … 10 s
S2c = 10 … 100 s
S2d = 100 … 1000 s
S2e = 1000 … 10000 s

For the Reed relay, by whose contacts the lamp is connected and disconnected, the following conditions must be met: operating voltage does not exceed 12 volts and current draw is not greater than 200 mA. If Reed relay’s working voltage is below 12 volts, then you can connect a resistance Rx in series with the relay’s coil.

Eventually it can be used a triac in place of the Reed relay. In this case the gate is controlled by Rx (here Rx = 150 ohms).

You should to consider a proper insulation of the time delay relay installation and wiring.

Time delay relay circuit schematic

time delay relay circuit schematic


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    pls can anyone help me with a 555 timer circuit that stay active after a 10 seconds delay(that can switch 12v relay)

  • mark joseph enclona

    can i ask what is the value for rx. what watt can i use to my resistor?

  • Henryh

    Hello, I like the article and comments. I´d like to ask about two simple modifications. A)I want a roughly 10 to 45 minutes timer, Can I eliminate the a-d S1 capacitors with no problems? eliminate the switch and direct-connect the 4700microF B)How can I add an indicator LED light to show it is ON? That is, a red light when the relay is connected and 220V on the external load (a heater)
    Thank you.

  • Digi Components

    Some content not understand, tomorrow to see it again.

  • anthony

    teach me how to connect the triac on the circuit help cant delay or even turn on a fluorescent bulb.

    • Jim Keith

      Troubleshooting tips:
      Connect resistor and LED to pin 3 of 555. If it does not light as expected, find wiring error.

      Apply approx 100mA DC gate signal to make sure that it turns on in both polarities. Do this by connecting 100Ω resistor between gate and +12VDC. This should turn on TRIAC. Test with incandescent lamp if you have problems with fluorescent lamp starting properly.

      Diode previously mentioned is for relay coil. Since the TRIAC has no inductance, no diode is required.

    • anthony

      therefore I have 2 resistors connecting on GATE of my TRIAC. parallel 100ohms 12vdc to GATE with RX=150ohms from pin 3 to GATE ?

      RX got burn maybe because I connect A2 to 12vdc ground or i’d use 1/2 watt to all my resistor.

    • Jim Keith

      100Ω is for experiment only while 150Ω is disconnected.

      Yes, swapping A1 & A2 could burn out both resistor and 555. TRIAC may not survive either.

  • anthony

    can i replace:
    1. 500k potentiometer on 2M pot
    2. BT 136 on reed relay
    3. 12v 200mA only transformer

    thanks !

    • sunil somaiya

      Dear Sir,

      I want delay circuit of 1 minute, and do not want to auto-reset. So what changes I should do in this circuit.

      Pls reply Sir…

      Thank you so much..

      Sunil Somaiya

    • Jim Keith

      These changes are OK. Another issue I have with the circuit is the absence of a back diode across the relay coil, so place a 1N4004 diode across the relay coil with the cathode up. The diode provides a path for the inductive discharge rather than through the IC. While it may work without this diode, some 555 devices may function properly.

    • anthony

      thanks for your reply !
      i replace my pot to 1Mohms.
      also, if i replace BT 136 on reed relay, correct me if i’m wrong on pins: M1 to ground; M2 to AC load; Gate to 150ohm resistor.
      with your diode on pin GATE pointing up.

    • Jim Keith

      Looks good to me. Let us know if it works OK.

  • ponpon

    and how i connect the triac in the circuit ? sorry i’m quite noob, this is for my minor subject 🙂

  • ponpon

    nice work ! perfect for my subject proposal.
    In place of the Reed relay, what TRiac rating i’ll be use ? thanks !

  • anthony

    what is B25C500 ??

    • navan dev

      What is the abbreviation of rp1

    • Jim Keith

      B25C500 is a full-wave bridge rectifier. Virtually any bridge rectifier will work for this application, or you may make your own with (4) 1N4004 rectifiers. Just remember that all arrows point in the same direction or to the right in this case.

      Regarding the 10,000sec time delay, I would dispense with the 4700uF capacitor because it is very unlikely that the 2M resistor can charge a capacitor of this size due to its leakage current.

  • tranqspogi

    this is good man!!! i i finally to do this right… basis the time relay… you must depending the capacitor.. this is great and meaningful!….