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Help needed to make 555 timer circuit throw relay.

Discussion in 'General Electronics Discussion' started by atsijs, Dec 12, 2012.

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  1. atsijs


    Dec 12, 2012

    I'm trying to make an automatic control circuit for a model railway shuttle. I've read the principles elsewhere and it all sounds good in theory. There are basically three parts to it - all of which I have working independently, and two together, but I can't get the last one to click (literally).

    So I have set up my track circuit (using isolators and diodes to stop the train running off the end) and my switching circuit (a DPDT relay which reverses the polarity on the tracks - this works with a 9v battery applied directly to the relay).

    The idea was to use a 555 timer to throw the relay periodically. I bought some cheap generic 555 kits, the circuit of which is here:

    I've put this together in the astable configuration and it makes the LED flash at the intervals I'm after (after some switching of capacitors and resistors).

    I naively assumed I would simply be able to connect my relay to the 'output' on the circuit. Apparently the pin 3 current is not sufficient. I have tried using a transistor off the pin to amplify the current to no avail. I've read something about the need to put in a diode but that makes no sense to me.

    Please can anyone tell me if this is possible, or whether I've just wasted a few evenings trying to make it work (albeit enjoyable and educational evenings). Or am I just doing something wrong that should be obvious to me?

    Thanks in advance,

  2. BobK


    Jan 5, 2010
    What are the voltage and current requirements of the relay?

    The diode goes in reverse across the coil of the relay. It's purpose is to suppress a high voltage spike that occurs when the coil is turned off.

  3. Rleo6965


    Jan 22, 2012
    Try to removed load R4 and LED from NE555 pin3 output for better driving base of npn transistor. You can connect other end of R4 to +9V and other end of LED to collector of 2N2222A. See diagram post by Relayer. His diagram will surely solve your problem.
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    That circuit should work. Also, the 555 can source and sink 200 mA so it should be able to drive the relay coil directly. You should ALWAYS put a diode across the relay coil (see BobK's explanation).

    I'm not sure what you're doing wrong, but it COULD be that your 555 circuit is only generating a short pulse each time that you want the relay to change over. To make a working circuit, you have to set it up so that when the 555 output is in one state, that's like your DPDT changeover switch being in one position, and when it's in the other state, it's like the DPDT switch being in the other position.

    You need to drive the relay with a signal that is continuously ON for a time, to run the train in one direction, then continuously OFF for a time, to run the train in the other direction. If your 555 circuit just blinks the LED briefly, this won't work.

    You also need to use a DPDT relay, and wire its contacts with the same crossover arrangement that you used for the DPDT switch. The relay contacts need to replace the switch.

    I've attached a diagram that should do what you want. I've suggested some relays. Some have a 9V coil and some have a 12V coil. 12V relays are more widely available and cheaper. You can run the circuit at either 9V or 12V, to suit the relay coil.

    I've shown the power supply for the train separately from the power supply for the circuit, but you can join them together.

    VR1 is a trimpot. It sets the length of time for each run. If that value doesn't give you a long enough run time, you can increase R1 and/or C1.


    Edit: That circuit isn't complete. You should add some decoupling capacitors to ensure that the 555 runs reliably. Add a 0.1 uF capacitor from pin 5 to ground, and a 0.1 uF and a 10 uF in parallel across the power supply rails to the 555 (from pin 8 to pin 1). Connect them (especially the 0.1 uF ones) close to the 555 with short leads.

    Attached Files:

    Last edited: Dec 17, 2012
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