555 Timer Edge Triggering Options...

Discussion in 'The Projects Forum' started by NFA Fabrication, Apr 27, 2016.

  1. NFA Fabrication

    Thread Starter Member

    Aug 12, 2012
    104
    3
    OK, I have a 12V DC system that once it turns on, I need a secondary part to turn on roughly 30 seconds later. I was thinking an edge triggered 555 timer like the diagram I have here in my trusty book. The secondary system is already on a relay, but taxes the battery as it takes a ton of juice to get it going. (This is basically a gas engine powered/generator/air compressor). The problem is that as soon as I try to start the engine, the electric air compressors try to run immediately taking all my cranking power. If the engine was allowed to start, and stabilize for 30 seconds and get recharging before the electric air compressors kicked on, it would solve my no start issue that happens too often. And unplugging the air compressors everytime is obviously a pain in the a**. I have it all figured out, but I do not have the 1N914 diodes right now (I have ordered 500 of them to add to my bins). Is there a suitable standard diode that can be substituted, or is there another way to do an edge triggered 555 that I am missing?

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  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,813
    1,105
    Almost any diode could be used in that circuit. Note that you will need a pull-up resistor from the left side of C1 to the positive rail, unless whatever is providing the trigger signal can pull the input up (otherwise C1 will have no way of discharging except via leakage).
     
  3. NFA Fabrication

    Thread Starter Member

    Aug 12, 2012
    104
    3
    Thanks! I already built it last night and just left that diode out for now. I did add a 1K resistor to the left of C1 as well. I wasn't 100% sure if it was needed, but I knew it wouldn't hurt it. I'll throw in a 1N4001 and go give it a test! Thanks!
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    Your circuit outputs a pulse. Don't you want a delay and then the output stays high?
    Below is the LTspice simulation of a circuit that does that.

    You will need to add a driver at the output to drive the relay.
    What is the relay coil resistance?

    Alternately, could just use a MOSFET that drives the relay with an RC delay at the input. But you may have to do some tweaking with the RC values to get the desired delay due to threshold variations between MOSFETs.

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    Last edited: Apr 28, 2016
    AnalogKid likes this.
  5. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    Below is a simple delay ON circuit using a MOSFET, which is driving the relay.
    The N-MOSFET can be just about any standard device (not a logic-level type since those have a low threshold voltage which will give a short delay time).
    You will likely have to adjust the resistor value of R1 to get the desired delay.

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  6. NFA Fabrication

    Thread Starter Member

    Aug 12, 2012
    104
    3
    When I have a little more time, I will look at building these to test them, Thanks!Of course, mine did not work, so i came up with something else. I ended using an LM358 Op-Amp, and using a 1M resistor to charge a 22uf Capacitor. I input the voltage of the capacitor on one leg of the comparator, The used a 5K potentiometer so I could set the voltage (Delay) on the other pin. I ran the output through a 1k resistor to a 2n2222 to drive the relay. I used an LM7805 5V regulator to stabilize the voltage at a level that the system should not ever drop below. This worked great!

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