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[on:]

I'm trying to find a relay (perhaps a pulse-timer relay) that energizes a relay's coil upon activation but does NOT begin to "count down" until after the trigger signal is removed. Does anyone know of a relay or circuit that does that or how to modify an existing pulse-timer relay to do so?

Most PTR's I'm finding will energize a coil and start counting down at the same time. The problem with this is that if my trigger lasts 10 seconds but the PTR is set for 2 seconds, the relay coil will only energize for 2 seconds despite the 10 second trigger. Once the trigger is removed nothing happens since the PTR is waiting for a new trigger signal.

Conversely, if I take a negative trigger and go directly to the 555 timer's trigger pin with a 10 second trigger, the relay will energize for the duration of the trigger but de-energizes upon removal of the trigger. In other words, I "lose" the count down.

I've built a circuit for an automotive application. To avoid having the circuit powered for a constant power line, I would like to use a custom PTR to provide power from the vehicle's 'constant' wire to the circuit upon being triggered from the vehicle's ignition or accessory power, and to stay on for a while after the ign/acc is removed. I hope that makes sense.

[Reply #1 on:]

something like this (partial code, will not run)

Code:

unsigned long countdown = 2000;
unsigned long start = 0;

void loop()
{
  if (digitalRead(TRIGGERPIN) == HIGH)
  {
    digitalWrite(RELAYPIN, HIGH);
  }
  else  // trigger == low
  {
    if (start == 0 && digitalRead(RELAYPIN) == HIGH)  // only when relay has started
    {
      start = millis();  // remember the time the trigger went low
    }
  }


  if (millis() - start > countdown)  // wait until countdown has passed
  {
    digitalWrite(RELAYPIN, LOW);
    start = 0;                           // reset
  }
}

   

[Reply #2 on:]

Mind explaining how this works? It seems I would need a microcontroller powered constantly to run this, no?

[Reply #3 on:]

no?

yes!

Think even the smallest arduino (tiny85 ?) can do this.

To understand the algorithm, follow it with paper and pencil and write out what happens in  ACTION -> EFFECT lines.

0) start and do nothing  -> relay switches off (no state change)
1) press trigger -> relay switches on
2) ...

[Reply #4 on:]

I guess I should have specified better. I would like to avoid having to power  a microcontroller to avoid a current draw. This is the reason I'm using a trigger to energize a relay that provides power to the microcontroller. Once the trigger (that powers the microcontroller via a relay) goes away, the relay should stay on for a duration of time continuing to power to microcontroller until the relay times out.

[Reply #5 on:]

add a capacitor that can hold the relay for some time ...

[Reply #6 on:]

add a capacitor that can hold the relay for some time ...

You know, as an EE major you'd think I would have thought of that lol. What a great idea. This is what happens when we try to over complicate things. I'll definitely give it a try. Thanks Rob!

[Reply #7 on:]

quick google -> http://forum.allaboutcircuits.com/showthread.php?t=16210

[Reply #8 on:]

quick google -> http://forum.allaboutcircuits. com/showthread.php?t=16210

Thanks again for the help Rob.

I just wired up a reed relay with a coil resistance of 1050 ohms and a 1000uF electrolitic capacitor across the coil. Pulsing the coil with power and ground keeps it energized for about 2 seconds. Being that this is a reed relay and I'm planning to use it in an automotive application I'd rather use a standard Bosch style relay. Thanks to you and the link you provided I ended up finding this which incorporates just that.

http://www.the12volt.com/relays/page5.asp