Old Geezer needs help with Interval Timer Circuit

Its been a LONG time since I've done any real work with electronics, and I need to come up with an "interval" timer. This device needs to work on 12V, and produce a 1-2 second pulse (relay closure) when triggered. That part of it is easy, but I also need this timer to NOT cycle for 5 or 10 seconds after the last trigger pulse is recieved. The triggers may be short (half a second) or may be continuous for a minute or so.

Any help that anyone can offer will be greatly appreciated.
THANKS,
Maurice

Give me an example of something that you've built so I can gauge your abilities...

To start off though, here's how I'd probably set things up:

1) start with a capacitivly coupled switch - this will do two things: A) It will cause the circuit to trigger at the start of the button press, regardless of how long the button is pressed down. B) It will allow you to set a minimum time between button presses by setting adjusting the RC time constant.

2)The actual pulse itself can be generated by a diode, capacitor, transistor (and a couple resistors). Take the pulse generated by the capacitively coupled switch and use it to turn on a transistor. If the relay on-time needs to be consistent, then there will need to be another transistor there.

Of course this depends on your inclination/skills. Some would instantly throw a microcontroller in there just so they have the utmost flexibility, while others might lean towards other solutions...

First question is easy...electronic abilities are LOW.

The pulse will be from a 12volt source...a relay closure. (specifically, the output from a VOX circuit. What I had drawn out, was having the pulse trigger an SCR, which would be connected in series with a zener (to provide a more reliable cutoff) and the relay coil. This line would charge a capacitor, instead of being grounded, and as the capacitor charged to a certain point, the zener would stop conducting, and which would cause the SCR to and the coil relay to drop out.

I'm new at this forum...maybe I can draw out what i have...and post the image here??

Once the values are set for this circuit, they dont really need to be adjustable, only dependable, so I didnt want to go to the extra sophistication of digital circuits.

Thanks!!!!!

Okay, then if this thing is essentially going to modify some other circuit's relay drive, then the solutions change a bit... The relay is currently being driven by a transistor or something? A schematic of the stuff right in front of the relay would be useful.

James

Ok..Im trying to upload a schematic of something I had in mind. It is attached as delayrelay.jpg.

When a 12 volt trigger is applied to the SCR, it begins to conduct, and immediately trips the relay, which begins to draw current from the C4 (sized to allow for 1-2 seconds of current. D1 provides a sharp, dependable cutoff for the relay. Anytime a pulse is present (it can be anywhere from .25 seconds or so to continous), it charges C3, which will prevent the relay from tripping for a set time period after the last pulse.

What do you think??
Thanks again for your help!!

eek.

'scuse me.

I don't think the relay will ever turn on - since D1 will never conduct any reasonable amount of current. Also, is 4.7uF enough to power a relay for 1-2 seconds? I'd guess that the you might be off by an order of magnitude (or two).

Any better ideas, even starting from scratch? One problem, is that the capacitor sizes need to be kept fairly small...this project is for automotive use, and I have to use either tantalum caps or automotive ones for vibration resistance.

I started drawing out a circuit, but then realized that it's easier to just "say the correct name" and point you to someone else's pretty pictures. In this case, the keyword is "monostable multivibrator" to generate the 1-2 second pulse.

If you want to just use transistors, https://en.wikipedia.org/wiki/Multivibrator has a monostable circuit - two transistors, 3 resistors and a capacitor, or alternatively a 555 based circuit would also work. (Just google for "555 monostable" and you'll hit a pile of links).

So that circuit will generate a consistent output pulse, and the only thing left to do is to figure out how to do the input side - as well as control the timeouts. Now there's one bit of confusion in the specification you gave - is it supposed to wait 5-10 seconds between the start of the pulse, or is it supposed to wait 5-10 seconds after the first pulse has gone low?

James

Choice B...I don't want the circuit to be able to fire again for 5-10 seconds after the last pulse has gone low. Initially, I thought I'd be able to make up a very simple circuit, the one that i posted, but I see its going to have to get just a little less simple. I'll post it when done, and maybe you can see what you think about it. THanks for all your help!!!

no prob.

BTW, if that is the input functionality you need, then you will only need 1 capacitor, 3 resistors and a diode or two, in addition to the parts for the monostable pulse generator. (The other choice would require a little more thought...)

good luck.