Timer circuit advice

BenST · 22 Jul 2007 at 19:56

Hey,

I'm looking for some help. I need a timer circuit that when turned on (power is supplied) it will idle for 2 seconds and then output +ve for 1-5 seconds (variable).

It will operate from 12v DC. I was thinking of a 555 circuit but I'm not too great with these things....

Thanks in advance! :-)

BenST · 22 Jul 2007 at 20:04

A small relay, coil resistance of 250ohms. I can work out transistor values and such, but I'm having trouble with my timing issue.

BenST · 22 Jul 2007 at 20:12

But how would I get my two-second delay before triggering the monostable?

BenST · 22 Jul 2007 at 20:50

sniffy said:
The monostable provides the delay.

?

I need a delay of two seconds before a +ve output is activated for 1~5 seconds (variable). How would a bistable achieve this?

Six6siX said:
That any use?

The problem is your circuit keeps the output permanent. I don't want it to be permanent.

BenST · 22 Jul 2007 at 21:07

How would a bistable provide a delay? Can you please explain further?

BenST · 22 Jul 2007 at 22:08

But then surely the astable will just keep triggering the monostable? I think we're getting there though

Thanks for all your help by the way

[edit] How about two monostables in series, would that work? Possibly, but how would I make the trigger for the first monostable upon powering the device?

BenST · 24 Jul 2007 at 19:25

Still can't figure this out :'-(

BenST · 24 Jul 2007 at 20:01

I'm (was) an A level electronics student and I still suck.

I understand what you're saying, but the monostable needs a positive-edge trigger, which needs to be a shorter pulse than the required output, so you're looking for 0ve -> +ve and back down to -ve - which isn't going to happen when I power the circuit up.

BenST · 24 Jul 2007 at 20:36

That's cool mate, have some sleep

sniffy said:
Have you thought of hooking up a RC circuit to the input of the 555? I'm sure you can arrange the 555 so the output triggers when the input is greater than 1/3 the supply voltage. Don't know if you run into the same problem as you mentioned above.

Yeah I have thought about an RC circuit but I'm still concerned about the edge-triggering, unless I'm getting it totally wrong

BenST · 24 Jul 2007 at 20:42

I have a breadboard / IC's etc, I may have a play

I don't know of any simulation software unfortunately.

Thanks for your help mate!

BenST · 24 Jul 2007 at 20:51

Cheers fella

BenST · 3 Aug 2007 at 21:01

A helpful gent from another forum posted this solution using NAND gates;

It works fine with a LED however I'm having much trouble trying to get it to trigger a relay. I think the transistor I'm using doesn't have a high enough HFE. The resistor coil is 250 ohms and AFAIK the max output of CMOS logic is 1mA?

Any help appreciated!