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! :-)

Six6siX · 22 Jul 2007 at 20:03

What's it going to be driving?

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.

benjo plz. · 22 Jul 2007 at 20:05

Use a bistable to control a monostable, or use a relay.

BenST · 22 Jul 2007 at 20:12

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

sniffy · 22 Jul 2007 at 20:12

Phnom_Penh said:
Use a bistable to control a monostable

I would personally do that.

Edit

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

The monostable provides the delay.

Glaucus · 22 Jul 2007 at 20:13

just google 555/556 timer circuits. Loads out there.

Six6siX · 22 Jul 2007 at 20:14

I'd go the direct route without the IC's...makes things a bit simpler.

Saving me typing it out again, have a look at this circuit I made for a similar purpose:

Click!

Basically, ignore the 5V line and that circuit gives you variable delays up to about 5 seconds with the variable resistor on max.

That any use?

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.

benjo plz. · 22 Jul 2007 at 20:51

BenST · 22 Jul 2007 at 21:07

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

benjo plz. · 22 Jul 2007 at 21:53

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

ah no, I'd go with an astable instead, you set the astable to a 0.5 hz frequency, and then have the monostable on the output of the astable, so that after 2 seconds, the monostable is triggered, and will stay on for the time you want it to.

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 :'-(

sniffy · 24 Jul 2007 at 19:53

BenST said:
?

I need a delay of two seconds before a +ve output is activated for 1~5 seconds (variable).

Yeah sorry I didn't read it properly.

Edit

What I posted was ****. Let me make another post.

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.

sniffy · 24 Jul 2007 at 20:33

I'm almost falling asleep here so finding it hard to provide you with anything meaningful lol.

BenST said:
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.

I didn't know that, certainly didn't cover it when we used 555's in our projects.

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.

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

sniffy · 24 Jul 2007 at 20:41

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

Hopefully someone with some more info can comment on that.

Have you thought of downloading simulation software and trying out some circuits you've got in mind? I would personally try that out. They aren't amazing but usually highlight problems like this. Better yet, get hold of a breadboard and have a little play

I'll have another look for you tomorrow when I can think straight

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!