I'm trying to design a circuit where a pulse on the input toggles the output, but if the output remains high for a certain time (e.g. 2 seconds) it will go low by itself. Low output remains low until another input pulse occurs. (Again I'm looking to use low-voltage ICs with a 3V power supply, if possible.)

The best I've been able to come up with is something like this:

input --> toggle stage -(final output)-> missing pulse detector --> feed back and combine with input

It seems a missing pulse detector is done with a 555 timer; and the toggle stage could use the other 555 on a dual chip, which might save space. That's still a lot of components though, including an extra IC for the logic gate.

I'm thinking it would probably be easier to do this with a microcontroller. But there might be a simple circuit that I've overlooked. (Perhaps there's a clever single-stage 555 setup that will do this.) Does anyone here know of one?

 

Maybe this will help?
http://home.cogeco.ca/~rpaisley4/LM555.html#22

 

Thanks, but I already read that page. It helps fill out part of the design I posted, but I'm trying to replace that with something smaller, which I don't think this page covers. The first two circuits are basic missing pulse detectors, and the others only seem to detect the first missing pulse?

 

Hi,

I think if you look at the datasheet for a CD4047 one-shot, you will be able to get it to perform your function.

 

I've been thinking about that very hard, and I'm afraid I can't see it. May I have a further clue?

 

The way you describe it sounds more like you are after a monostable with a 2 second timing period rather than a toggle..?
http://www.zen22142.zen.co.uk/Circuits/Timing/555mono.htm

 

OK, I re-read your question and perhaps you want the output to toggle unless there is a pause in the proceedings for more than 2 seconds, in which case you want the toggle to reset. A quick drawing of the good ol 4013 with the output also being diverted to a basic R/C time constant which triggers the Reset pin oughta do the trick.
http://img.photobucket.com/albums/v474/pe9le9/timedtoggle.gif

the 4000 CMOS series are supposed to work as low as 3volts, but thats right on the very edge of their specs. Component count is 4, plus you have a spare "D" flip flop to play with. I havn't worked out the values of C and R but use a low value Cap, and high value resistor then work on the finer details of the time constant. This is just outta my head, so I havn't tried it, but can't see why it wouldn't work.... btw, if size is an issue, all these bits are commonly available in SMD.

 

OK, I re-read your question and perhaps you want the output to toggle unless there is a pause in the proceedings for more than 2 seconds, in which case you want the toggle to reset.

Yes, that's what I meant =).

btw, if size is an issue, all these bits are commonly available in SMD.

I don't have the equipment for using SMD, so size is an issue in the large-scale sense. Also I'm trying to minimise the number of solder joints.

A quick drawing of the good ol 4013 with the output also being diverted to a basic R/C time constant which triggers the Reset pin oughta do the trick
...
the 4000 CMOS series are supposed to work as low as 3volts, but thats right on the very edge of their specs. Component count is 4, plus you have a spare "D" flip flop to play with.

Thanks! That looks possible. The usual story for me- I'm having quite a struggle finding the CD4013 in a small quantity for a reasonable price. There's one on eBay for £2.25 which I could buy grudgingly =(

EDIT: Maplin have got the 4027 (JK flip-flop) which I think might be able to do the same thing. Not sure though...

EDIT2: Yes, that should work. I think the equivalent circuit is obtained by removing the feedback loop from /Q, and tying J and K to power.

 

Yup, your onto it. Should be fine. Slightly larger chip (by 2 pins) but similar availability. I always carry some of each in my parts tray, along with the inverters, Nands, Nors etc etc etc.
Updated circuit...
http://img.photobucket.com/albums/v474/pe9le9/toggle2.gif

The 4013 retails here at $1.00 NZ (I think thats 33p UK money....I assume there are 100p in a Pound nowadays...??).
the 4027 is the same price. (less in quantity).
There must be a hobbiest somewhere with em...
Good Luck

 

Yes, your diagram matches what I was thinking.

I'm going to get a small number of 4027 from Maplin at 83p each (they don't stock the 4013). If/when I next want £30 of stuff from Farnell, I can obtain both ICs for less than 30p- but that is some time away yet. (The 4013/4027 look really useful for other things too.)

Thanks for your help. Hopefully I'll be able to try this out soon and see how it behaves in practice.

 

It's working great! Thank you very much!

 

No Probs. Theres still the odd thing you can do without getting a PIC and programmer out....