Bill, Let me know when you post please I would like to see it. And how difficult would it be to add digital led or lcd ? I'm just posing a question. ? thanks Jim P.S. In this post you did say I could share batt. and piezo buzzer http://forum.allaboutcircuits.com/showpost.php?p=269134&postcount=10 I'm thinking I can just run jumpers from board to board . Is this correct? Jim

 

I haven't tested the latest layout, but here it is...

The old drawing were full of bugs, now to wire this and test it. I'm not really finished with the drawing, there are a couple of tweaks I want to do first, and as I sit here looking at it I found another bug.

Yes, it is entirely possible to build a count down timer, but it will be a lot more complex. There is a family of chips, called microcontrollers or µC, that can be programmed to do it and are a lot simpler, but I'm not into those at all. They are basically a computer on a chip.

If I were to do this with a 3 digit display old school (with individual chips) it would take around 8 or so chips, maybe a few more.

 

Hopefully, this is the final version.

 

Bill, can I make this circuit easier by installing only one SPST switch and if so where would I have to place it? jim

 

You could do this. I was thinking push buttons, but using a simple goggle switch the design can be radically simplified, eliminating the transistors completely. I'll post the new drawing when I can.

I had missed your last post, and have been working on the variation last seen. The last schematic had a bug, which has been fixed with this design.

I'll be back soon.

 

OK, the design you might be looking for looks like this...

This design is used by 555 Monostable project, you can lift the layout from the project for the most part.

Adjust the resistors R1 and R2 to the same values used earlier for maximum brightness.

I notice I missed the pot on this drawing, it was a quick and dirty effort. You'll need to add it.

If I don't get back with a post after a day or so feel free to PM me.

.

 

Tired computing bites again...