Using a countdown timer without a microcontroller

Hi Everyone.

Can anyone tell me. How could i get this to work without using a microcontroller.

Has anyone done this before. If someone did. Do have a wired circuit diagram that you created that works.

What I want to do is.. take a cheap kitchen digital DC countdown timer and connect it to my photo developer and wire the timer inline to the developers AC wiring, So when i set the time on the timer it will start counting down and when it stops on zero the lights will shut off automatically on the photo developer.

P.S I do NOT want to build a unit from scratch. Can anyone Help!

thanks.

countdown timer schematic

The basic core you need is a set/reset flip-flop. Check out this URL if you are unfamilar with the operation.

http://hyperphysics.phy-astr.gsu.edu/Hbase/electronic/setreset.html

In addition to the flip-flop, you have to tap into the start button of the kitchen timer. With a sample of this button, you have to buffer then signal and then use it to set the flip-flop. The output of the flip-flop would be used to turn on a relay which you control your exposure light.
Next, you have to tap into the beeper circuit of the kitchen timer and buffer this signal before using it to reset the flip-flop.

Again, the core is the set/reset flip-flop. This by design provides the debouncing you need and hold its previous state until the next trigger event.

Finally, you need an manual override to allow you to reset the flop if it powers up in the ON state. You can add an OR gate to the beeper signal. Another option with minimal hardware is to just set a dummy 1 sec exposure if the light comes up in the wrong state.

countdown timer circuit

banjo


Can you provide a simple diagram of how this circuit and setup is going too work and look like, and how many circuits and compotents are needed to make this retrofit.
Like i said i do not want to build a complete unit or a complicated circuit to make this work and be safe to use. Can spend more then $15 to make circuit.

I need just one simple circuit to interconnect with the timer and photo developer.

The timer i have is just a simple dc battery operated kitchen countdown timer you can buy at radio shack . The photo developer i build myself with 2 bulbs a electromic ballast kit i bought off the shelf and a on/off switch LED INDCIATOR wired for AC 115 powered house current thats it. Simple.

Can you please elaborate more on this retrofit inline circuit.

All help and information is appreciated and greatful. I thank you for replying to this post.

thanks:

countdown timer schematic diagram

I'm sorry, but this is not a straight forward project. To accomplish this, you have to do a fair amount of experimenting and testing. The difficulty is that you need to interface with an unknown circuit. The keypad interface of the timer can use any one of a number of interface techniques. Likewise, the buzzer output could be a simple on/off, or a pulse, or a waveform. Until the interface is known, its hard to know what the exact circuit would look like.

You mentioned the timer is from Radio Shack. Some Radio Shack devices came with schematics or the schematic is sometimes available on the Internet. If you can point the forum to a schematic, we can be of more help.

digital countdown timer circuit diagram

what the microcontroller you are going to use.

exposure timer circuit

Azka said:

what the microcontroller you are going to use.

Click to expand...

always read the topic carefully he told that he never want to use Uc

regards
Fragrance