ok, let me explain this thing first: what i want to do is build a circuit that will start then stop a stop watch when a LED goes on then goes out. so the operation would be like this: LED turns on -> start watch; (after a while) LED goes off -> stop watch.

now my plan is to get a working stop watch then build a circuit that will preform the tasks listed above. the problem is that i have absolutely no idea of where to start, which is why i came here. because the stop watch is activated by connecting a circuit, and then stopped by connecting that same circuit; all i need is a circuit that will close a circuit for a short time then open the connection, when power is applied to the inputs. then when power is disconnected from the inputs the circuit will close the circuit again for a short time then open it. i know this sounds hard and confusing but its really way over my head. if anyone has any circuit that will do this please post, any help is welcome!

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w00t!

 

You need two monostable multivibrator e.g. CD4528. One section configured to rising edge, the other to falling edge.

 

I have a bunch of questions:
1. Is the light from the LED used to start and stop the watch?
2. How close to the LED can the light detector be?
3. Is the signal that lights the LED available?
4. Why do you want to do this?

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