Monostable circuit, triggered by an ldr.

[James Taylor]

Hi folks,
I'm looking to make a small light that comes on in the dark, and then goes off after a set amount of time.

I figured that a monostable circuit triggered by a LDR would be the trick, however When I have tried the circuit out on circuit wizard, I found hat the timer didn't work while it was still dark.

is there any way around this problem?


Any circuits or suggestions re welcome.

Thanks

James

 

[Søren]

I'm looking to make a small light that comes on in the dark, and then goes off after a set amount of time.

I figured that a monostable circuit triggered by a LDR would be the trick, however When I have tried the circuit out on circuit wizard, I found hat the timer didn't work while it was still dark.

is there any way around this problem?

There's always a way!

Show what you did and where, mind-reading is not my strong side

First... What "circuit wizard"? (Google comes up with 863000 hits).
Second... What monostable circuit? (there are so many ways to make a mono-flop).
Third... What does your LDR measure at the point where you want it to turn on the 'flop?

If you use a 555 and the LDR (connected from Vcc to pin2) is say 20kOhm at the darkness level where you want it to switch on, you'd need a pull down of 10kOhm from pin 2 to ground.
Since it won't be a "nice" value in a real application, you need a resistor in series with a trimming potentiometer/variable resistor to dial in the switching point.