Hi all.
I was trying to build a simple circuit with one IC only (555 timer), but didn't get to a circuit that will work as I want.
I want to make some LEDs light for a short time (monostable..) when a sudden darkness happens. I thought about using a photoresistor and a resistor (with suitable values) to make a voltage divider at the trigger pin (2) of the 555 timer. The photoresistor is on the positive side and the resistor on the ground side...

I know that the timer triggers when a negative edge (sudden high to low..) is sensed on pin 2. So I tried the circuit choosing the timing components I need. When a sudden when I turned off the room lamp (it's dark), the LED lighted but all the time. I found that the problem is with triggering the 555 timer, the voltage at pin 2 should decrease suddenly and then increase, in my case it still low as there is darkness.

I think the overall idea can be made using one 555 but there is a trick. I also think it can be done using one two comparators (using LM358 for example). Or few transistors and especially mosfets like 2N7000 (which is on hand) with resistor and capacitor parallel between gate and ground... There are many ideas that will work but I'm looking for a simple small circuit.

Regards,
Hazim

 

If you post your current schematic we can help you better. From you description, just an additional resistor and a capacitor will do the trick. About 10k from pin 2 to Vcc, and a 0.1uf capacitor from the junction of the photocell and the series resistor to pin 2 of the 555.

 

Have you looked at this...

555 Monostable

Pay special attention to the signal conditioner description. It sounds like you don't have it.