This is a Dark Detector circuit that utilizes 1) the astable ocillator that you can make with a 555 to drive a piezo and 2) the reset threshold of the chip.
Credits to Tony van Roon for the circuit diagram.
I'm planning to either
1) combine it with a strobe light so with each "off" cycle, it screeches or
2) Put it over the doorbell (so it can be used seasonally too! : ] )
Step 1: Step One: Gather the Parts
555 Timer IC (I used a Texas Instruments NE555P, you can use pretty much any other brand)
A breadboard (doesn't have to be big)
Some wire; bare or stripped (you can use staples if they are thin enough)
One 1 megaohm resistor
One 100K resistor
One 100 ohm resistor (a 100 ohm potentiometer is better but optional)
One 1000 picofarad capacitor (in nanofarads: 1 nF in microfarads: .001 uF)
A Cds cell
A Piezo siren/buzzer
The resistor values can be experimented with but I reccomend these values for the most sucess.
The piezo can also be a speaker.
The capacitor value should probably stay the same.
You can try different Cds cells.
A great place to find components is the Electronic Goldmine: http://www.goldmine-elec.com
Step 2: Step Two: Put the timer in
Step 3: Step Three: Stick the negative and positive rails in.
Connect the 1st pin with the negative rail.
Then, connect the 8th pin with the positive rail.
If you need help with the pinouts, see the Pin Diagram picture.