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Step 1: Materials/Supplies
1. A single 1000uF Capacitor
2. A 5K Trimpot (larger values will work)
3. CdS Photocell (Cadmium Sulfide Cell)
4. Some Perforated Board
5. A 9v battery and clip
6. The 2N3904 Transistor
7. Several Small mirrors
8. About 5-12VDC Piezo Siren (102dB)
9. Any General Laser (650nm 5mw)
8. An L7805 5v Regulator
9. The Project Case
10. A 5 - 9 volt Adapter
Step 2: How Does it Work?
We'll be using this large resistance change (about 10k ohm in daylight to about 1M ohm in pitch dark) to switch on/off a transistor for detecting the dark (when the beam is broken).
They capacitor is used to run the buzzer for several seconds (depending on the size of the cap) even in the beam is broken for only a fraction of a second. To get the siren to go off longer, just use a bigger capacitor (1000 uF) or simply put more capacitors in series with the current one.