Light Activated Wireless Burglar Alarm 1
This project is a burglar alarm wireless. It consumes no current from the battery and is dormant when the CdS cell is in the dark. But in light, the circuit emits a shrill tone through the radio band AM. Place a radio near the installation and you will know quickly what we mean.
An application of this circuit is the safety. Such a circuit can be used in a drawer, a locked room, etc. A radio placed near, but not necessarily in the same room, can be used as alarm monitoring.
In the darkness, the photoelectric CdS cell has a high resistance so that no base bias current can flow when the transistor is off. The 10K resistor through B-E of the transistor ensures that the transistor is turned off in this condition. Under a sufficiently powerful light for adequately reducing the resistance of the photoelectric CdS cell, a bias is supplied to the base of the transistor through the resistor 100K and the CdS cell.
The resulting oscillations are energy peaks that have signal components through the whole AM radio band. You will not need an antenna if you put the radio within 20 feet of this circuit.