Light alarm circuit for detecting the presence and intensity of light

Light alarm - 1

The circuit works when the photoresistor receives light.

When no light falls on the light dependent resistor (LDR), its resistance is high, which makes the transistor driving the speaker non-conductive.

When light falls on the light-dependent resistor (LDR), its resistance decreases, which causes the collector potential of the second transistor to drop. Through the second 100n capacitor, the first transistor is turned off. At this time, the 47K resistor and the first 100n capacitor accelerate the conduction of the second transistor, outputting a spike pulse in the speaker. This continues until the first 100n capacitor is fully charged, and the collector potential of the second transistor rises, which forces the first transistor to perform the same process, and so on, forming an audio oscillation.

Light alarm - 2

This circuit is similar to the Light Alarm-1, but since the circuit is connected directly to the speaker, a loud audio output will be produced.

The circuit is basically a high gain amplifier. Initially, the transistor starts to conduct when the LDR charges the 10n capacitor, and the positive feedback of the capacitor accelerates the conduction. When the potentials at both ends of the capacitor are equal, the transistor is quickly cut off, and then the cycle is repeated to form audio oscillation.

Light alarm - 3 (motion detector)

The circuit is very sensitive and can be placed in a room to detect the motion of people.

The front end of this circuit is connected to a high gain amplifier (from the first to the third transistor) to amplify the photoelectric signal received by the photoresistor (LDR) or photosensitive Darlington transistor. The third transistor charges the 100u capacitor through a diode and provides the oscillator with a turn-on voltage. Using an LDR instead of the phototransistor in this circuit will have the same sensitivity.