Sensitive Light Controlled Switch 1
Here is a nice little project that will detect light intensity change and indicate the presence of this change by flashing lamp or lighting it either.
On one hand, this project is similar to the Light Hum and Noise Detector see the Amplifiers section that made you hear the roar and the sound in the earpiece. The main difference is that it is more sensitive and can react to a wide range of changes in light and noise.
The buzz of characteristics running / stop of a fluorescent lamp keeps the lamp burning. Incandescent lamps and sunlight require some interruption or change in light level or remained lamp off. Because of this, you can use these light sources for various things accomplished.
For example, while using sunlight or an incandescent lamp, you can temporarily switch your hand between the light and the photoelectric cell, and the lamp will light for a short time. If the movement of your hand is very slow, nothing will happen. When you use a fluorescent light, the photoelectric cell must be in the shadow of something or the lamp will remain on.
We discussed the various circuits used for this project elsewhere, so we'll give a brief explanation here.
Resistance of 47K and the CdS cell form a voltage divider for the 9 volts. The voltage across the CdS cell then depends on the change that the light due to the voltage division for these two resistors.
The Q1 is a Class C amplifier stage zero-bias which amplifies the voltage loads appearing through the CdS cell. The Circle of three is an audio amplifier output transformer coupled. The germanium diode (Ge) changes the AC signal to DC that is filtered by the 10 uF and applied as base bias in Q2 to turn it on. The relay is wired to light the lamp when a signal is present.