Here is a circuit of light controlled lamp. This is basically a Schmitt Trigger which receives input from a cadmium sulfide photo cell and controls a relay that can be used to switch a lamp on and off at dawn and dusk. The photo cell should be shielded from the lamp to prevent feedback so the lamp light does not strike the photo cell and switch off the lamp.
The photo cell is wired in series with a potentiometer VR1, so the voltage at the base of transistor Q1 can be adjusted to about half the supply, at the desired ambient light level. The two PNP transistors are connected with a common emitter resistor to produce a gap between the on and off voltages - called the HYSTERESIS GAP.
Under dark conditions, the photo cell resistance will be high producing a voltage on the base of Q1 that is higher than the base voltage on Q2. This causes Q2 to conduct and activate the relay.
The switching points are about 8 volts and 4 volts using the resistor values shown but could be brought closer together by using a lower value for R3. A value of 3k3 would move the levels to about 3.5v and 5.5v.
The photo cell is wired in series with a potentiometer VR1, so the voltage at the base of transistor Q1 can be adjusted to about half the supply, at the desired ambient light level. The two PNP transistors are connected with a common emitter resistor to produce a gap between the on and off voltages - called the HYSTERESIS GAP.
Under dark conditions, the photo cell resistance will be high producing a voltage on the base of Q1 that is higher than the base voltage on Q2. This causes Q2 to conduct and activate the relay.
The switching points are about 8 volts and 4 volts using the resistor values shown but could be brought closer together by using a lower value for R3. A value of 3k3 would move the levels to about 3.5v and 5.5v.
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