Compare Switch Types
BJT switches are fast and the transistors are not expensive. A minus point is the current needed to operate the switch.
MOSFETS are even faster but they cost more. A plus point is the minute current needed to operate the switch - nearly zero.
OP-Amp comparators are slow, more expensive, but much more sensitive to input voltage changes.
Logic GATE switches are fast and cheap but not very sensitive to small input voltage changes.
Mechanical switches are less reliable due to the moving parts.
Electromagnetic relays are less reliable due to the moving parts.
Mechanical switches and relays can switch high voltages and currents and also alternating current.
Switches can be build around logic gate inputs. In this example a NOT gate is used.
If it's light, the LDR resistance will be low and the NOT gate input will be high.
It the light sensing voltage divider output is more than half the power supply voltage, this is a logic ONE.
The NOT gate input will be high.
The NOT gate output will be low.
The electromagnetic realy will be off.
The high voltage lamp will be off.
If it's dark, everything is reversed and the high voltage lamp comes on.
It's better to use a Schmitt trigger NOT gate. Without this, the lamp might flash on and off rapidly.
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