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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.