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How does one make a reverse photocell? Answered

How can you use a photocell to make a circuit whose resistance is proportional to incident light? I'm looking for an open circuit in the dark and a short circuit in lighted conditions.

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Simple circuit.

A photo cell to a NPN transistor base and emitter.

Light strikes the photo cell and completes the circuit making a dead short between collector and emitter of the transistor.

But when dark there is no current flow to the base of the transistor so the circuit is open.

The NPN transistor depends on the current and voltage of the circuit.

Joe

Hi Joe - What is a NPN transister and where can I get one? Also, is it good for a 120 Volt and up to 500 Watts? Thanks.

Look up light and dark detector circuits.

Yes, a standard CdS cell has that property.