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CdS Cell Light Meter with Transistor Amplifier
Électronique / Electronic

The purpose of this circuit is to measure the relative intensity of light similar to a light meter as used with cameras. The circuit uses a Transistor amplifier to obtain greater sensitivity than what is obtained with many light meters. The heart of this circuit is the cadmium sulfide (CdS) Cell which controls the amount of deflection obtained on the Meter.

A Control is included to allow measurements of relative light intensity from near darkness to full sunlight. The CdS Cell is effectively a Resistor (as its schematic symbol implies) that changes value depending on light energy present. In darkness its resistance is very high (in the megohms range) and in bright sunlight it is very low (about 100 ohms or so).

The CdS Cell is placed in the base-bias circuit of the Transistor so that an increase in light causes an increase in base-bias voltage. Recall that collector current is determined by base-bias current, increased base-bias causes increased collector current.

The collector current is caused to flow through the Meter to produce readout:

The 10K base Resistor protects the Transistor from excessive base current in bright light when the 50K Control is set at maximum.

The 1K Resistor is included to protect the Meter from excessive current should the Control be set too high, or the light too bright.

The Control should be kept low (near zero on the dial) unless a higher setting is definitely needed to get a usable Meter reading. You can use your VOM to check the resistance of the CdS Cell (when not connected into the circuit), use the ohm meter function.

You can use your VOM to see the change in voltage across the 50K Control, use the voltmeter function. For best results set the Control at zero for this test. The Control settings and Meter readings may be recorded for comparison of light levels. If a calibrated light meter is available, you can make a chart of Control settings and Meter readings that is accurate enough for most light meter requirements.







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