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1. Build the
breadboard circuit shown above. This is a dark sensor and relies on
several components, in particular the transistor. When the LDR is
covered the LED will light, although there may be a need to alter the
setting of the preset resistor.
How it works:
When light shines into the LDR its resistance is high and consequently
current cannot flow from positive 9 volts to negative 0 volts. If the
LDR is completely covered its resistance falls dramatically. Current
then flows into the base of the transistor switching it on. Consequently
current can flow through the collector and emitter - therefore, the LED
lights.
2. Try building a light sensor, that
is a circuit in which the LED lights if light shines into the LDR. HINT;
try swapping round the LDR and the preset resistor.
3. Try building a similar circuit
but this time add an arrangement of transistors called a
Darlington
Pair. Do you find any difference in the operation of the circuit ?
4. Try building a similar dark
sensor but this time with a relay rather than an LED. |
