680 ohm resistor to protect the LED.
1K resistor from LDR to the base of the NPN transistor.
One BFY50 npn transistor (try any alternative).
One 10K preset resistor.
PIN LAYOUT OF NPN TRANSISTOR
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
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
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.