Data Sheet

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119. Latency of a Photoresistor
Build the circuit shown on the left and turn on the switch (62). Whenever
light shines on the photoresistor (68), the colorful LED (72) will light the
ber tree (40). Cover the photoresistor (68) with your nger and the
colorful LED (72) may get a little dimmer, but even the little current
entering the Base of the NPN transistor (50) is enough to allow enough
current to ow from the Collector to the Emitter to light the colorful
LED (72).
Photoresistors exhibit latency between exposure to light and the
subsequent decrease in resistance. This latency is usually on the order
of 10 milliseconds. The latency in going from a lit to dark environment
is even greater.
120. Wake Up Alarm
Build the circuit shown on the left and turn on the switch (62). Whenever
light shines on the photoresistor (68), the alarm (78) sounds. Cover the
photoresistor (68) with your nger and the alarm (78) will go off.
This circuit could be used as a wake-up alarm, where the alarm (78)
goes off when the sun shines into your room.
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