Data Sheet
123. Reverse Control using Photoresistors
Build the circuit shown and turn on the switch (62). Now, whenever light
shines on the photoresistor (68), the heart LED (69) will be off. Cover the
photoresistor (68) with your nger and the heart LED (69) will turn on. You
may need to take your circuit into a dark room to see the heart LED (69) light.
This circuit does the reverse of the circuit in project #116 by turning
on the heart LED (69) in darkness and turning off the heart LED (69) in
light. This is done by using the NPN transistor (50) as a switch. When
light shines on the photoresistor (68), this creates a very low resistance
path from the Base of the NPN transistor (50) to ground (0V), and
thus very little current will ow through the Base. This means very
little current will ow from Collector to Emitter of the NPN transistor
(50) and thus the heart LED (69) does not light. But if you cover the
photoresistor (68) with your nger making it dark, this creates a very
high resistance path from Base to ground and thus the current through
the 100kW resistor now ows through the Base of the NPN transistor
(50), turning it “ON” (allowing a large current to ow from Collector to
Emitter) which turns on the heart LED (69).
124. Street Lights
Replace the heart LED (69) with the star LED (70) and turn on the switch
(62). Whenever light shines on the photoresistor (68), the star LED (70)
will be off. Cover the photoresistor (68) with your nger and the star LED
(70) will turn on. This circuit could be used to control street lights, where
during the day when light shines it turns off the street lights, but once the
sun goes down and it gets dark, the street lights turn on.
125. Audio Compressors
Build the circuit to the left and turn on the switch (62). Whenever light
shines on the photoresistor (68), the alarm (78) will be off. Cover the
photoresistor (68) with your nger and the alarm (78) with sound.
Photoresistors are sometimes used as audio compressors to reduce
the gain of an amplier when the audio signal level is above a threshold.
This can be done by using an LED that indicates changes in audio signal
levels, and feed this into a photoresistor that adjusts the gain in the
amplier (transistor) based on the LED brightness (which indicates audio
signal level). Due to the slow response time of the photoresistor (68), it
is believed that this technique can provide smooth audio compression.
-75-
+
–
+
–
+
–