Elenco Electronics 202 - Manual (Page 53)

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Project #97
OBJECTIVE: To show how light is used to turn an alarm.
The alarm will sound, as long as light is present. Slowly cover the
photoresistor (RP), and the volume goes down. If you turn off the
lights, the alarm will stop. The amount of light changes the resistance
of the photoresistor (less light means more resistance). The
photoresistor and transistor (Q2) act like a dimmer switch, adjusting
the voltage applied to the alarm.
This type of circuit is used in alarm systems to detect light. If an
intruder turned on a light or hit the sensor with a flashlight beam, the
alarm would trigger and probably force the intruder to leave.
Light-controlled Alarm
Project #98 Fading Siren
OBJECTIVE: To produce sound of a siren driving away into the
distance.
Press the press switch (S2), the integrated circuit (U2) should make
the sound of an up-down siren that gets weaker with time. The fading
is produced by the charging of the 470
μF capacitor (C5), After it is
charged the current stops and the sound is very weak.
To repeat this effect you must release the press switch, remove the
capacitor, and discharge it by placing it across the snaps on the
bottom bar marked A & B. Then, replace the capacitor and press the
press switch again.