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Project #320
OBJECTIVE: To build a music alarm.
The alarm circuit activates when you remove the jumper wire from
points A & B. The jumper wire shorts the SCR’s (Q3) gate to ground
and the SCR does not conduct. Removing the jumper wire places a
voltage on the gate and the SCR conducts. This connects the battery
to the music IC (U1) and music is played.
Construct the circuit and you should hear no music. Now remove the
jumper wire and the music starts playing.
Music Alarm
Project #321
OBJECTIVE: To build a light-music alarm.
Light-Music Alarm
Use the circuit in project 320. Replace the resistor R3 with the photo
resistor (RP) and remove the jumper wire. Cover the photo resistor
with your hand. Now slowly remove your hand. When enough light
hits the resistor, the music plays.
Project #322
OBJECTIVE: To build a circuit that activates a bulb and motor
with the amount of light present.
Cover the photo resistor (RP) with your finger. Turn on the switch
(S1), and only the LED (D1) lights. The relay (S3) connects the motor
(M1) and the bulb (L2) to the batteries, but the motor and bulb are
powerless until a voltage is applied to the SCR's gate.
Remove your finger, as light hits the photo resistor, its resistance
decreases and a voltage appears on the gate of the SCR (Q3). The
SCR conducts and the motor and bulb work now.
Light-Controlled SCR
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