Elenco Projects 102-305 - Manual (Page 56)

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Project #255
OBJECTIVE: To build an alarm that plays music on the radio.
Place the circuit next to an AM radio. Tune the radio so no stations are
heard. Set the slide switch (S1) on. You should hear the song play.
The red LED (D1) should also be lit. Adjust the variable capacitor (CV)
for the loudest signal.
Connect a jumper wire across points A & B and the music stops. The
transistor (Q2) acts like a switch connecting power to the music IC
(U1). Positive voltage on the base turns on the switch and negative
voltage opens it. Connect a string to the jumper wire and the other end
of the string to a door or window. Turn the slide switch on. If a thief
comes in through the door or window, the string pulls the jumper off
and the music plays on the radio.
Radio Music
Burglar Alarm
Project #256
OBJECTIVE: To build a light dimmer.
Press the press switch (S2) to complete the current’s path flow. You
might expect the LED (D1) to light instantly but it doesn’t. The
charging current flows into the 100
μ F capacitor (C4) first. As the
capacitor charges, the charging current decreases, input current to the
PNP transistor (Q1) increases. So current begins to flow to the LED
and the LED gradually brightens.
Now release the press switch. The capacitor begins to discharge,
sending input current to the transistor. As the capacitor discharges,
the input current reduces to zero and gradually turns off the LED and
the transistor.
Light Dimmer