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Project #255
OBJECTIVE: To build an alarm that plays music on
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 should also be lit. Adjust the variable capacitor (CV) for
the loudest signal.
Connect a jumper wire across points A and B and the music stops.
The transistor acts like a switch connecting power to the music IC.
Positive voltage on the base closes 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. Set the 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
Press the press switch (S2) to complete the current's path flow. You
might expect the LED to light instantly but it doesn't. The charging
current flows into the 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
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