Radio Wave Alarm
This simple circuit is sure to have the police beating a path to your door -
however, it has the added advantage of alerting you to their presence even
before their footsteps fall on the doormat.
The circuit transmits on Medium Wave (this is the small problem with the
police). IC1a, together with a sensor (try a 20cm x 20cm sheet of tin foil)
oscillates at just over 1MHz. This is modulated by an audio frequency (a
continuous beep) produced by IC1b. When a hand or a foot
approaches the sensor, the frequency of the transmitter (IC1a) drops
appreciably.
Suppose now that the circuit transmits at 1MHz. Suppose also that your radio is
tuned to a frequency just below this. The 1MHz transmission will therefore not
be heard by the radio. But bring a hand or a foot near to the sensor, and the
transmitter's frequency will drop, and a beep will be heard from the radio.
Attach the antenna to a multiplug adapter that is
plugged into the mains, and you will find that the Medium Wave transmission
radiates from every wire in your house. Now place a suitably tuned Medium Wave
radio near some wires or a plug point in your house, and an early-warning
system is set up.
Instead of using the sheet of tin foil as the sensor, you could use a doorknob,
or burglar bars. Or you could use a pushbutton and series resistor (wired in
series with the 33K resistor - the pushbutton would short it out) to decrease
the frequency of IC1a, so activating the system by means of a pushbutton
switch. In this case, the radio would be tuned to a frequency just below that
of the transmitter.
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