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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. |
