Radio
Wave Alarm
by Rev. Thomas Scarborough - scarboro@iafrica.com
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.
