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
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,
would be tuned to a frequency just below that of the transmitter.