This very simple alarm 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 alarm circuit transmits on MW (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 4093 CMOS IC1b.
When a hand or a foot approaches the sensor, the frequency of the transmitter (CMOS IC1a) drops appreciably.
Suppose now that the alarm 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.