Figure 1 represents a cheap and simple Gate Alarm, that is
intended to run off a small universal AC-DC power supply.
IC1a is a fast oscillator, and IC1b a slow oscillator, which
are combined through IC1c to emit a high pip-pip-pip warning
sound when a gate (or window, etc.) is opened. The circuit is
intended not so much to sound like a siren or warning device, but
rather to give the impression: “You have been noticed.” R1 and
D1 may be omitted, and the value of R2 perhaps reduced, to make
the Gate Alarm sound more like a warning device. VR1
adjusts the frequency of the sound emitted.
IC1d is a timer which causes the Gate Alarm to emit some 20 to
30 further pips after the gate has been closed again, before it
falls silent, as if to say: “I`m more clever than a simple on-off
device.” Piezo disk S1 may be replaced with a LED if desired,
the LED being wired in series with a 1K resistor.