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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.
Figure 2 shows how an ordinary reed
switch may be converted to close (a "normally closed"
switch) when the gate is opened. A continuity tester
makes the work easy. Note that many reed switches are
delicate, and therefore wires which are soldered to the
reed switch should not be flexed at all near the switch.
Other types of switches, such as micro switches, may
also be used.
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