figure 1 represents a cheap simple gate alarm, that is intended to run off a small universal ac-dc power supply. ic1a is a fast oscillat, ic1b a slow oscillat, which are combined through ic1c to emit a high pip-pip-pip warning sound when a gate ( window, etc.) is opened. the circuit is intended not so much to sound like a siren warning device, but rather to give the impression: "you have been noticed." r1 d1 may be omitted, the value of r2 perhaps reduced, to make the gate alarm sound me 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, befe it falls silent, as if to say: "i'm me 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 resist. figure 2 shows how an dinary reed switch may be converted to close (a "nmally closed" switch) when the gate is opened. a continuity tester makes the wk easy. note that many reed switches are delicate, therefe wires which are soldered to the reed switch should not be flexed at all near the switch. other types of switches, such as microswitches, may also be used.
