Low-cost Gated Alarm Circuit

Low-cost Gated Alarm Circuit Project build very easily and simple. The circuit shown here employs just four components and a piezo sounder and is unlikely to be out-done for simplicity. While it does not offer the most powerful output, it is likely to be adequate for many applications.

Circuit Diagram:

Circuit Details:

A dual C.M.O.S timer IC type 7556 is used for the purpose; with each of its two halves being wired as a simple as a standard 556 IC will not work in this circuit, or will two standard 555’s. Note that the “CMOS7556” is supplied by many different manufacturers, each using their own type code prefix and suffix. The relevant Texas Instruments product, for instance, will be marked ‘TLC556CN’.

The circuit configuration used here is seldom seen, due probably to the inability of this oscillator to be more than lightly loaded without disturbing the timing. However, it is particularly useful for high impedance logic inputs, since it provides a simple means of obtaining a square wave with 1:1 mark-space ratio, which the ‘orthodox’ configuration does not so easily provide.

IC1.A is a slow oscillator which is enabled when reset pin 4 is taken High, and inhibited when it is taken Low. Out-put pin 5 of IC1.A pulses audio oscillator IC1.B, which is similarly enabled when reset pin 10 is taken High, and inhibited when it is taken Low.