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Inside the prototype, with the PCB swung out so you can see the components. The NiCad cells and siren are visible underneath.

&nd activate a latching Circuit, in the case of the power to this unit bcing in-terrupted. The latching Circuit when op-crated enablcs an oscillator which in turn drivcs sv\itching transistors to pro-duce an AC output voltagc. The AC output from this stage is then applied to a voltagc multipler circuit which pro-duces sufficient output voltage to drive the 12V piczo siren. Once the latching circuit is operated the only external way of stopping siren operation is by operat-ing the kcy switch (SW1).

In morę dctail, the input voltage which is derived from the main alarm is applied via Rl, Dl, R2 and D3 to the 4.8V nicad battery pack (4x1.2V penlite batteries). RI and R2 limit the charging

ELECTRONICS Australia. September 1987

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