square wave. When these signals are both high, the negative lead is held high and the sounder is off. When either signal is low, the sounder will be energized The result is a tone ot about 5 kHz that is keyed at a 1-Hz rate and modulated at a 10Hz rate.

The timer requires clock signals of 1 pulse per minute to drive the counter, 1 pulse per second the key the alarm, and 10 pulses per second to modulate the alarm, These signals are produced by a divider chain driven by an astable multivibrator, as shown rn Fig 4. The multivibrator output is a nonsymmetrical square wave whose high time, t|, is 60 ms and whose low time, t^. is 40 ms. The period, t! + t>, is 100 ms and the frequency is thus 10

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