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A two tone Alternate Alarm Circuit

2016-02-20 10:47  
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This two-zone alarm has automatic exit, entry and siren cut-off timers. It was developed for the Beginner's Guide To Cmos Timers. So it has a particularly detailed circuit description. There's an optional One-Time-Only module. It will force the siren to switch off after the first ten minutes - and prevent the alarm from activating a second time.A two-tone generator that is alternately switched ON provides a high/low output as might be heard from a traffic vehicle like a police car or ambulance. IC1,  CD4011 , quad 2-input NAND gate is a two-tone oscillator in which each side, pins 1 through 7 and 8 through 13 set the tone frequencies. Changing the values of C9 and C] determines the high/low tones. The outout frequencies are coupled to IC2, CD4011, of which one side (pins 1 through 6) acts as a buffer. The buffer is necessary to prevent loading on the outputs that would occur if one tried to go directly to the LM386 amplifier. The other side of IC2, pins 8 through 13, is a slow pulse oscillator of approximately 8 Hz per second.

Alternate Tone Alarm Circuit 
 

The output at pin 10 is connected to IC4 as a clock. IC4, CD4027 , is a dual J-K master-slave flip-flop that is wired to perform as a toggle switch in which Ql and 15, and Ql (NOT) pin 14, go high and low alternately (flip-flop). The clock input from IC2 pin 10 is connected to pin 13 of IC4, and the outputs at pins 15 and 14 changes the flip/flop state with each positive pulse transition. The CD4027 functions in toggle mode when the set and reset inputs, pins 9 and 12, are held low or grounded. Also, J-K inputs, pins 10 and 11, must be held high or to the positive. When this alarm is activated its Siren will sound once - for up to 20-minutes. Then it will switch off and remain off.