AUDIBLE_TIMER

Published:2009/7/14 5:30:00 Author:May | From:SeekIC

The audible timer relies on the principle of obtaining a relatively long timing period by dividing down the frequency of a high-frequency astable multivibrator. The chip used for the multivibrator is the well-known 555 timer in its CMOS form. The preset resistor VR1 allows the frequency to be set to 273 Hz. This provides the alarm tone. The 273-Hz signal is divided by the 14-stage counter IC2. At pin 8 of the counter, a signal at approximately 1 Hz is used to make the note intermittent. The timing frequency comes from pin 14 of the IC, at which the astable frequency is divided by 214, (16,384), giving a frequency of 1/60 Hz (1 count per minute). The output from pin 14 is inverted by transistor TR1 and fed to the clock input of a second counter (IC3).The outputs of this counter are normally at logical high, except that just one of the outputs is low at each stage of counting. When the counter is reset, output 0 goes high. On the next positive-going clock input, output 0 goes low and output 1 goes high. At each successive high-going clock input, the outputs from 0 to 7 go high, in turn, repeating. The rotary switch (S3) selects the output to be used to indicate the termination of the timing period. The circuit is reset by pressing button S2, which resets both counters and also the flip-flop formed by gates IC4a and IC4d. This turns off the NPN transistor TR2, which drives the loudspeaker, but turns on the PNP transistor (TR3), causing the LED to light.While the RESET button is held, the output from IC2 pin 3 is low, turning off TR1. The counter is incremented every minute until the output selected by S3 goes high. The high level from the selected output sets the flip-flop. The result is a note at 273 Hz, pulsing at the rate of 1 Hz. This is heard from the loudspeaker. The LED flashes on at the same time. Because the mark-space ratio of the astable output is high, the LED is turned on at almost full brightness during the ON periods.



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