POWER FAILURE ALARM DRIVER USING A LONG PERIOD TIMER FOR MUTING


Here is a circuit for the occasion when you want an alarm that can be muted but not permanently. Perhaps for a power loss alarm for a critical refrigerator with a DC control unit. The power fails and the alarm is annoying but switching it off completely could lead to somebody forgetting about it. Note the inputs are a  + DC voltage with reference to the circuit ground.  Most timer circuits are based on the 555 however for a long period timer the RC values get to be impractically large. This is solved in the ICM7242 as it counts 128 RC time periods in its one shot mode. The CD4093B is a nand Schmitt trigger but the normal 4011 nand gate should work as well. The nand gate has a 15V maximum so to protect it a zener was used to limit the supply voltage. For better noise immunity the 68K and 10K resistors in the input circuits could be replaced by 6.8K and 1K if the signal impedence permitted. 

This circuit is still under development and has not been tested. When it has been I will provide a PDF of the circuit board.


The circuit above provides a battery operated alarm driver with the ability to mute the audible warning for a some period controlled by the components connected to pin 7 of the counter (approximate 20 minute period shown). The time is calculated from uf  x ohms x 128 in seconds. Thus a 10M would give about 1 hour. Transistors T1 and T2 isolate the circuit from the input levels and turn on when their inputs rise above about 4V. The ALARM operates on a low input which is the expected condition of power fail. The mute input is selectable for positive or negative operation by the link. As shown a low on MUTE starts the time counter. Cut the link and connect to the other side for positive operation. The alarm is muted while the mute time counter (ICM7242) is running. The mute time counter is stopped at the end of a time period or on the return (even if momentarily) of a high on the alarm trigger.

The output
pin 3 of the time counter when not running is high making pin 9 low and holding the counter reset with a high on its reset pin.

When the alarm Trigger is low nand gate pin 1 goes high and as pin 2 is high pin 3 is low turning on The BD682. T3 is a darlington and will source at least 200ma.

A low on MUTE causes a high on the timer trigger (Trigger overrides reset). The time counter starts and the output goes low holding nand gate pin 2 low and thus pin 3 high turning off the BD682. When the counter reaches 128 periods pin 3 rises and the low on pin 9 causes a high on the reset and the time counter stops and the alarm comes back on as nand pins 1 and 2 are high.

If the power returns while the time counter is running the low on pin 8 forces a high on pin 10 reseting it. Thus MUTE is only active for the set period or until the return of power whichever occurs first.

Note In a real setting particular attention will need to be given to preventing the MUTE from false triggering. Some RC time delay into the trigger of the ICM7242 would help. The ICM7242 starts working with a power supply of only a couple of volts so you should ensure that if it is powered up on a power fail the trigger imput is held low while the power rises. This would not be the case in the above circuit if the MUTE input was pulled up to the circuit positive because T2 would not conduct until about 4V and the trigger would be active from 2V as the power came on.