EDN Access--01.01.98 Pulsed sensor extends battery life

-January 01, 1998

January 1, 1998

Pulsed sensor extends battery life

Terry Millward, Maxim Integrated Products, Reading, UK

Activating the sensor circuit in Figure 1 for 1 sec every 30 minutes lowers its 20-mA supply current to an average of 70 µA. For a battery comprising three AA Duracells, this pulsed operation extends the battery's life to several years. The sensor uses an optocoupler and an IR-emitting diode. Designed to monitor the level of salt crystals in a water softener, the sensor relies on a reflection from the crystals to generate a "no-alarm" level of emitter current in the Q3 phototransistor. As the salt level drops past the sensor's position, this current level makes a step-change downward.

When the drop across R7 equals the reference voltage in the comparator/reference device, IC2, comparator IC2B's output switches high and releases the manual reset on the voltage monitor, IC3. After a minimum reset-delay interval of 140 msec, Q4 turns on and sounds the buzzer. Comparator IC2A monitors the battery voltage via R1 and R2; for levels above 3V, the comparator activates the IR-emitting diode, D3, by turning on the constant-current sink comprising Q2 and associated components. Thus, the buzzer sounds for 1 sec every 30 minutes if the battery voltage is below 3V or if the salt level is low.

Power for the sensor is available only when Q1 turns on. IC1, a µP supervisor configured as a timebase generator, controls Q1. IC1 consumes less power and has a smaller footprint than the alternative 5556 timer or a 555 timer with multistage counters. IC1 also eliminates the large capacitors otherwise required. Connected directly across the battery, IC1 draws 60 µA at 3V. IC1's external connections cause its internal watchdog timer to cycle repeatedly. With C2=1.5 µF, the internal time-out is 3.6 sec; connecting WDS high multiplies this value by 500, extending it to 30 minutes. Each time-out produces a reset pulse that applies power to the remaining circuitry by turning on Q1 for approximately 1 sec. From IC1's data sheet, tRESET=2.67C1=1.25 sec, and tWATCHDOG= 2.67C2x500=30 minutes. (DI #2139)

Figure 1
Powered by three AA cells, this optoelectronic sensor has a battery life of several years.

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