Chip takes the fear out of designing alarm circuits

Alarm circuits are used in a variety of applications from smoke alarms and medical equipment to industrial sensing and monitoring. Most alarm ICs require off-chip sensing devices and output indicators such as buzzers or piezoelectric horns, in addition to external circuitry to drive the audible devices. Now, a low-power CMOS alarm IC is available that integrates the sensor processing and drive circuitry into one chip.

The CMOS alarm IC simplifies interfacing an alarm-level voltage to a piezoelectric horn. With a low average current draw, the part is suited to battery operation, and its high-impedance inputs let it connect to a variety of systems and input signals. Also, placing the drive circuitry on-chip replaces the need for discrete components, saving board space and cutting down on design time.

The IC can be simply described as a comparator that compares a monitored voltage level with a user-defined trip point to determine whether an alarm condition exists. A simple two-resistor voltage divider can be used to set the trip point. An external resistor and capacitor set the chip's clock frequency. The internal oscillator's period is 1.65 sec (f = 0.6Hz) during no-alarm conditions. Each 1.65 sec, internal power is applied to the entire IC, and a check is made for an alarm input level.

During an alarm condition, the oscillator period becomes 41.67 msec (f = 24Hz), and the piezoelectric horn oscillator circuit enables. The horn output modulates between 167 msec on and 83 msec off, or a duty cycle of 2. During the off time, the alarm-detect input is again checked and inhibits further horn output if no alarm condition is sensed. During alarm conditions, the low-battery chirp circuit is disabled, and the LED pulses once per second.

An internal monitor circuit checks the 9-V battery level at 40-sec intervals during the LED pulse. Checks are made every 24 clock cycles for low battery by comparing Vdd to an internal zener voltage. A voltage divider connected between Vdd and Vss internally sets the low-battery voltage level. This voltage can be altered by external resistors connected from pin 3 to either Vdd or Vss. Connecting a resistor to Vdd decreases the threshold while a resistor to ground increases it. An internal voltage divider supplies a fixed reference voltage which is compared to the battery voltage, Vdd, and when the battery is low, the horn chirps.

Information for this article was contributed by Chad Madden, Motorola Sensor Products Div., 5005 East McDowell Rd., Phoenix, AZ 85008, (602) 244-5417, Fax: (602) 244-4002, http://mot.sps.com/sensors