Because ionization-based smoke detectors are cheap and efficient, they are often installed in the garage and other outbuildings as well as in the house. Those locations pose a need to transmit the alarm signals back into the house or central home security system.
The circuit shown in Figure 1 provides a remote output for such detectors, by sensing the supply-current increase associated with activation of the detector's internal sounder.
Figure 1 depicts this situation:
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Figure 1: By sensing an increase in battery current when the smoke detector's sounder activates, this circuit produces an open-drain remote output for the detector.
Typical current values are 50 microamps during monitor mode and greater than 3 mA when the sounder is active. The circuit is powered by the smoke detector's battery, with minimal impact on the detector.
The comparator shown is chosen for its wide supply-voltage range, low supply current and internal voltage reference. Typical devices draw less than 3 μA of quiescent supply current. The circuit output is an open-drain FET that can drive the home security system or trigger an RF module that communicates back to the residence. The FET is a robust power device (VN88ADF) that supports continuous load currents greater than 1 A, and the complete circuit draws about 3.1 μA.
Voltage across the sense resistor is greater than 30 mV when the alarm is active, so the circuit trip point is set at 18 mV. Because the threshold between active and inactive operation is so great, the hysteresis pin is connected for minimum hysteresis in the comparator. The capacitors prevent false triggers by reducing noise and by reducing the sensitivity to RF noise. (Note: The 4.7-nF capacitor at the 18-mV trip point should have a low-leakage dielectric, such as polyester.)
If the smoke detector connects to the home security system, note that a nearly discharged battery, which intermittently activates the sounder, will also activate the remote output.
About the author
Kevin Bilke is with Maxim Integrated Products Inc.