Provide telemetry and alarm design solutions for smoke detectors
Ion-based smoke detectors are cheap and effective, except for installation indoors. It is also often installed in garages and other outdoor buildings. These designs need to send the alarm signal back to the room or home security system. The circuit shown in Figure 1 provides the telemetry output of the detector by detecting the operating current when the Sensor is alarming. Under normal monitoring mode, the working current is only 50μA. When the alarm is triggered, the current exceeds 3mA. This circuit is powered by the internal battery of the smoke detector and has little effect on the detector. The comparator shown in the figure has a wide operating voltage range, low operating current and built-in voltage reference.
Typical quiescent operating current is less than 3μA. The output structure of the circuit is an open-drain FET, which can drive the home security system or trigger the radio frequency module to send the signal back to the residence. VN88ADF 'type FET can continuously drive more than 1A current. The working current of the whole circuit is less than 3.1μA.
After the alarm is triggered. The voltage after the detection resistor is greater than 30mV. Therefore, the circuit trigger point is set to 18mV. Because the active and inactive states differ greatly, the hysteresis voltage of the comparator is minimal. The capacitor plays a role in reducing noise and radio frequency noise sensitivity (4.7nF at the point of 18mV. The capacitor should use a low leakage polyester capacitor). If this smoke detector is connected to a home security system, be aware that the alarm telemetry output will also be activated at the same time that the alarm is activated intermittently when the battery is almost empty.
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