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Using the RE46C140 circuit can be designed a very simple smoke detector alarm using few external electronic components . The RE46C140 IC is a low power CMOS photoelectric type smoke detector IC that will provide all the required features for a photoelectric type smoke detector project . This smoke detector alarm design incorporates a gain selectable photo amplifier for use with an infrared emitter detector pair. An internal oscillator strobes power to the smoke detection circuitry for 100us every 10 seconds to keep standby current to a minimum. If smoke is sensed the detection rate is increased to verify an alarm condition. Smoke Detector Alarm Circuit Diagram An interconnect pin allows multiple detectors to be connected such that when one units alarms, all units will sound. In standby the LED is pulsed on for 10mS every 43 seconds . In a local alarm condition or the push to test alarm the LED pulse frequency is increased to once every 5 seconds. In the case of a remote alarm the LED n...

This causes the resistance of LDR to increase and the voltage at the base of the transistor is pulled high due to which the supply to the COB (chip-on-board) is completed. The sensitivity of the smoke detector depends on the distance between bulb and LDR as well as setting of preset VR1. Thus by placing the bulb and the LDR at appropriate distances, one may vary preset VR1 to get optimum sensitivity. We have not build this smoke alarm detector. For smoke detector installation and maintenance use a near electrician.