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Circuit Solutions Notebook
Last Updated On:  Wednesday June 02, 2021

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Ultra Low Power Human Finger Detector


Could a slotted or reflected optical sensor be used in an ultra low power circuit to detect a human finger?


The circuit below seems to work well and averages a current drain of only 10 microamps from a lithium cell.  It uses a single CA4013 dual D flip/flop.  One section is wired as a pulse generator.  It applies 10us 10ma pulses to a LED inside a slotted optical sensor at a rate of about 50Hz.  The phototransistor in the second half of the sensor detects the pulses and sends a pulse to the data input of the second flip/flop in the 4013 package.  If there is nothing between the LED and the phototransistor, the inverting output of the 4013 remains low.  If the light beam is broken by a human finger, the flip/flop changes state sending a logic high level to a n-channel FET.  The open drain output could be sent to a computer system, a relay or a beeper.  The circuit has been used as a safety switch for machine operators and for wheel chair controls.  


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