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