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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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