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Challenge:
Devise a circuit which can determine if a one inch diameter piece of clear
tubing has alcohol in it or not.
Solution:
An optical technique seemed like the best choice for this application.
The simple circuit below works well but only when the light detector can be shielded from
bright ambient light. A visible red LED launches light pulses through the side of the
clear tubing and emerges at the opposite side. A phototransistor forms a light detector.
The circuit takes advantage of the lens effect from any clear fluid inside the tubing.
When there is only air in the tubing, the light scatters and spreads out quickly before it
reaches the phototransistor. When the tube is full of clear fluid, it forms a
cylindrical lens, focusing more light onto the detector. By carefully selecting the
value of R6, the output voltage appearing at the “Data” input of the flip flop is sufficient
to change the state of the device at the trailing edge of the clock pulse. The second
half of the flip/flop is configured into a pulse generator. The average current of the
circuit draws only about 10 microamps, so it can be powered from a battery for many years.
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