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More  Alarms, Indicators, LED Circuits

Time to Dust Indicator (May 7, 2006)
designed by David A. Johnson, P.E.

I have used the circuit below for many different applications, which require ultra low power drain.  It uses a single CA4013 flip/flop.  One section forms a pulse generator.  The generator produces 100uS pulses at a rate of about 1 per second.  The pulse drives an infrared LED.  Light from that LED spills out onto a flat surface.  A matching phototransistor, positioned at a 90-degree angle from the LED monitors the light reflected off the dust particles on the center surface as the drawing below depicts. 

A circular plastic shield prevents most ambient light from reaching the phototransistor.  The second flip/flop within the CA4013 package, changes state whenever there is sufficient light reflected off the dust in the middle of the sensing device.  When the flip/flop changes state, it drives an n-channel FET, which can be used to turn on a flashing light.  After the other surfaces have been cleaned, a quick wipe with a dust cloth will prepare the device for another monitoring cycle.  The average current drain from a 3v lithium cell battery is about 5 microamps, so one battery should power the circuit for many years.
 

Click on Drawing Below to view PDF version of Schematic

Time to Dust designed by David Johnson, P.E.

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