Design a battery powered device, which will alert a hotel maid or cleaning
person when it is time to remove dust from the surfaces.
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.