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