Circuits designed by David Johnson, P.E.
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List of Dave's Circuit Designs
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Water Heater Flame Detector
Designed by Dave Johnson - August 5, 2012
|Problem: Design a device that detected whether the gas flame of the heater was off or on - whether it was
heating water or not - and convey that information to the bathroom. With a readout in the bathroom, one would know if there were enough hot water to take a
Solution: A battery powered circuit would be best for his situation. The flame detector circuit could be connected to a battery powered LED flasher circuit. The actual LED could be mounted next to the thermostat, using the same wire pathway as the furnace thermostat wires, located nearby. A remote flashing LED would
change color from green to red, whenever the flame was on. A red
flashing indicator would alert the person that water in the heaterís tank was
not up to temperature and would mean that there may not be enough water to
complete a shower use. When the LED color changed to green, the heaterís
flame was off, indicating that the tank was full of hot water.
My flasher circuit is shown
below. It could be housed in a plastic box with a battery compartment.
The compartment would house two AAA cells. Two 1.5v alkaline AAA cells
would yield 3v, enough voltage to power the circuit and drive the two remote
LEDs. A three leaded, two color LED assembly would
be connected to the box. A miniature twisted shielded cable would link the box to
the LED assembly. The assembly could be some 100 feet from the box. Another
twisted pair of wires would like the box to a phototransistor, which is positioned through
a hole, which can view the light from the gas flame. The device can be 12 inches
from the flame. Be sure to use a hole which is not too close to the flame so the
device will not be subjected to too much heat. The 3v battery in the flashing
circuit should be able to power the circuit for about a year.