I built this to see how long my how water heater was running which would tell me about how much money I am spending on hot water.  It has also proven its worth by telling me when it has recovered from a shower so the next person has hot water for their shower. 

The major factors driving the design choices were...

1.  Cost:  Money was a little tight so I wanted to build it out of what I already had on hand.

2.  Not pulling wire:  The water heater is in the garage, but I want an indication in the house.  There was also no convenient way to run wire into the house without drilling holes completely through a wall, which I didn’t want to do.

The water heater is against the wall in the garage.  The other side of that wall is the hallway in the house.  One side of the wall to the other is about 6 inches.  At that small distance, I decided to hang a coil of wire on the garage side of the wall as a transmitter antenna, and another coil on the hall side as a receiving antenna, and use the pair like a very inefficient transformer.  An alternating current in the transmitting coil, induces a small alternating voltage across the receiving coil.



Figure 1, Transmitter

The top section of the circuit is the power supply.  The 230 VAC is taken from the power connections to the heating element.  That way, when the heater is on, the transmitter is on, and it goes off when the heater goes off.  Since the bottom heater element is disconnected, I only have to worry about one element coming on.  I did not show it, but I did put a 1/4 Amp fuse on the primary side of the transformer.  Diodes D3, D4, D5, and D6 are actually a single bridge rectifier module I had laying around.  1N4001 diodes would surely work there also.


The 555 chip is configured as a square wave (roughly) oscillator.  The output from the 555 drives a transistor that switches on and off, which quickly starts and stops current flow in the transmitter coil.  Capacitor C2 is 0.0068 uF, which with the resistors shown, gives a frequency of about 10 kHz.  I found the higher the frequency, the better the receiver could pick it up.



Figure 2, Receiver

When the transmitter is on, a voltage in induced across the receiver coil, L1.  This signal then goes through 2 amplifier steps.  The first one multiplies the signal 10 times, and the second multiplies that by another 100 times.  Coupling capacitors C1 and C3 are 0.0068uF and C2 is 0.002uF (sorry, I forgot to put those values on the drawing.)  The signal, which is an AC signal, is converted to a pulsating DC signal by diode D1.  This signal then charges capacitor C4.  When the voltage on C4 gets high enough, it turns on the output transistor, which turns on the LED, D2.


The coils.

The the transmitter coil is 4 conductor telephone wire in a coil of 14 turns with a diameter of about 4 inches.  The receiver coil is a 4 conductor phone wire wound 2 times.  Both coils have the inner conductors wired to each other in series to multiply the turns by a factor of 4.  Full disclosure here, I got this idea from the late TJ Byers in Nuts and Volts magazine.


Figure 3.







Why did he do that?

What is D2 for on the transmitter?  Because the transmitter coil makes an inductor, this resists a rapid change in current.  When the transistor shuts off, the current in the coil does not easily want to stop, so it “slams” against the transistor, causing a voltage spike.  Normally one puts a single diode in reverse parallel the the coil, so the current can loop back around to avoid slamming into the transistor.

Well, when I first put the circuit together, I forgot the diode, but got away with it, and it worked.  When I remembered it, I checked, and then saw the transistor saw a 30 volt voltage spike.  That was a bit more that I was comfortable with, so I put a single transistor, D1 in the circuit to allow the current to bleed off.  However, when I did that, I could hardly detect the signal at the receiver.  I compromised and put in the second diode as well in the opposite direction as D1.  It is a zener diode, which stops the flow “against the arrow” until its voltage setpoint is reached (in this case, 12V,) then it allows current to flow.  That allowed decent signal transmission, without slamming the transistor so hard.

Why is he using a 9V battery for the receiver power supply?  When I first prototyped this on my workbench using my (homemade) DC power supply, the power supply noise made the LED come on weather of not the transmitter was on.  A battery fixed that problem.  I probably need to figure out how to clean up that noise, but I haven't gotten around to it yet.  So far the battery has lasted almost a month.