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Doggie Water Dispenser Alarm June 5, 2012

Doggie Water Dispenser Alarm
I have two Beagle dogs.  They drink a lot of water; they go through about 3 gallons a week.  They have two water dispensers.  One is on the inside and second unit on the outside of my house.  Often, I forget to fill one of their two jugs and discover them completely empty.  I do worry that my dogs might get dehydrated if I ever forget to fill both of their containers.  To help prevent this, I decided to design and build a battery powered alarm system which would monitor the level of water in the jugs and sound an alarm when the water level dropped below a certain volume.

Water Dispenser 1 Water Dispenser 2
I experimented a bit with several different water level sensing techniques.  The classic method would be to install a couple of stainless steel electrodes through the plastic jug container but I really didnít want to drill holes in the container, which could later cause the plastic material to crack and leak.  The illustration below is what I plan to use.

I will first cut a short piece of Ĺ inch PVC pipe and drill some holes in it.  I plan to cap both ends of the pipe but will drill holes in the caps as well.  Before I mount the caps, I will glue a strong super magnet to a piece of hard plastic foam, cut to size so it would slide easily inside the plastic pipe, with the magnet facing down.  The foam will then at as a float, moving the magnet up and down with the water level, held inside the plastic pipe.  With the float in place, I will install the two pipe caps and will then mount the pipe assembly inside the plastic jug with some aggressive water proof sticky pads.  I will position the pipe so its bottom will be at the water level I want to call the alarm condition. On the outside of the plastic jug, at the same alarm level, I will mount a small reed relay.  With everything in place, the reed relay contacts should close when the magnet moves to the bottom of the pipe, which should only occur when the water level is at or below the critical point.

The electronics for this alarm project is pretty simple.  The whole circuit is powered by a pair of alkaline AAA cells, housed inside a nice plastic case.  A low power voltage comparator is configured as a 0.5Hz oscillator.  It has a diode and resistor in the feedback network, which causes the oscillator to produce a short 50ms pulse every two seconds.  The pulse output drives an n-channel FET, which is connected to a low voltage Beeper, available from www.jameco.com.  The reed relay is wired so it switches +3v to the oscillator and transistor circuit.  When the magnet activates the reed relay, power is routed to the circuit, which causes the beeper to produce a short chirp sound, every 2 seconds.  There is no on/off switch.  The whole circuit is in the off state until the reed relay is activated. 

Click on Drawing Below to view PDF version of Schematic

Dog Water Dispenser Alarm designed by David Johnson, P.E.

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