I have a 5000 litre water tank next to the workshop, and it collects rain water from the workshop roof. 5000 litre's is enough to last me about 3 weeks, and with the rain we've had I've being using rain water for the last 4 months. I also have a jet pump and bore to supply water when there isn't enough rain.

There is also a 1000 litre header tank up on the hill behind the house. Its about 12 meters above the ground floor, so provides heaps of water pressure.

To pump water from the main tank to the header tank, I use a combination of wind driven pump, and a 12v electric pump. The windmill pumps about 100 litre's per day.

The 12v electric pump is supplied from the batteries kept charged by my generating windmills, and uses a float switch in the header tank to switch the pump off/on as needed. The float switch is about 1/4 way down the header tank, its there to keep the tank at least 3/4 full, the wind pump filling the rest.

I originally used the float switch to operate a relay for the electric water pump, but this proved a bit erratic. As the tank level rose the float switch would bob around and switch the pump off and on every few seconds. This was a bit harsh on the poor old pump, and could last several minutes.

So I designed this little pump controller.

The water pumping windmill, based on the blade design by Ed Lenz.

12v water pump.

To the right is the circuit diagram. Its a pretty simple circuit based on a PICAXE chip ( have I said how much I love these things? ). Click on the circuit to show full size.

There are 3 connectors: 12V DC input ( Battery ); Float Switch; Pump or Relay.

I wrote the program in the little chip to follow these rules.

1. If tank level low, turn on pump for 30 seconds and check level again. Continue to check level every 30 seconds.

2. If tank then becomes full, run pump for a further 60 seconds, then turn off. This gets past the bobbing float period.

3. If the pump has being running for more than 30 minutes, turn pump off, flash the LED once every couple of seconds. This means it has taken too long to top up the tank, so there must be something wrong, ie Main tank empty, leak in the line, float switch stuck. You need to turn the controller off and on to get back to normal operation.

4. If pump is on and battery voltage drops below 11 volts, turn off pump, flash led twice every couple of seconds, and wait for 20 minutes. After 20 minutes, check battery voltage again, if recovered enough, turn pump back on. This saves the battery, gives it a chance to recharge before starting pump again.

  I used my CNC router to make my circuit board. The CNC does a good job, sure beats vero-board or etch pens and ferric chloride.

The only adjustment is the low battery pot ( 5k ). To set this, I used a variable power supply adjusted to 10.5 volts, and turned the pot until the controller went into low battery mode.

The controller will come back online after 20 minutes and if the battery has reached over 11.5 or so volts. This is adjusted in the software and should only need to be set once.

If you want to use the controller for 24 volts, replace the 10k resistor in series with the 5k pot with a 22k resistor.

While the power MOSFET can handle the pump current ( about 10 amps ), I use it to switch a relay that in turn drives the pump. If you intend to use a mains powered pump, then you must use a relay to isolate the electronics.

Click here to download the PICAXE code.