All my mom ever wanted was a nice fountain in her outdoor pond. In the past, we had bought her a solar panel, a nice battery and an inverter to run the 120Vac pump she bought for it. There was no way Dad and I wanted to run a 120V line under the lawn for safety reasons. So the solar solution was conceived. It does work, but not for too long. Up in New England we don’t get too much sun due to many reasons (lack of sun, too many trees, etc.). This year, I came up with an upgrade to the solar system. I bought one of the 12Vdc motors I used for the hydroponics since it has a 3m head height. I also wanted to turn the pump off after dark. To do that, I made a simple circuit:
R2 is a photoresistor and M1 is the pump. I put in the D1 diode so that when the motor is turned off, all the inductive windup nasties are nicely taken care of. The MOSFET is a simple TIP3055 that I had lying around. It’s an NPN FET.
This circuit works like a champ. Turns off when I put a finger over the photoresistor, and turns on when I take it off. As a benefit, the fountain will only run on nice sunny days. The circuit uses 430mA when the pump is running with air (I’ll have to measure what happens with water) and 4mA when it’s off. If you have a 12V 20W solar panel like this one, you could potentially break even throughout the day. I’ll probably be putting a battery on there just to store any extra charge the solar panel might make.
Protip: If you want a circuit to activate when there is no sun, simply flip R1 and R2’s position. You might have to select a slightly different value for R1 for what amount of light should turn on the device.
I was tempted to add in a diode and capacitor before the gate of the FET so that there is a short delay when the sun’s going down so that the pump is not turned on/off/on/off. This is what I’m thinking of:
The problem with this latter circuit, is that the photoresistor will change slowly during the course of the day that it’s really not necessary. So the simple circuit above will do the trick.