I'm building an automatic dog water bowl filler powered by solar panels. It includes two voltage regulators one 7805 for the arduino and a 7812 for the solenoid that turns the water on and off, using a automatic sprinkler valve and some PVC pipe. The arduino controls a relay that turns on the 7812 circuit to power the solenoid. I need help at certain times during the day there is not enough power coming from the solar panels to power the device, I need a circuit that will take very little voltage and current to run, and check the power coming in. when the circuit is at the minimum required voltage and current, turn the main circuit on. Then turn off when it gets to a low voltage and current.
I guess what I could use is just a relay that has a high voltage rating to activate the coil. This relay would only activate when the solar panels reach 12v volts or higher.
Do you really need a voltage regulator for the solenoid? How much more than 12V can the panels produce, and how long do you turn the solenoid on for? You could probably dispense with the 7812 completely and just switch the solenoid from the Arduino with a mosfet. If you are still worried about overdriving the solenoid, you can use a BJT to turn the mosfet into a constant current source, which is as good as driving the solenoid from a constant 12V.
To monitor the voltage from the solar panels, just use a voltage divider to feed an Arduino analog input. If you build yourself a bare bones Arduino (i.e. no FTDI chip), you can make it run with very low current consumption, by using a lower clock frequency and putting it in one of the sleep modes for most of the time.
The Solenoid needs 12V .5A it is part of a automatic sprinkler valve found at HomeDepo. The concern I have is not the the solenoid itself but the hole circuit that drives the system there are no batteries in line with the solar panels it is connected directly to the 7805, the input voltage has to be 5V or more to drive the 7805. At Dawn and Dusk there is not enough light for the solar panels to power the system, so I need a on and off circuit in line with the panels to the 7805 circuit. I have the arduino using the A0 pin to keep an eye on the solar panels levels. The panels produce about 22V and can pull about 1.5A, the hole circuit takes about .8A to run. The solenoid only runs for about 30 seconds to a minute to fill the water bowl then shuts off. 7812 powers the solenoid and nothing else every thing else is done with the 7805 and the relay is used to just turn the 7812 on and off according to the water levels.
You really need a battery connected to the Solar Panels to provide a stable voltage.
Its not possible to know from just measuring the Solar Panels voltage when there is enough light available
to produce the current that your circuit needs.
Well I got it :). A couple of op-amps and a window comparator to control a BJT that dose the switching on and off so when there is enough voltage the comparator set at a certain voltage, kicks over the BJT to gives power to the rest of the components. I will post more as I go along, I'll have a circuit diagram and some pictures. Instuctables.com will have a tutorial if some one wants to make there own Automatic Dog Water Trough.
Thank You!