Hi
I'm not sure whether this belongs in "solar" or "light", so feel free to move it.
Today I installed an automagic relay for the anchor light on my yacht Magic. I had the same system on my old boat, and have also installed it on a couple of friend's boats. It would be equally suitable for controlling a night light outside a house. It could even be used with a small battery / PV combination for a night light by a remote gate etc.
The (anchor) light controller is simply a small relay actuated by the output of the solar panels. The light is connected through the normally closed terminals on the relay. When the sun comes up, the voltage on the output of the solar panels increases, the relay pulls and the light is switched off. When the sun goes down, the voltage from the solar panels decreases, the relay is released and the anchor light comes on.
The secret to avoiding the relay being activated by the batteries is to connect it between the solar panels and the controller, or if you don't use a controller, between the solar panels and the blocking diode.
Since the relay is pulling all day as long as there is output from the solar panels, it is of course important to use one which will not consume so much power that you don't gain anything. It is also important that the relay is rated for the current consumed by the anchor light. I used the Radio Shack SPDT Minature PC Relay (275-248) which is a single pole relay with both NC and NO contacts. It is rated 10A at 120VAC / 24VDC which should be adequate for most applications.
The nominal coil current is 30ma which means that, assuming a 10W lamp, the relay would have to be pulling for a total of around 28 hours before it consumed the same as the light would if left on for just one hour more than needed in the morning. Also, in the case of a yacht, there is no more need for switching the anchor light on before going ashore for happy hour.
Best of all, the relay will never consume any power from your batteries, it'll simply reduce the charge from your solar panels by around 30mA. That's 360mA in 12 hours of daylight. A 10W light left on would consume 10Ah in the same period.
I haven't had a chance to check the voltages on the Radio Shack relay yet, but the relay I used on my last boat would pull at 9V and release at 7V which worked very well with dusk and dawn. The one I have now released (switched the anchor light on) at dusk this evening just at the time it would be normal to switch on the anchor light. (If one could ever remember.) I'll probably be asleep when the relay pulls and switches the light off again tomorrow morning.
This is not by any means the first solar controlled anchor light in the world, but the beauty is that apart from a five dollar relay, you are using parts you already have. Nothing else to buy!
A nice safety feature is that the failure mode would be leaving the light always on. (Controlled by the existing anchor light switch) I have fitted an override switch to mine so that I can change a broken bulb in daylight and confirm that it's working.
PS. While I was at it, I replaced the blocking diode with a rectifier bridge to be able to share the output of the panels between the house battery bank and the starter battery. The positive output from the PV panels is connected to the negative terminal on the bridge, the house battery and starter battery are connected to the AC inputs. This simply uses two of the diodes in the bridge to split the output from the panels between the two battery banks. The reason I used a bridge rather than two diodes is that I had it, and that it is easier to install on a heat sink.
Regards,
Owen Morgan
Yacht Magic
Anchored in the lagoon, St Maarten
Click here for my latest reported position:
http://www.findu.com/cgi-bin/winlink.cgi?call=LA7QZ]
Owen, since you used "control" in the title on a post about controlling a light, it seems that controls is the proper section. Moved it there. TW.
