Automatic Anchor Light

Going to the local anchorage this weekend? Do you have an anchor light to warn off late arrivals? Or, maybe you need a nightlight (and radio) to switch on at dusk to give your boat that 'lived on' look? Or, do you want to turn OFF your 12V frig at night so it won't drain the batteries?

Then look at this very simple and inexpensive project ( under $10 US ! ) that uses your existing solar panel and an inexpensive electrical relay to create a very reliable Automatic Anchor Light.


These days it seems that just about every boat you see has a Solar Panel or two to keep the batteries fully charged. If you already have one, then a few dollars worth of electrical relay, fuse, terminal block, a bit of wire and a little electrical knowledge is all you need.


Virtually every solar panel comes equipped with a blocking diode that prevents the panel from discharging the battery when it's dark outside. The blocking diode can be found in the 'junction box' on the back of the panel. A typical one looks something like this:

Diagram Showing the Solar Panel Blocking Diode Location

At point 'A' the voltage output from the solar panel will rise at dawn and drop again at disk. Point 'B' is the battery side of the diode, so this voltage remains high at all times. Point 'C' is the negative or 'Common' side of both the solar panel and the battery.

If we connect a 12 volt relay coil between point 'A' and 'C' (see the diagram) then it will pull in whenever light is shining on the panel. At dusk, the voltage drops and the relay is de-energized. By selecting a relay with SPDT contacts, the normally closed (N.C.) contacts will be 'open' during daylight hours when the relay is energized and 'closed' at night. These contacts become our 'light controlled switch' that can be used to turn on an anchor light, a radio, or anything else! The normally open (N.O.) contacts do the opposite. They turn close during the day and open at dusk - which can be useful for turning off 12 volt equipment at night to prevent battery drain!


First you will need to obtain a small 12 volt SPDT relay. Select one that has contacts rated at 5 amps or more. ( In the US, you can find these at 'Radio Shack' or any electronics parts store.) These relays are usually small enough to be able to fit on the underside of the solar panel. A small terminal block will also be useful. You will have to solder some leads on the relay, then connect them as shown in the diagram below.

Diagram showing 12 Volt Relay connections to the Solar Panel

DAY Power & NIGHT Power

If you connect a light between the relays' NC terminal and the common or negative terminal "C", it will turn ON at Dusk and OFF at Dawn. And you have an Automatic Anchor Light! Likewise, you can run wires back into your boat and use this 'Night Power' for a night light or a radio - to give your boat that 'lived on' appearance while you're away.

'Day Power' can be obtained from the relays' NO terminal and the common or negative terminal "C". Power will turn ON at Dawn and OFF at dusk. This can be useful if you use your solar panel(s) to lessen the load of your 12 volt 'frig' during the day. Turning the frig off at night can save you from flat batteries in the morning.

On our boat, the anchor light comes on a dusk and goes off just after dawn. Just when it goes on or off depends on your solar panel and relay specifications. If we wanted to build anything more precise, with adjustable triggering at various light levels the project would have been more expensive and complex.


If you are not experienced with soldering electronic components, or are unsure about the connections seek the help of someone who is qualified.

1. General Note: ( This applies to every solar panel installation whether or not you are adding the circuit described in this article.) Be sure to install a fuseholder and properly sized fuse in the positive lead from your solar panel. All solar panel installations should include this fuse because any accidental shorting of the leads going from the solar panel to the battery can carry the full discharge current of the battery and present a potential fire hazard! Unfortunately this fact is missed in most installations (even if done by a professional). So be safe - use a fuse.

2. If you are going to switch loads higher than 2 or 3 amps, use a relay with contacts rated at 10 amps.

3. The current drawn by the relay coil is small - usually less that 50 mA. The power for its operation comes from the solar panel and it will not discharge your battery!

4. Make sure that you connect everything up as shown. Double check your work and give everything a thorough checkout.

5. When everything is working, you can 'mount' your relay to the underside of the solar panel with clear silicone sealer if you wish. This will keep the moisture out. While it isn't pretty, ours has been working this way for nearly three years.


You can add separate ON/OFF switches or use the output from the relay to trigger larger relays as needed. You now have a very reliable DAY/NIGHT switch that can be used in many applications - especially good when you have to row 'home' in your dinghy on a dark moonless night after an extended happy hour on a neighboring yacht!

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