When I bought my boat, it came with what I believe is an old Davis Mega light – one of those anchor/utility lights with a long cord that plugs into a 12 volt socket. It never worked right, only putting out a dim orange glow instead of a bright light and not turning on or off in the light/dark. As a result, it just sat in a locker, unused, for years.

Yesterday, I decided to try and repair it. It’s a simple light sensor circuit and, except for the bulb, I had whatever replacement components I might need at hand.

First, I plugged the light into a 12 volt socket and checked the voltage at the bulb. I was getting 12 volts. I unsoldered the bulb and connected it directly to the power leads. It lit up brightly. So I started unsoldering the other components one by one, checking to see if they were faulty.

In the process, I broke off one of the legs of the transistor. No problem. I had an identical one.

Eventually, I had disassembled the entire circuit and every component checked out okay. Puzzled, I put together a complete circuit on a breadboard, using all new components except for the bulb. I hooked it up to a battery and it worked like it should – it turned off in the light, on in the dark, and the bulb lit brightly.

So I began soldering everything together. While bending the legs of the transistor to position them correctly, I broke off one of them. Talk about being klutzy. That was two transistors I had ruined. Fortunately, I had another and they were cheap.

Got the circuit assembled and plugged the light into a 12 volt socket, fully expecting it to work. It had on the breadboard, after all. But all I got was what I had started with – a dim orange glow and no on-off in the light/dark.

I knew the circuit was good so, as a last resort, I checked the power leads. That’s when I discovered the plug had been wired incorrectly – the tip was negative and the side prongs were positive. Apparently I had overlooked the minus sign on the voltmeter when I had tested the voltage at the start. Which meant the original circuit had been fine. Instead of spending hours tearing it apart and building a new one, I could have fixed the light in minutes by simply reversing the power leads.

Oh, well, lesson learned. I unsoldered the circuit from the power leads so I could reverse polarity. But while fumbling around, I broke off one of the leads to the bulb. (It had a hard-wired bulb, unlike the current model which has a socket and replaceable bulb.) Now I had ruined the only component I didn’t have a replacement for and the one which, presumably, was necessary to meet the two mile visibility requirement for an anchor light.

I grabbed three white 5mm LEDs and a resistor and made a replacement “bulb” out of them. I pointed the LEDs outward (so the viewing angle was to the side) and positioned them 120 degrees apart. After checking to see that it worked, I coated all the exposed wires with liquid insulation.

The photo shows the assembled circuit (left), a close-up of the light turned on after dark (center), and a shot of the light from about 15 yards away (right). I walked about 100 yards away and it remained very visible. Not what I had planned and not USCG certified. But it looks like it will work as a low current draw automatic night light.