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12V at the Pin Box.


If you have a travel trailer or fifth wheel; one with a 7-pin trailer connector, did you know you have a source of 12VDC power? One of the pins on the trailer connector is typically connected to the battery so that the tow vehicle can charge the battery whilst towing. This is necessary because the battery must be charged for the RV's emergency brake system to work (that pull-switch with the cable on it).

But when you are not towing, the pin on that connector can be used for other purposes, such as powering a light or illuminating a sign.


All we need to do is build an adapter cable as shown at the right. The current should be limited with a fuse because this circuit comes right off the battery. I figure only to need 5A at the maximum so that will be the criteria I will use to size the wiring, fuse, and other components.

And like most of my recent electrical projects, I will emply a TVS Surge Suppressor diode in the adapter.

For the free end connector I intend to use an accessory/cigarette lighter socket because this provides a generic, almost universal connector for attachment of a whole host of different items.



From the schematic you can see this is a very simple circuit. The most difficult part is that I have had to use SMD (Surface-Mount Device) technology to get everything stuffed into the end of the connector. Otherwise, it is a fairly easy project. I did create a circuit board for the project as I wanted to be able to replace the fuse. However, if you forego the TVS diode, LED, and fuse (which I don't recommend), you could omit the circuit board as well. In that case, simply install the accessory socket to pins 1 and 4.

Pay particular attention to the color code for the socket. There are two standards; RV Standard and SAE Standard. The color code I show here is the RV standard. Just be aware that depending on the 7-Pin socket you purchase, the color code might be different.


The circuit board is designed to fit on the top of the plug and holds all of the components except for the LED.

The circuit board can be ordered from OSHpark here: Order from OSH Park

Note that you must purchase boards in a quantity of three, but I think you will agree that these are useful enough that you might want to make several or give them to friends.

The prototype circuit board shown in the video does not have the combo TVS diode pads. The circuit board shown here allows the use of a 1.5Kw or 400W TVS diode.

A Bill of Materials for the required components is shown below:


ItemPart NumberQTYSource
Fuse Holder01550900M 1DigiKey.Com
Fuse 0452005.MRL 2DigiKey.Com
TVS DiodeSMCJ16A 1DigiKey.Com
1K Resistor 1206RC1206JR-071KL 1DigiKey.Com
LEDOVLBR4C7 1DigiKey.Com
Hex standoff 1/2" Number 2203 1DigiKey.Com
JST ConnectorGeneric1Amazon.Com
Trailer Socket581501Amazon.Com
Accessory Outlet/CableAUTO-CR-001 1Amazon.Com


TVS Diode. The circuit board can handle either SMCJ or SOD-123 size TVS diodes. The diodes differ in the peak wattage they can handle. If you want a lower wattage TVS diode, replace the SMCJ (1.5Kw peak) with a SMF4L16A (400W peak). The only difference between using one TVS diode over the other is peak power handling capability.

A quantity of 2 is specified for the fuse so you will have a spare.

JST connector. On the prototype, the connector was mounted on the board. However, there can be some intereference, so I recommend a pigtail assembly with an in-line JST connector.

You will also need a couple of #4-40 screws, 3/8" or so long to attach the standoff to the socket as described in the video.


Populate the circuit board as follows:

  • Install the fuse holder in the area marked YELLOW.
  • Install a 1.5Kw TVS diode in the RED area - pay attention to the polarity.
  • Optionally, use a 400W TVS diode rather than the 1.5Kw diode in the GREEN area. Again pay attention to the polarity. Install 1 TVS diode only.
  • Connect a short piece of wire at locations marked PINK under the board to attach to the socket.
  • Install the 1K resistor at the location shown BLUE.
  • Connect the JST wire harness to the LED at the GREY location.
  • Connect the output cable to the pads marked ORANGE - pay attention to the polarity.





Bonus (see video)





Last reviewed and/or updated Mar 1, 2018