Back Porch Light (No it’s not a micro brew)
It might sound trivial, but I consider it rather important for my application. Sooner or later someone is going to come knocking on my back door when I am in the truck. It will probably be dark out, and it could be anything from the local cops to someone who lives there to some bad guys looking for something to steal. I need to be able to turn on my back porch light (even though I technically don’t have a back porch) and look through my peep hole to see who they are.
Alternatively, I need to be able to come up to my truck in complete darkness and fumble through my keys to gain entry. Either way, I need a light in the back, and I need to be able to turn it on or off from the outside as well as the inside.
Fortunately controlling one light with two switches is easily done. You just need two single pole double throw switches (SPDT). A SPDT switch has 3 terminals, one in the center that connects to one on each end depending on which way the switch is flipped. They are very common, both in the world of home AC wiring and in the world of electronics. The wiring schematic is often printed on the package containing the switch as you would find it in your local home improvement store. This same circuit is often used to control a central light from both ends of a room or hallway.
In my case, the back porch light is actually my left rear backup light. It is normally driven by the trucks electrical system, which in my case is separate from the house electrical system. They share a common ground, but that is all. The challenge is to have the light work as a back porch light controlled by two switches, and still work as a backup light as well. The solution is to use two diodes. This is the same technique as is often used to have a tow vehicle control the tail lights of the towed (sometimes called a “toad”) vehicle.
Each circuit drives the light through a diode. Current can go into the light from either circuit, but cannot go from the light back into either circuit. The result is sometimes called a “diode or” meaning the light will turn on if one “or” the other circuit tries to turn it on, and will only turn off when both of the driving circuits are off. It’s much harder to say with words than to draw a schematic.
I have one additional requirement. When two switches are used to control one light, you never know which position of either switch will be on or off, as it depends on the position of the other switch. This is not a problem controlling the light in a hallway as you can see the light from where you control either switch. In my case the back porch light (left backup light) is on the outside of the truck where it is not easily seen from the inside of the truck. I don’t want to accidentally leave it on and run the house batteries down. It only draws about 2 amps, so it would take a very long time to run the battery down, but still, there is no need to waste energy and burn out bulbs. I suppose I could just look out the peep hole, but it might not be night time or easy to see, so I wanted a small LED on the inside next to the switch that will light when the outside light is on.
That implies that I needed 3 wires from the inside switch to the outside switch, Two wires carry the output from the two possible inside switch positions, and one wire caries the result back from the outside switch to light my inside LED.
I ended up with a heavy duty panel mount switch mounted discretely on the back of the truck in a location where you would probably never see it unless you knew where to look. The inside switch is a standard AC wall switch of the SPDT type, mounted on the ceiling above the back door. The outside switch is mostly protected from the weather because of where it is mounted, but I also gave it a coat of Goop to help protect it from moisture. Time will tell, but it should be fine.
The schematic can be found, along with a bunch of pictures in the photo gallery under key word “Stealth”.
To be continued….
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