Hey guys, i have not seen a dedicated thread for a loop key tutorial so since i was making a couple yesterday, i decided to snap pics of how i did mine.
What are loop keys?
Loop keys are a budget friendly option for an on/off switch for your board. It’s also a physical disconnection of your circuit and is proven to be the most reliable switch alongside the AS150 connector.
Some antispark switches may eventually burn out or have other issues like a small current draw which eventually drains your battery pack even when off.
Why should you use a loop key?
If you are a college student on a budget, and can’t afford $40 for an antispark switch, a loop key circuit would cost you aroooound $5 US give and take.
Some pics of the wiring and how it works before we start
Ok let’s do this. I will be using a 3D print for the loop key made by @ervinelin on the builders forum. I find that this is the cleanest loop key design that also gives an area where you can pull easily.
Model here :
I was tired of struggling to unplug my XT90 antispark loopkey and decided to design one which allowed me to grip it easily. Solderup the XT90 as thinly as possible as per the photograph. Here I soldered using a 12AWG wire (without silicone). Print…
Here’s what you need for the loop key section
A Male XT90 and a female XT90S
About 4~5cm of 12AWG silicone wire
Do note that the female connector has a green L on the connector. It’s a resistor that kinda acts as a fuse as well in case you decided to mess up your battery polarity yea that happened to me
Good thing is that the loop key and male connector will be the one that pops, and hopefully everything else will be fine. Do not use a female XT90 without the resistor as a loop key.
Trim off a bit of the silicone revealing the wire
Flux and tin it, this holds one end of the wires together.
Cut and reveal about 3cm of the wire.
Bend and slightly flatten the wire in the middle, making the overall thickness thinner by spreading it out by a bit.
Dip it in some flux
Add some solder and have it flow around the wire a bit, do not put too much
Use a wire snip and cut the wire a bit after the flat-ish area.
The reason for the solder is to partly solidify the wires. If not it’ll still be flexible and be prone to bending back straight.
Now grab your pliers and hold it like so, and you’re gonna angle it on your work table and bend it 90 deg
Then, use your pliers and press the edge to be round, since the corners are flat after snipping it
Now the wire is too long, we need to snip it to the correct length
Now for the next bend, have it somewhere in the middle of the bullet connector, right here.
The wire should sit nicely and rest on the walls of the connector with the right length
Now its time to solder. Apply some flux around the wire and connector
Also a good time to put the male connector on. This ensures the bullets don’t move or slant when it gets too hot when soldering. (good practice when soldering all XT30/XT60/XT90)
Have your clip hold the bottom while you work on it.
Once soldered, let it cool and clean any flux away with a solvent and a cloth
Solder should be nice and even, and no big globs
Alternatively, use a round file and file through the two pins, and drop a stripped 12AWG wire thru the middle! I recommend this actually. Courtesy of @glyphiks
While i was soldering, i’ve also printed the loop key.
Do note that the lip of the connector protrudes out by a bit from the holder, so that the connector can push and connect fully.
Now you will need a M3x20 socket cap screw and a M3 nut
The bolt will rest on the plastic part of the connector and prevent the connector from backing into the holder. The bolt does not touch the wire nor bullets. Now is a good time to glue your connector to the holder. Use epoxy or conformal coating.
The reason i flattened the wire slightly is because the space in the holder is tight, so it tends to hit the lid. I also lightly heated the lid so it will conform to the top of the wire.
Glue the lid on after the fit is right, and you are done!
Spray some bedliner to make it look sexier.
No 3D printer? No problem! You can also do the traditional loop key by using a longer 12AWG wire and the cap for the XT90. This however protrudes out a bit more so if your loop key location is at the side of the board it has a chance of being hit/yanked.
And that’s your loop key done!
Alternatively, there are also other loop key holder designs that you might prefer.
Now that the key is done, you need to wire up the connectors that goes in the middle of your battery and ESC.
You will need a male XT90, which connects to the loop key, and a female and male XT90/XT60 (depending on which connectors you use)
There was a discussion on which side the loop key should be, either on the negative side of positive. I find that there’s no difference since the loop key acts as a physical disconnection so i placed mine on the positive.
And if everything is done right…
You will have a nice clean looking XT90 loop key switch, which will turn your board on when connected, and turn off when pulled out. How’s that for $5 and 20 mins?
A couple things to note when using loop keys
Ensure the connector is pushed all the way in. If the connector is not 100% in, the loop key may short.
No metal thingys touching the two bullets on the enclosure connector pls. You can print a plug to cover it when not in use. And if you have children at home, that would be a good idea. You’ll never know when they decide to stick a screwdriver in there.
So there, that’s my tutorial on how i do my loop keys. If there’s a more efficient way to go about doing it or if you would like to contribute to this thread please do so. Thank you for reading!