When you’re away from home, it can be frustrating when one of your devices runs out of battery power. It is even more frustrating when your other devices still have power but you have no way to recharge the dead device from the live one. To solve this problem, I created a simple keychain using a scrap of brass and an old micro-USB cable that will allow you to recharge almost any micro-USB device from any other device.

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Why this works: USB On-The-Go (OTG) allows a device to act as a host when no other host is present. By bridging pins 4 and 5 on one side and connecting them to pin 5 on the other side, the cable tells the charged device to supply power to the discharged device. You then connect pin 1 to pin 1 and you’re ready to go.

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The brass piece adds a touch of style and keeps the cable from falling off of your keychain. You could use a variety of materials depending on your own personal tastes.

Important note: if you want to create a cable compatible with Apple’s Lightning connector for iPhone and iPad devices, you’ll have to make a few small changes. First of all, during soldering, connect pin 2 with pin 2 and pin 3 with pin 3. These pins create a data connection between devices. Then simply add a micro-USB to Lightning adapter and you’re all set. The cable will still work with non-Apple devices if you connect these pins, so if you’re not sure, go ahead and make the connections.