Adding Wireless Charging the XRP: The Electronics

2024-08-20 | By Ramaditya Kotha

License: See Original Project

The XRP is a great robotics platform! However, I often find myself running out of AA batteries, and my ‎XRP ends up on the shelf until I get more. To fix this problem, I created a wireless charging system with ‎rechargeable Li-ion batteries and Adafruit boards so that my XRP is always charged and ready for my ‎next project! ‎

In this project, I’ll show you how to create the circuitry to wirelessly charge your XRP. In another post, ‎Shruti G. will show you how to design and 3D print a receiver case, as well as a docking station to tidy up ‎and hide away our circuitry!‎

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Before we get our hands dirty, let’s talk about and understand the high-level working principle of a ‎wireless charger. Wireless chargers work on the principle of coupled inductors. Inductors are electrical ‎components that store energy in a magnetic field. Two inductors that are aligned and close to each other ‎can transfer energy. ‎

inductors_2

Luckily Adafruit has a convenient module that takes care of the circuitry and theory of wireless charging ‎for us! All we need to do is provide the transmission board with a 9-volt source, and we’ll get 5 volts at ‎up to 1.2 amps on the other side!‎

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Because the XRP requires between 5 and 11 volts, we need to connect two of these 3.7-volt Li-ion cells ‎in series (7.4 volts nominal).

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To charge the batteries, we’ll be using a Li-Ion charger from Adafruit.‎

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One issue with this setup is that most off-the-shelf Li-Ion battery chargers can only charge 1 cell at a ‎time, and do not support charging two cells in series. Thankfully, we can use this clever wiring diagram ‎from Adafruit to wire up our batteries so we can work around this limitation.‎

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In order to create the circuit above, we’ll need a 3PDT switch. A 3PDT has three different switches inside ‎it, with each switch having two possible positions. When we want the XRP to run, we can set the switch ‎to “running mode,” which will put the batteries in series. When we want the XRP to charge, we can set ‎the switch to “charging mode,” which will connect the batteries in parallel to the charging module. Lastly, ‎we will power the Li-Ion charger with our wireless charging module, leaving us with the following full ‎circuit:

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With the planning out of the way, we can now get to creating the circuitry! Following the wiring diagram, ‎lay out the wires where they go! Take care not to mix up the battery wires. ‎

batterywires_8 ‎ ‎

Solder a female JST PH connector to the charging input wires. This is necessary because the Adafruit ‎board has a male JST PH connector. We could solder the wires directly to the board, but I do not ‎recommend this, as it will make testing and troubleshooting the circuitry much more difficult.‎

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Then, solder two male JST connectors to where the batteries connect to the switch! This is much better ‎than soldering the batteries directly to the switch, as it will allow us to swap out batteries if we ever want ‎to.

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Finally, solder a 2.1mm barrel jack connector to the output. This will allow us to plug our circuit directly ‎into the XRP.

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Now that our circuitry is mostly done, we should probe a few points to make sure the circuitry is good! ‎First and foremost, use the continuity feature on your multimeter to ensure that all the connections are ‎correct. ‎

After, you should check that the voltage outputs are correct. When the switch is set to running, you ‎should see close to 7.4V at the output (it’s normal to be slightly higher). When the switch is set to ‎charging, you should see no voltage at the output, and you should see 3.7V across the female JST ‎connector (the one that goes to the Adafruit board).‎

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Now, let’s solder the receiver module to the Adafruit board’s DCIN port. I chose to extend the length of ‎the wire going to the receiver coil during this step. If you choose to do the same, be sure to heat up the ‎wire well before soldering it! Otherwise, you’ll have a tough time getting the solder to tin the wires well. ‎If you would like to add rechargeable batteries to your XRP, you could exclude the wireless charging ‎module and simply charge your batteries via the USB cable on the charging module.

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You can now plug your 2.1mm power connector into the XRP. Use a power supply or a 9V battery on the ‎transmitter coil to test the circuitry. A 9V battery won’t last long, so just use it to briefly test. I ended up ‎using a power supply.

xrp_14 ‎

Now you can place the coils on top of each other and set your power supply to 9V! In the charging ‎position, you should see the charging lights on your Adafruit charging module light up. In the running ‎position, the LEDs on the XRP should light up!

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Unless you want to lug around a benchtop power supply everywhere you go, I highly suggest swapping ‎out the 9V power supply for a 9V wall adapter. To be safe, make sure it supports at least 10 Watts!‎

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And voila! Your XRP can now wirelessly charge! You could take this project further by designing a custom ‎PCB to tidy up the wiring. You could also experiment with using relays or even MOSFETs to have the XRP ‎autonomously switch between charging and running modes.