My work has taken me down the road of using Raspberry Pis as data collection devices.  This means I need to power a Raspberry Pi in the field.  I’ve had trouble finding a reasonably-priced 12V to 5V USB adapter that I could easily and safely fit into a box with a RPi.  So I designed one in KiCAD and built it.  The design is on my work github account.

I’m ultimately designing something that will connect to a battery, and batteries can explode if mistreated.  Testing is critical, as is circuit protection (the fuse).  I’m envisioning this to be in a box on the top of a pole with a camera, so the lead going from the battery (which will likely be on the ground) will be fused in case the wire gets cut.  This is critical for the same reason it is necessary in a car – to protect the battery from short circuiting should something happen.

Empty PCB

Test fit components at the office.

In putting these together at home, I tested these in every way I could think of, and assembly and testing went something like this:

1. Solder SMD C2 and R1
   1. Test resistance from 7805 output to LED positive solder hole, should be 330 ohm (I used 330 ohm resistors instead of 310, since I don’t happen to have any 310 ohm).
   2. Test continuity from 7805 output to ground via connected to C2. Should show no continuity.
2. Solder USB connectors and C1
   1. Test capacitance from 7805 input to to ground via near C1. Should show a reading (mine all showed around 1000 uF, which is high, but my understanding is that multimeters are notoriously bad at capacitance)
3. Add input headers, fuse holder, and LED
   1. Test continuity between inputs – should immediately beep, and then drop to no continuity after capacitors charge
   2. Apply voltage, LED should light, all magic smoke should remain contained in devices
   3. Test voltage from 12V- to 7805 output – should be 5.0v (mine showed something like 5.007v)

The one thing I was unable to test was the actual USB output voltages, but it seems to me that they should be okay.

IT LIVES!!! This is one of 5 I built.

I have five blank PCBs left for additional builds should I need it, although I’d have to have work buy more components.  Maybe I could get some larger 7805s that would fit the ground pad…

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