Sunday, February 17, 2013

External Power Supply for Digispark Projects

While searching Google for info about an LM567 tone decoder chip I wanted to use in an Amateur Radio project, I came across a vendor's site that had some interesting kits.

NightFire Electronics has a great little 5V power supply that is built to plug right into your breadboard. It sells for just under $8, but for just a few dollars more you can get one that is fully built and tested. I really didn't need another unfinished project staring at me, so I chose to spend the $12 and get the assembled one.

I couldn't resist the temptation to purchase a few more items from NightFire. I got two more breadboards, several small protoboards for when I what to make my projects permanent. And a huge bag of miscellaneous parts including a speaker, a microphone, transistors and ICs for only $10. Shipping wasn't free, so it was better to get more items. Everything arrived at my mailbox about a week later.

A few of the components were bent over when I removed it from the mailing packet, but they were easy to straighten. 

The underside has two sets of pins that plug into the side rails on a solderless breadboard putting regulated 5V and GND right where you need them.

There are two screw down terminals for the input voltage and ground. The spec sheet that came with the board says you can input either DC or AC through these terminals. It warns you to use no more than 25V DC and no more than 15V AC.

Knowing that, I dug through my junk box and selected one of the many old 'wall wart' transformers that I have accumulated over the years. I chose one that was labeled as 12V DC. It was a nice hefty one that could handle a good load. If you are planning to do the same thing, make sure you pay attention to the polarity diagram usually printed on the case. Most of these use a barrel connector with the inside connector + and the outside -. But there are some out there that do the opposite.

There is no jack on the power supply that would accept the standard barrel connector so I cut the plug off the wires.

If you do the same, you will need to figure out the polarity of the two wires. Usually there will be something that distinguishes the connectors. You might be lucky enough to find red and black wires running inside the same outside insulation. The one I chose has two black wires molded together. One side is marked by a broken white line. I stripped some insulation off one of the wires that was still attached to the plug. Then I used my multimeter to see if it was connected to the inside connector, or the outside barrel.

I found out that the wire with the white lines was connected to the inside connector which means it is the positive lead. I separated a couple of inches of the two wires on leads still connected to the transformer and marked this one with red tape so that there would be no confusion later.

I stripped about half an inch of insulation off each wire, twisted the braids and tinned them with solder to make them stiff and tight. These fit neatly into the + and - screw terminals on the board and were nice and secure after I tightened them down.

Before hooking Sparky up to the board, I tested the voltages to make sure that everything was safe and the polarity was correct. The wall wart was rated at 12V, but measuring the actual voltage across the two screw terminals, I found that it was really pumping 16V into the board. That is still well within the limits of the power supply. I measured the output on my power rails and found that to be 4.95V. That's close enough to our 5V needs.

I am real happy with the result, and think it is well worth the $12. The Pros on this item are the price, the way it fits tightly on the breadboard and available power. I might even use it on my Raspberry Pi. You can even hook it up to a 9V battery! I would say there are two Cons. A barrel connector jack would have been more convenient and there is no On/Off switch. I would call this gadget: 'A Buy.'

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