Wednesday, 10 September 2014

Arduino as ISP for ATTiny85 – PCB Revisited

I have revisited the PCB design for the ATTiny85 ISP shield after trying it out a couple of times.

My design goal was to create a shield module for my Arduino R3 that I can use to program an ATTiny85 and that makes use of the indicator LED that are included in the ArduinoISP sketch (that comes with the Arduino IDE).

The bill of materials is fairly small and should be fairly inexpensive.

  • 1 x 8 Pin DIL socket ($0.17)
  • 1 x ATTiny 85 chip ($2.00)
  • 3 x 3mm LED (green, red, blue) ($0.12)
  • 3 x pin headers (2, 4 and 6 pin) ($0.12)
  • 3 x 220 Ω resistors ($0.03)
  • 1 x  40 x 53.5 mm copper clad F4 board ($0.17)

For a total of $2.61 (based on the cost of purchasing these items from eBay).

I plan to use this over and over again, so the cost of the ATTiny85 isn’t really part of the bill of materials, but I’ve included it for completeness sake.

Fritzing ATTiny85 ICSP_pcb

Also, there are some header pins that are only included to give the board a bit more mechanical structure (Arduino connections to pin 6, 3.3V and a GND connection). If you wanted to, you could save yourself $0.03 and not include them, but the shield would be wobbly.

While designing the PCB, I had the Arduino R3 part on in the design so that I could align the header pins correctly. I’ve printed the design out so that I can confirm the alignment. Also, if you really wanted to, you could remove the 3 x LED and associated 220 Ω resistors, but again, why not use them?

The PCB is single sided, so all of the traces should end up on the bottom of the board. The thing that I’ve been struggling with is that the header pins are soldered into the bottom of the board. This makes it a bit challenging to solder as the plastic block for the header pins will make contact with the copper side of the board, making it more difficult to solder. I will push the pins further into the plastic block and put the block on the component side of the PCB. However, that means that the solder fillet will interfere with the insertion of the pins into the Arduino. All that that really means is that the board won’t sit flush against the Arduino … meh.

I’ve put big friendly text on the PCB so that it is all pretty lookin’ when I’m done.

The solderless breadboard edition of the ATTiny 85 / ArduinoISP works pretty well, so I’ll be starting this project tonight. The only thing holding me up at this stage is that I’m out of Hydrogen Peroxide and Hydrochloric Acid at the moment, and I have to go to the Chemist and the Hardware store to restock (bummer). Well, the plan for tonight is to do the transfer printing of the PCB copper side. I won’t do the component side as it will just smudge when I use acetone to clean off the toner from the copper side.

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