Thursday, 11 September 2014

Arduino ATTiny85 ISP Shield – PCB – Part 2

Okay, I’ve now got some etchant, yay!

I tried to use the old etchant, but it was not working well at all. I went and bought some peroxide and some hydrochloric acid. The acid is from the same company that I usually buy, but now it seems they’ve added some iron salts to it (it’s a brick cleaner) and that results in the acid turning red when it mixes with the hydrogen peroxide. It still etches well enough, so, no harm. Interestingly, the hardware store no longer sells the 500ml bottles, their smallest bottle is 2.5L.

Anyway, I etched the board (and another so that I wasn’t wasting the etchant) and it turned out pretty well. I’ve also learned that agitating the acid bath makes it work better (faster) and the etch took just under 10 minutes.


I then went ahead and did a transfer of the silk side.


Which looked like crap.

So I came up with the idea to simply print the silk out and glue it to the top face of the PCB. I painted on a watered down solution of PVA first and then laid the paper on the board, next, I painted the PVA over the top of the paper too, so that it would wear better.


Now I can actually read the silk side better. Nice. I don’t know that it will stand soldering very well though.


The pins were put into the board and the plastic blocks were slid up the headers to allow space for soldering.


After soldering, the plastic blocks were returned to their previous position.


I then tested that the pin headers matched up correctly with the Freetronics Eleven r3 (Arduino clone). They fit nicely, so it’s on to populating the board.


I started with the resistors, then the 8 pin DIL and finally the LED. There was a bit of a problem with the LED, but it doesn’t stop the board working as an ISP. Actually, two problems. 1) the copper came away from the board, and 2) I got the LED backwards (face palm).


Next, I plugged the shield into the Arduino UNO R3 (genuine) and it fitted well too.

I plugged it all in, loaded the Arduino ISP sketch onto the UNO and then tried to load the blink sketch onto the ATTiny85. No success. My multimeter is in another state at the moment, so I can’t figure out where the problem is. Oh, well … I’ll wait until next week to test the board properly and find out where I went wrong.

Well … I’ve made another PCB for this circuit (with the LED around the right way) and I no longer get the sync error, so that does make a difference. I’m getting an invalid signature error on the new board so it looks like there may be a continuity error somewhere. That’ll definitely have to wait until I’m with my multimeter again.

Now that I’ve done a continuity test of the circuit with the multimeter, I’ve found that the MOSI connection (Arduino Pin 11 to ATTiny85 PB1) is broken. I’m going to need to either solder over the break or get a conductive pen to fix it. Jaycar has a silver conductive pen for $30 … it seems a bit expensive. I think that I’ll go with solder.

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