As you can see above the new circuit boards have arrived from Alberta Printed Circuit Boards. These guys have some of the quickest turn around I’ve seen – you do pay for it though. I’ve designed this board to not deviate too much from the smaller circuit boards I initially built – mainly to speed development of the V3.
A previous post showed a circuit board with an atmega32u4 chip and the associated circuits to support it. Considering I do not have a reflow over I opted for the simpler approach of only supporting the analog and wireless on this PCB and leaving the regulator and micro-controller on an Arduino pro micro. You can see how I broke out the ISP to make it more convenient to program.
Both of the boards mount to this main one – one via pins and the other via surface mount. I haven’t, but I should, incorporate power planes but I consciously did not include them as the AP2 module suggests not having anything metal under the F-Antenna. I probably took this to the extreme but after the difficulties with V2 maintaining a good signal I felt this was a good choice. The analog components are very very low power, on the order of micro-amps of current draw at 3.3v and I never had issues with the little circuit boards so I’m comfortable.
The voltage divider used on V2 is replaced with a precision voltage reference, the LM4140.
This will go on a bike without too much problem but having seen the size of the “Stages” power meter I’m keen to drive the size down. This would have been a great time to know some electronic engineer who works at any of the mobile phone companies as they can pack some power into a small space.
I’m keen to use the AP2 module as the nRF51422 hasn’t become available yet. I unfortunately missed my opportunity to get a free one as I had to renew my passport (Expired on the 25th of Sept, and the Boston conference was on the 24th). So I couldn’t book tickets or sign up for the tour, and as a result I’m waiting for nuhorizons to get in the evaluation kit.
I just submitted my order for parts with Digikey just then, so maybe a new prototype by the end of the week, but likely next week some time.