I've been trying to come up with a simple and cheap way to connect an AVR to nrf24l01 modules to make coin-cell powered wireless sensor nodes. By cheap I mean < $1 for the interconnection parts. After mulling over lots of ideas my first actual attempt is attached. Sorry for the poor focus - my phone camera doesn't have macro mode.
It's a picture of an ATtiny88-AU hot glued on top of the module. Soldering the wires to the qfp has been a lot harder than expected, espectially since the soldering iron tends to melt some of the hot glue near the leads which gums up the leads. My plan was to hot glue a CR2032 holder on top after I finish connecting the leads, and solder the + and - from the battery holder to Vcc and Gnd. Here's the cost breakdown:
nrf module: 93c (qty 10@aliexpress)
CR2032: 22c (qty 25@fasttech)
2032 holder: 10c (qty 1@tayda)
ATtiny88-au: 56c (qty 25@newark)
(wire and hot glue are cheap enough not to count)
I could go with a slightly more expensive but easier to solder ATtiny85-SU, and hot glue it upside down so the glue doesn't get on the leads.
Another idea I thought of was to unsolder the 8-pin header, hot glue an ATtiny to the bottom of the NRF module with wires going to the solder pads where the 8-pin header goes. I could get the pins pushed flush to the PCB, but even wit some solder wick I couldn't get the pins out.
Another idea I have is to make an adapter out of perf board and an 8-pin female header, with an ATtiny85-PU on the perf board.
Another idea is to solder wires from the nrf header to the pins on an ATtin85. Given the differences in the pin order, I think it will be a bit of an octopus.
My last idea is to go pro, and design a PCB for an ATtiny88-AU with a breakout for the nrf 8-pin connector. I've designed & etched simple through-hole boards before, but never designed a PCB from scratch with a cad package. For this reason and because of the cost overhead, I think this is the least feasible of my ideas.
I stared at pinout diagrams for hours now looking for ways to line up the pins of the nrf module with the pins of an AVR. The closest I can come up with is an upside-down attiny84. The problem(though I guess not that bad) is the SPI pins wouldn't line up with the USI pins, so I'd have to bitbang the communication to the nrf module.
So what way do those of you who have done more than a couple circuits think? If you were doing it and only had a budget of a few dollars, how would you do it?