Designing a simple and cheap temperature logger. Part 5: New prototype PCB.

It has options to be powered by a Li-Ion battery (rechargeable via the USB port, with a MCP73831), as well as by a CR2032 coin cell. There’s an other option for the 1,8V EEPROM SDO line pull-up voltage adaptation.

All the options are selectable by fitting or not 0 Ohm resistors and adding the dedicated components.

In the Li-Ion version, it’s supposed to fit into a case like that (but with a hole for a button and a LED). It’s 19mm x 40mm.

The blue connector is the ICSP program port. I designed it to be “off-snappable”, as it’s connected only by three bits of PCB, where the signals pass.

USB Temperature Logger - Prototype 2 - Face


USB Temperature Logger - Prototype 2 - Back

NB: Some of my 3D models aren’t exact (like the battery socket, the 3D model is SMT instead of the through hole I have)


Here, an other PCB, Li-Ion only. Just for fun, I wanted to see how small the PCB (19mm x 38mm) can be routed with the components on the same face. On the bottom, there’s only a contact-style connector for ICSP:

Mini USB Temperature Logger - Prototype 3 - Face


Mini USB Temperature Logger - Prototype 3 - Back

The components names couldn’t fit on the top side, so I put them on the bottom.

The PCB could maybe get even smaller with a smaller 3,3V regulator, in a SOT23 package instead of the SOT223. The 32,768KHz crystal could also use a smaller package. But here, I hit the limits of the 2 layers, 6mil/6mil routing and I’m not sure some tracks won’t interfere each other.

The prototype on the first two images is the one I’ll be using to continue the software development.


2 thoughts on “Designing a simple and cheap temperature logger. Part 5: New prototype PCB.

  1. It’s awesome the build you’re doing. I like the nice idea of the detachable icsp header, only used for programming. BTW, thanks for showing the MCP73831, I’ll put it in my to-try list for future designs 🙂

    • Thanks!
      I’m not super sure about the ICSP header actually. This board is still a prototype, so there’s no need to snap the connector off. And for a production board, I’d just use a row of contacts (like on the second image) or even easier, OTP microcontroller. Let’s say here, it allows me to get a proper ICSP connector and as I’ll get several boards (from iTeadstudio), I’ll be able to snap it off to get a better idea of the board size and try it into enclosures.
      Anyway, I just wanted to try it. And I find it funny 🙂
      Now, I’m waiting for the PCBs to arrive from China…
      For the MCP7381, I still have to find the right battery. I want a thin one, 20*40mm. Probably form The coin cell battery solution is convenient (and you can sell your product cheaper) but it’s impossible to come up with a solution for the enclosure.

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