• A review of NextPCB PCB service

    10/19/2020 at 01:01 0 comments

    My attention was drawn to the NextPCB factory by a new customer offer where you only pay shipping on the first set of 5 or 10 boards in return for allowing them to put their logo on it. I decided to use them for #6 segment 2 digit LED display board and later #7 segment 2 digit LED display board to make 6 cm tall digits, and in the case of the former project, to have a custom font. Only 2 digits will fit into the bargain 100x100 mm limitation so I made the boards cascadable.

    There isn't a lot to remark on their service and I mean this in a good way. Their web order page has the usual options. In this pandemic era, all shipping has been slowed. The first board took about 8 weeks to arrive and the second about 4 weeks. Part of the time tjhey were sitting in the departure airport in China. In the first case I could see from the tracking it was moved from one airport to another, probably due to the shortage of flights. Of course if you pay more for courier service you can get faster delivery. But I'm cheap as the idea of paying more than few dollars per board for what are just my hobby projects irks me. So I'm willing to overlap projects and work on something else while waiting. The long wait time is also incentive to double check and triple check the design files before submission.

    Here are a couple of close-ups of the boards I received, excuse my soldering:

    Disclaimer: My boards are not complex, I'm still using THT for the most part so I didn't push the envelope. I do have some SMT pads on the front side that I didn't use and those came out ok too. Do your own qualification to find out if they can handle the precision you need.

    What I liked about their work is that the solder on the pads were very easy to melt solder into. It has been a bit more difficult with boards from some fabs. There shouldn't be any difference as 60/40 solder should have the same melting point everywhere. Maybe I have a better iron tip now, who knows?

    As for the logo it was simply the word NEXTPCB on the bottom right of my boards. Not annoying at all.

  • A quick note on DS3231 RTC modules

    09/23/2020 at 13:27 0 comments

    I have been using DS3231 RTC modules purchased from the Internet as time sources. The DS3231 has very good accuracy, obsoleting many older RTC chips, and will work at 3.3V or 5V—the I/O pins are 5V tolerant.

    One commonly found model looks like this:

    The picture is actually many times life size. I should have realised it from the 0.1 inch pitch connector but I was still surprised. That cell is a CR1025 so it's 10 mm across and 2.5 mm thick.

    The other commonly found model looks like this:

    This model costs a bit more than the first. The differences are:

    • It doesn't come with a lithium cell, you have to provide one
    • There is a serial EEPROM included
    • There are a couple more pins for SQW and 32K outputs
    • The A0-A2 jumpers allow you to select a different I2C address from the default for the EEPROM

    There is another gotcha with the second type. They included a charging circuit comprising a 1N4148 and a 200Ω resistor. This is explained in this article.

    Originally this was intended for charging a LIR2032, a rechargeable version of the CR2032. However these are much more expensive and not justified when a normal CR2032 will power the RTC for years. It seems they never updated the module design to omit this charging circuit.

    If you are powering the RTC module from 3.3V you don't have to do anything as the diode will not be forward biased.

    If you are powering the RTC module from 5V, it will attempt to charge the CR2032. You can disable this by unsoldering either the diode or the resistor, or cutting the trace to the cell's positive terminal.

  • Embedder's Handbook

    09/08/2020 at 13:50 0 comments

    From EEVblog