• First Hardware Iteration Assembled!

    Michael Kafarowski02/23/2019 at 21:30 0 comments

    I recently ordered my revision 1 PCB, and got it back yesterday. With a few minor tweaks to the hardware and software, the system worked great on the first try!

    Last-Minute Tweaks to the Rev 1 board

    • I forgot to add load capacitors to the 32.768 MHz RTC crystal. The particular crystal I had purchased specified a 12.5pF capacitor. The closest I had to this were 10pF ceramic caps, which I surgically inserted between each lead and a nearby ground pin. The mismatched caps will probably change the crystal's frequency slightly and cause the clock to run a little slow or fast. I'll keep an eye on this.
    • Removed two mosfets that were to have formed a half H-bridge for the haptic motor. In testing between when I ordered and received the PCB I didn't feel any difference between passive and electrical braking of the motor.
    • Some pin changes were needed in software from the breadboard version.
    • My breadboard version used a different I2C RTC - the DS3231 instead of the 7940N on the PCB. A 7940N library dropped right in and worked like a charm.

    Next Steps

    There are definitely a few improvements to make on the next revision. Now that I am confident with the circuit I'm going to try to reduce it's footprint, that means replacing the massive DIP-28 and beeper packages with smaller, probably surface mount parts. I'm also going to implement the battery backup on the RTC. I also have plans for other features, such as a clap detector circuit or a low-energy conversion so that the circuit can be used with batteries (perhaps as a watch).