• Simulating a microphone

    04/17/2021 at 16:03 0 comments

    I was given a microphone spec:

    • Vout (RMS) = 0.25mV
    • Zout = 9.5 Ohms

    And I wish to use an audio generator to simulate such microphone. This can be accomplished by a voltage divider with the right resistor values.

    Given the low voltage on the output as well as the low impedance, we could have a hunch that R2 should be very close to 9,5 ohms, but let's do some math.
    Note: Zout on my generator is predominantly resistive, hence I've added Z and R without further ado.
    Choosing 1V(RMS) for our source (that we can control anyway) and a Zi of 40 Ohms and solving the matrix we have:

    No surprise here, but the math might be very useful to simulate other sources.

    On the scope of this project I should use a 9,1 Ohm resistor (or even a 8,2 Ohm) for R2 and a 39KOhm for R1 and adjust my Vo to have .25mv on the output.


  • Screw Arduino!

    12/14/2020 at 12:20 2 comments

    This week H-A-D featured an article with a home-built racing sim. While watching the video I have seen that the author did many connections on an Arduino board just inserting the wires on the pin header.

    I have seen that before on a considerable amount of projects. It's not something I would rely on for a long term operation circuit, then I realized that there is room for a screw terminal shield. (Eagle Files):

  • Ultimate MSX Caps Lock cartridge

    11/23/2020 at 02:21 0 comments

    The Expert is a Brazilian manufactured in the 80's and it is compatible with MSX standard. It uses a detachable keyboard but it lacks the Caps Lock LED on the keyboard because the 13 pin connector used ran out of pins (even the shield is used as a 14th conductor for GND). 

    One day I heard about that issue on my favorite podcast, and just for fun I have designed a Caps Lock cartrige for the Expert. Some time later I have added another feature to the cartridge that is not implemented on the computer:  a RESET button

    I have designed PCBs for the MSX cartriges before, like the MSXDOS2 cartride that contains a ROM with the MSXDOS2 kernel with some glue logic to perform the bankswitching (the circuit was not designed by me though). Such cartridge allows the Expert - an MSX1 machine - to run  MSXDOS2 without a Memory Mapper

    On this sunday afternoon I have merged the two projects and created the Ultimate MSX Caps Lock cartridge:

    The PCB and schematics files are available on Github. I have just routed the board, not assembled the circuit yet, sou use it at your own risk.