One wire reads the keypad from the APRS radio mic

[Shane Burrell] decided to spend some time learning how the keypad on the his Kenwood TM-710A APRS radio mic works. It uses a different technique than you might think. Normally a grid of buttons is scanned as a matrix to detect keypresses, but this hardware actually counts pulses on a serial wire to take each reading.

The stock radio sends a steady digital pulse to the handset and with each pulse the mic pulls the line low. It then uses a 4017 decade counter to see what comes back. If the edge count matches it means nothing is pressed, but a change in the number of pulses returning to the base unit can be used to extrapolate which button has been pressed.

[Shane] went on to implement this control technique using an AVR chip in place of the  radio base unit. He used the data gained from measuring the pulse behavior using an oscilloscope to write the firmware for the project. He filmed a bit of a demo after the break which shows his findings.

We’re not quite sure how this would translate into your own home-brew projects, but the thought of scanning a keypad with two pins of a uC is quite desirable. Sure there is the 555-timer frequency technique, but we’re always down with new ideas.

Comments

  1. chango says:

    Hmm… If only there were such a two-wire bus that was easy to interface to micros, that talked to a large set of peripherals including port expanders…

    I Too See that the world needs such a thing.

    (Kenwood, good use of jellybean logic parts…)

  2. Michal says:

    That mic has nothing to do with APRS. Misleading title of the article.
    It’s just a mic with some serial connection to the radio.

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