Motorized Magic Over HDMI

There is a certain warmth that seems to emanate from stereo receivers of the 70s, 80s, and 90s. Despite their large footprint and considerable heft, the soft glow of the indicator lights and solid kerthunk of switches provide a sense of coziness. When [Tom] recently swapped his receiver for a 1970s Pioneer SX-950, he found himself getting up from the couch to adjust the volume when watching TV far too often for his liking. Resolving to do something about it, he added some magic in the form of a motorized volume knob. One of the coolest tricks for stereos was to have a small motor attached to the volume knob so that it could turn the volume up or down via a remote.

The first obstacle came when [Tom] had to forgo the center tap on the potentiometer to get a motorized one. This meant the volume compensation feature would be disabled, which is but a small price to pay for convenience. After scouring the internet, he finally had the part in hand only to discover some troublesome capacitors in the way. The new pot had a rather large motor hanging off the back that the previous one didn’t have. Fortunately, there was a good bit of space between the PCB and the bottom of the chassis, so Tom was able to just flip the capacitors to the underside of the board and bend them on their sides.

The next problem to solve was how to change the volume remotely. IR was considered as well as optical cable control signals. What [Tom] did instead was to implement HDMI CEC (consumer electronics control). CEC was well documented and seemed simple to implement on an ATTINY4313 with the help of a half-H driver. The CEC protocol implemented by [Tom’s] TV seemed to be very sensitive to timing, so an external crystal was used to get more precise timing and additional handshaking was implemented to get the TV to accept the microcontroller as valid. A few fail-safes were added to make sure the motor didn’t burn out if something went wrong with the CEC protocol and a nice enclosure wrapped up the build quite nicely.

We’ve seen CEC implemented before on a PIC 18F87J50, but as a sender of CEC commands not a receiver. [Tom’s] code is available on GitHub and might prove useful if you’re looking to implement CEC on an AVR.

Thanks [Tom] for sending this one in!

12 thoughts on “Motorized Magic Over HDMI

  1. this is awesome. Bummer motorized pots are so expensive– they are pretty much my favorite electronic component ever since i was a kid and saw a super high-end mixer with motorized slide pots adjust all of its levels to a preset.

    I am already mourning the death of tactile control interfaces. It’s nice to see someone make analog ‘smart’

    1. I prefer rotary encoders paired with a display, but these days we don’t get either…. half the time we don’t even get a digital controls, just non-configurable fixed crap!

  2. A great amp, (protect against any shorts in speaker lines though) keep it going. You know that only the last volume control in a chain is the one to use, all those virtual screen “pots” need to be left at full but not beyond.

    Sound in movies today is an exercise in motivation and response to the bored state, just like Muzak used to do to work music. Up tempo, now down. Repeat.

  3. Nice. I have an old McIntosh C37 (paired with an MC7200) with the servo volume control over standard IR remote, which made setting it up on my Harmony trivial. I (vastly) prefer servo controls over attenuation, so I think I might try this out for my office setup, which is an early-2000’s (still made in Japan) Denon AVR-3801 to a Parasound stereo amp. Thanks for the tip!

  4. Reminds me of a project I tried to build many years ago. Tried to build a Power Supply with a Motorized pot,H-Bridge and an Arduino. It worked but couldn’t get the adjustable Voltage range from 0 to 30 Volts, was lucky enough to get from 4.5 Volts to 19 Volts.

  5. I just took a deep sigh seeing an old Pioneer in the Title Photo!
    I didn’t keep track of the number of audio output TO-3s I replaced in those. Sometimes the driver to the Finals as well.

    Yes, [echodelta] is spot on about not shorting the speaker wires!

  6. That’s a cool project! I thought he was going to install an external motor, hidden in the back of his stereo equipment, connected to the knob using a long rubber belt and pulleys.

  7. i dig the stereo
    those were built at a time when residential sub-woofers were unheard-of.
    it was assumed that if you wanted deep-er bass, you purchased large-er speakers,
    so the amp doubled as a subwoofer amp.

    i got a simillar one free once, one channel was bad.
    each channel was socketed so you could remove the bad one and power it back up after replaceing the respective fuses.

    it sounded so good but smoked many speakers with that.
    must have been logarithmic (non-linear) volume knob.

    the main power-supply capacitors (the big ones) really make a noise when shorted,
    i mean reeeally make a noise, 50000 uF (thats 50 thousand-micro aka 50 milli-farad) @ 50 volts. just remember to dis-connect before shorting, electronics wont like power flowing OUT-TO the supply.

    1. normally i’d say there could have been a DC riding on that volume knob but i checked, no DC on speaker, just a whole lot of 20-30hz when fed with a whole lot of 20-30hz “music”, the speakers in question were not even remotely suitable

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