Analog scanning and display hardware

These are the hardware chips that will be responsible for scanning the potentiometers.

2 x  CD4067 16 to 1 Analog Multiplexer

1 x TLC549 8-bit Serial A/D Converter.



The multiplexers will enable the PC to choose 1 of 32 analog inputs for A/D conversation by the TLC549 A/D converter. The endpoints of the potentiometers are tied to +5v and Ground and the center of each are routed to an analog input of the CD4067. The frontpanel program scans through each of the pots and registers any changes, transmitting these as MIDI-CC.


The display part is a standard 20×4 LCD with a yellow backlight connected to the PC with a 4-bit interface enabling the frontpanel program to display any characters or graphic on it.


The display is very easy to interface with thousands of examples on the Internet.

The frontpanel buttons are connected to the standard PC-keyboard scanner resulting in normal keypresses to the frontpanel program.

Here is the complete schematics with the 4 chips and display connected to the parallelport.


The 4th chip is just an 74LS04 Inverter used to extend the adress range for the multiplexers to 5bit linear adress  instead of 4bits per chip.

There is actually a bonus using an A/D converter, it converts analog voltage to a MIDI-CC so if we would like to have a CV input, no problem, just connect a jack to one of the multiplexer inputs. It’s that easy.



2 Responses to “Analog scanning and display hardware”

  1. fabrizio Says:

    Great work, Jan!
    Thanks to your examples, me too I’m planning some sort of DIY virtual analog, but…I’m mainly a musician, so while I can follow you about the software (and the carpentry…), I’m not such an electronic expert!
    Could you gently explain something more about this last step?
    For example, A0, A1, A2 and A3 will be connected at the same time with the parallel port AND the LCD? or it’s a synoptic scheme?
    And how the use of the logic inverter switches “the traffic” between the software and the right multiplexer? (pardon for beeing a cave man!)
    Thanks for sharing your experience & skills,
    Greetings from Italy

  2. janostman Says:


    Building something like this is hard without needing some electronics however the electronics in this synth is on a very basic beginners level so you should be able to put it together.

    The thing with this one is that i’m using 29 knobs on the front panel and finding a computer with 29 A/D converters will be hard so we need to multiplex (select one at a time) these into just 1 ADC.

    Please read my latest post for an explanation on how all the chips can work from the same wires.


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