I've designed a new circuit for controlling MSSIAH, Cynthcart, and other C64 software using potentiometers.

This circuit features sub-miniature trim-pots for calibrating the range of the knobs' rotations. No longer are dead-zones a problem when using pot control - this new circuit allows the full range of the pots to be utilised. The controls may be calibrated using MSSIAH's Diagnostics page, and once set, can be left alone.

I have also fixed the issue of pot tapering. This is the effect of the pots moving the on-screen pointers at different speeds, depending on how far the knobs are turned. This new circuit has a reverse logarithmic behaviour that approximates how the C64 operates closely.

Note that the circuit must be doubled to allow four pots to be used, with two pots on each of the Commodore 64 joystick ports.

I have constructed the circuit, twice, on a piece of stripboard measuring 2"x2". The mod may be housed inside a C64 along with case-mounted pots, or, as I have done, in an external controller box which connects to the joystick ports. All the parts are standard and can be obtained cheaply.

VR1 and VR2 are the main pots, while VR3 and VR4 are the respective trim-pots for calibration.

I would very much like C64 users to try this out. If you decide to assemble this new improved circuit, please post your results here. Thanks.

Oh, and it works with Cynthcart, Prophet64 and any other C64 software which uses pot/paddle control.

Last edited by InactiveX (Jul 6, 2011 4:44 pm)

This is awesome. Thanks!

smile I'll try and get some photos of it in my external controller box.

I might try to build this one soon...

If i wanted to make this circuit easy to turn on and off while being internally build (so i can also use other joysticks)
Which connections should I disconnect? all four?

You should be able to do it by disconnecting either 7 or 8.

BUT... I would be very careful that you don't switch it when the C64 is on. You might be fine, but it's not recommended to disconnect stuff while the C64 is working.

Switching on and off while powered down will be fine.

Timbob wrote:

...(so i can also use other joysticks)
Which connections should I disconnect? all four?

Actually, thinking about it, if you're using switch joysticks then it shouldn't need a switch. The joystick will most likely work with the pot mod.

You would, however, run into trouble using a mouse as the mouse uses the POTX and POTY signals that the pot mod is using. The switch would allow both to be used.

I'm just guessing here though. Haven't tried it.

Last edited by InactiveX (Jul 6, 2011 8:54 pm)

Ah, thanks smile

I've got a mouse too, so i need to be able to switch it anyhow.

OK. A simple SPST switch to break either line 7 or 8 should do the job then.

I've been having better results with calibrating the trim-pots using this program:

20 POKE 56333,127: REM TURN OFF KBD
30 POKE 56320,64: REM SWITCH POT#1
40 PRINT TAB(1)"X1:"PEEK(54297);
50 PRINT TAB(11)"Y1:"PEEK(54298);
60 POKE 56320,128: REM SWITCH POT#2
70 PRINT TAB(21)"X2:"PEEK(54297);
80 PRINT TAB(31)"Y2:"PEEK(54298)
90 POKE 56333,129: REM TURN ON KBD
100 GOTO 20

It gives a digital numeric readout, which offers more precision than MSSIAH's diagnostics page.

Here's a photo (thanks to my photographer friend Andy!) of the board. The empty rows of holes in the foreground are for connecting to the joystick lines and the main pots.

Fantastic work.

Some user reports and stuff in here.

Excuse me if I'm mistaken, but that schematic looks a bit weird. I believe the ground connection is incorrect. In all schematics I've seen so far, you connect ground (pin 8) to the counter-clockwise terminal, +5V to the clockwise terminal and the input to the wiper. Your only ground connection is through a capacitor, which will stabilize the voltage reading, but the circuit will not in itself form a voltage divider, but relies on the C64 side to provide the bottom half of the voltage divider. Is this intentional?

Hey Nitro2k01, sorry for the late reply; real life etc....

I've put a response up on the MSSIAH forum:



Well I tried to message InactiveX through PM and email here but I got no reply, so I am forced to post here in hopes somebody can help as I gotta make this modification real soon.

I don't fully understand the diagram posted on the top of the thread. My confusion comes on where the resistors are plugged. Are they connecting to the POT data line in one end and to ground before the capacitor on the other? And where does the trimmer connect in all that? Does it go straight to the data line or does it connect to the resistor first? I just can't figure out the component hierarchy because I'm an electronics noob.
Everything else is pretty straightforward I believe but that part had me confused.

Any help appreciated!

not sure if i understand your question correctly, but as it's about what is connected to what: when you zoom into the picture, you may notice little dots on the junctions of the components.
The dot means there is a connection.

so the left side of R1 is connected to "9" and the center lugs of VR1 and VR3. The right side of R1 is connected to the left side of VR1, VR3 and C1.

have this on the "to build"-list for ages, but can't decide if i want internal or breakout box, so i don't know which pin is GND etc.
hope this helps

Last edited by shizcake (Apr 9, 2013 3:15 pm)

Hmm, so yeah, you confirm what I thought, I guess my diagram is right. Thanks a lot.
I will test it out as soon as all parts come and hopefully post some pictures that might help others.

Last edited by akira^8GB (Apr 9, 2013 3:39 pm)

Well it's me again.
I am trying to get the parts and I am having problems sourcing electrolytic 4700pF capacitors. Can I use ceramic ones, the ones shaped as little discs? I read somewhere they might introduce noise to analog signals so they might not be the best thing to use...