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Software Support => Automated Projects => Topic started by: BadMouth on October 29, 2013, 12:29:34 pm

Title: What diodes should I use for a button matrix, etc?
Post by: BadMouth on October 29, 2013, 12:29:34 pm

What diodes should I use for a button matrix, etc?
Is there a standard go-to when working with devices using 5v USB sources?

If it's relevant, this is for two different projects:
One is a USB Global VR driving cab I/O board that uses a 4x4 matrix to get the 16 total buttons that show up on the PC.
The manual has a wiring diagram, but doesn't say what value diodes to use.

The other is a circuit I'm working on to interface a switch based gear shifter in place of a potentiometer based Logitech one.
The Logitech board reads the wiper voltage from each pot and registers the appropriate gear based on that.  I've tested with an external power supply and confirmed that it looks at nothing but the wiper voltage.
My plan is to have a switch for each gear that will pass 5v through to a circuit that will send the appropriate voltages to each axis for that gear.
I need the diodes to prevent voltage from going back through the circuits for the other gears since they're all hooked up to the same two inputs.

Attached is the general idea.  Two gears shown, but there will be 6 total, some needing three resistors to get the appropriate voltages.
Neutral will be daisy chained through all the NC tabs on the switches, so if none of them are pressed the appropriate voltages for neutral will present.
Feel free to tell me if something is horribly wrong.  :lol

Title: Re: What diodes should I use for a button matrix, etc?
Post by: DaOld Man on October 29, 2013, 06:57:56 pm

Hello BadMouth.
I think just about any diode will work, however you have to remember that a diode has voltage drop across it. Most are around 0.6 volts, but some can be higher.
Not sure if this will affect what you are doing, but it looks like it may.
You can probably use germanium diodes, they have a much smaller volt drop (typical around 0.2 volts.)

I think you could also use a variable voltage regulator to do what you want.

Title: Re: What diodes should I use for a button matrix, etc?
Post by: BadMouth on October 29, 2013, 07:54:22 pm

Thanks.  I have an Elenco brand diode kit with various types, so I guess I'll play around and see what works best.
I wasn't aware of the voltage drop, so I'll have to go back and recalculate the value resistors needed.

1N4004 seems to be popular for a lot of different applications, so I guess I'll start with that and similar models.

EDIT: Finally got around to getting the parts together to breadboard.  Open the Elenco kit and I have a strip of a dozen 1N4004 diodes stuck in the front of it leftover from a recoil project.  :applaud: