Jul 152017

So… last weekend I got to trying out the I/O modules. I rigged up wiring harnesses for all five push-buttons and their associated LED strings.

I used CAT5e since I’ve got loads of it around… and ran four strands up to each button, carrying: +12V, MOSFET drain, GPIO and 0V. I ran a resistor between +12V and GPIO to pull it high, the switch NO and common connected between GPIO and 0V. The button’s illumination LEDs and the LED string both connected in parallel between +12V and MOSFET drain.

So far so good. I have just a 3-pin connection on the I/O module, carrying all but the +12V. That remaining wire I hooked to +12V directly on the input feed. Not having a 0V feed going direct to the power supply though was my mistake.

If this happens, the 0V reference on the I/O module is open-circuit, and the zeners, meant to protect the MCU from +12V, don’t do anything. So I suspect a MC14066 copped a belt of +12V by mistake!

Not sure, but I thought I’d play it safe and build a new one anyway. I started on it earlier this week and finished it this afternoon. This time around I opted to put LEDs on the outputs of the 74HC574. No current-limiting resistors, since they’re meant to be ⅛ duty cycle anyway, and if they smoke, well, who cares?

This highlighted a glitch on the GPIO_EN signal during programming, LEDs would illuminate during MCU programming. A pull-up helps here.

I tested using a 9V supply from my electronics kit, which has AA cells in it (somewhat stale ones) this time around. Handily, the plugs for the LED strings have LEDs in them themselves, and they work at 9V, so we can use those to see what the LED string would do.

On the software side, I finally fixed polyphonics and a key-sticking issue. One of my I/O modules has stopped working for whatever reason, but the others are working fine, as can be seen here:

The other issue I have to chase is some leakage on the blue and white buttons: the plugs for them are not meant to be glowing unless pressed, but you note they are glowing significantly, and get brighter when those buttons are pressed. I might have to re-visit those connections. Otherwise though, very good progress today.