P1 opto input concepts?
Hello all. On the first image, this is what I have done on lots of this particular board for external buttons/contact closures, etc. I run 5V from the main Switching power supply out to buttons over shielded 22G stranded wire. Tie the drain to the main board GND terminals. The 5V returns from the switch and is held low with a resistor. Before the Propeller input is a 1.5K Resistor and .1uF to GND. This R value seems low for 5V but I have never damaged an input with 5V to 1000+ P1's with this method.
The current method requires an external INPUT board with screw terminals, that connects with a 12" cat5. I want to simplify the whole thing and move the screw terminals for inputs onto the main board, reduce the work and cables. There once was a reason to park the external board outside the main box, now I put it all inside.
The new concept is to simplify and dummy proof the install for others. Ideally, I'd like to get rid of the shielded cable requirement. But since the 5V in this new idea still uses the main 5V power from the switching power supply chip, I'm concerned about noise if running multiple cable runs to switches. The Prop is on an LDO 3v3 reg. One thought is to add another 5V 7805 style switching reg ($5) and use that 5V rail to go to all external buttons, maybe that would keep noise separate from the main board? As shown, the external switch just takes the low side of the LED input to GND after the series resistor so the 5V is not full current at the remote switch.
I have never had a damaged P1 input using this. So why not just keep the same method and move the screw terminals to the main P1 board? Forget the optos? The only reason is to avoid installers having to track down 2 or 4 Con 22G shielded which is not always local. Use 2 con any type wire, unshielded, even 18G stranded simpler for installers.