I ordered (most of the) parts and a PICDEM FS USB Demo Board (PIC18F45K50), Microchip part #DM163025-1. Getting used to MPLAB X, the usual compiler headaches. Starting to grok a bit how their configuration tools look. It seems like a pretty standard HAL setup for the PIC18, a bunch of EXTERN definitions for the Microchip MLA USB stack, setup functions which just fill in some structs which are declared VOLATILE, etc. Pretty standard C stuff; there were a few things with #define some_function() some_label that then led back to the HAL that had me scratching my head a bit - if it's a macro in C, why bother with the function style if it's just text replacement? and it just pointed to some volatile bit - doesn't look like much of a function call to me. Maybe it's just style. Whatever, the debugger will answer this. I had an ICE 4 sitting around from some previous consulting, might as well put it to good use. One thing to note is "Chapter 6. Reconfiguring the PICDEM™ FS USB Hardware" of the PICDEM FS USB - if you want to not use the legacy pin setup, you have to apply cuts & jumpers to the demo board. That reeks of poor planning to me, but fine. I'll have to apply this step when I get the board. Good to know.  Fine, looks pretty standard; demo application built. I'm sure there will be some goofy problem when I start playing with the board. There's always some goofy "gotcha". Anyway, foam board printout looks good, when the parts get here I'll do some fit tests.  I'm having a bit of trouble sourcing the keys for what I'm willing to pay. I've contacted Kailh in China to see about getting a bag of switches for (hopefully) less than what you can currently be gouged for online. Stay tuned on that front.   

Hi PADAKATHU If your ESP32 voltmeter is sending overvoltage message, there is a problem. There may be a wrong connection or a faulty component. I made some modifications for better results. R2 = 18 ohms, C2 = 470nF, C6 = 470nF, P1 = 20K pot. IN LOW pin connected to ground. You can test the PCF8574 on the Protoboard first. Without any connection to the TLC7135, change the jumpers in the inputs and see the byte in the message output.https://www.flickr.com/photos/jgustavoam/39420646405/in/photostream/https://www.flickr.com/photos/jgustavoam/39420488295  

Thank you sir,  i will follow up you for troubleshooting, i connected 1.5v AA dry cell in input of ICL7135 please find the photo attached out put still overvoltages.  Mean time i will make another fresh circuit follow exchatly posted circuit. 

Padakathu ,  The reading is correctt ! You put the ESP32 Voltmeter in over voltage condition.  You can only mesure voltages between +2V and -2V. Read carefully the project.  "Tried to connect 2amp 5v usb power and measure the voltage still in -VCC Appears -3.22 between input terminals pin 9-10 measured near zero 4mv. output at serial monitor showing "Over voltage" For test, connect 1.5V AA battery at input of ESP32 Voltmeter.  

sorry writing mistakes in above mentioned lines ICL7135 pin 01 (-VCC) measured -3.22V ICL7135 pin 02 (Vref) measured 1.000V ICL 7135 pin 11 (+VCC) measured +3.3V