This is the Analog front end of the 80C39 Process Controller. The analog input is protected by a Zener barrier, low leakage. You could use clamping diodes too.
The non-inverting low-offset amp offers high input impedance. After further amplification it reaches the VCO LM331. The pulse train from the VCO reaches the uC port and is gated and measured by the MCS48 firmware. The voltage is deduced from the Frequency or Pulse width.
You can get a resolution near to a 8-10 bit A/D converter. It does not work for negative voltages. It is a low cost Voltmeter or Process Display solution.
80C39 and MCS48 based Process Controller is the main circuit that has the LED 7 segment display for output and push keys for input.
This shows how to OR gate two 555, when one 555 cycles at a low frequency a valve turns on an off, the second 555 stretches the ON duration of the pulse with a diode OR gate.
Digital Timers Counters and Clocks
The OR output uses sample and hold to get the stable analog data from a sensor after the actuator has gone OFF, this ensures correct reading.
555 is a fundamental Mixed Signal Circuit as it can be made into a VCO using Pin-5. If you see old exar databooks, you can see 555 and PLL and Tone decoders all applications compiled in one base. I feel the Venerable Signetics 555 “Architecture” and Intersil ICL8038 ‘CMOS’ were inspiration behind early communication chip designs, Moving from Bakelite Telephones to Compact Push Button Electronic Phones and more.
This circuit is an User Interface part of a Security Alertness Monitor that i designed decades ago. The circuits are here – Digital Circuits – Part 1 The RAM and RTC part is missing, i will add later. It is without uC or Software. Only CMOS Logic.
Every Hour “+V UR” Goes High for 120 Seconds. The Buzzer Sounds and a Red LED turns on. The Guard has to respond by Pushing the Switch. The Green Light Flashes and the external Flip-Flop logic brings “+V UR” Low, The Buzzer Sound Stops and Red Light Goes off..
If not pressed the Sound Stops after 120 Seconds and even the Red light goes off then. This records a Non-Alert Hour in the RAM for that Day. The Ram Stores 9 Days alertness status.
You could try porting this project into a 89C2051 for learning product and interface design.
This is a Voltage to pulse converter using opamps for measuring speed or rate from a Tacho Motor mechanical coupled to a Conveyor System.
See the complete circuit in my site to study other sections. The motor DC out is filtered and scaled. Then the clean safe dc is fed to this VCO based on a generic application note design.
The pulses can be used to advance digital counter or mechanical counter using a solenoid driver transistor circuit. The Voltage Vin can be further scaled and fed to a Digital Panel meter to read the rate.
VCO – Voltage Controlled Oscillator using Opamps. LM339 is a quad Comparator.
The complete PDF circuit and other such circuits at my Mixed Circuits Analog with Digital