Searching through my Babani Publishing books, came across a way of using 2 inverter gates as a Schmitt Trigger with a resistive divider feedback. Could be used to solve a problem with the ldr sensor for the Dome Nodes Project. The circuit basically relies on 2 trip points set by 1% -2% metal film resistors. Problems here could be from the reduced supply voltage 5v restricting the hysterisis range used that would still be accepted by the microcontroller/pi board. Hoping it will be within 33.33% and 66.66% Vdd used by HC logic family.
Here are 2 formulae for the trip point voltages:
Vh(Utp) =V/2(1+Rin/Rfedb)
Vl(Ltp) =V/2(1-Rin/Rfedb)
Suggest making Rin/Rfedb ratio 1:47 or 1:5 if the acceptable logic voltage allows. Have a wide variety of ldr which may not help matters.Resistor values Rin 680k, Rfeb 1M0 with NORP12 ldr plus 200k to 250k preset pot as a supply rail divider. If this doesn’t work, may resort to a 74HC132 ic without the resistors. At present using 74HC00.
Update: The higher voltage circuit (9-12v) using the CA3140 op-- amp does work. Experimented with it this evening 8/11/19.It works with 3 different ldr’s ,one being the common ORP12. Needs voltage translation stage to work with microcontroller board. It may have a probable inverted logic sense which could be solved by changing over the position of the ldr and preset potentiometer in the circuit. Could be used as an extreme remote sensor with 20m to 50m of cable using 5 pin Din connectors. Circuit block/stage was copied from the February 1981 edition of Everyday Electronics. Experiment with direct ldr analogue sensing next week. I think this is only a get round circuit as it only offers the on/ off function.