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Without any Values of the Parts, You can't calculate any Voltages or Currents.
I try to analyze that thing.
I Think it is a signal conditioner / protection unit for rotary encoders.
RA and D1 are a Pullup Cicuit to give the TTL Input a proper signal when both Transistors on the left side are opened.
D1 prevents some current flow back to TTL side when the rotary encoder Supply Voltage is higher than 5V
RB and ZD are an overvoltage Protection for the TTL Input when the rotary Switch output Voltage is higher than 5V.
D2 allows a fast dischargeing of the input Capacitance at the TTL Gate to increase the slew rate.
You see? - The TTL CMOS Input had to be a Schmitt Trigger one!
To get a basic idea what is going on you have to assume some input capacitance of the CMOS gate. Then solve for the points where the diodes turn on and turn off, then calculate the charge time of the input to the CMOS gate.
This circuit looks like some sort of delay circuit where they want to generate a delay for the high going edge of the input signal. Only for the high going edge because D2 and Rb create an asymmetrical drive for the CMOS gate.
The zener could be there to help perform a level conversion or just protection in case the input goes too high for some reason, including a spike on the line.
D1 is a little less clear, but may be there to provide fast spike protection.
So the way to start the analysis would be to assume the input is zero, then calculate the different voltages, then allow the input to go high, then calculate the voltages again. Since the voltage of the battery is not given, you might consider three different possibilities and analyze for each case separately:
1. Battery voltage is less than the CMOS gate power supply.
2. Battery voltage is more than the CMOS gate power supply.
3. Battery voltage is equal to the CMOS gate power supply.
All the while you have to consider the possible threshold voltages for the SMOS Schmitt Trigger input as well as the input capacitance.
You might get different results for each case especially for certain battery voltages.
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