DC Input Protector - Level Shifter

1. Measure the voltage dividers and adjust for a 5V output.
2. Connect to a digital microprocessor and test.
3. Build a second circuit with the op-amps powered from a lab source. Measure the Vout signal from both circuits to find the delay of the op-amps startup.
4. The Vce of the 2N3904 is being exceeded. Substitue a suitable replacement with higher Vce rating.
5. Add transient filtering on the input.
6. Develop a circuit that could handle 125VDC. There is not much switchgear with 125VDC logic signals, but there are some breaker solenoids that operate at that level. It would be handy to be able to bring those signals in for an event history also.
7. Design a final output stage with isolation. This could be done with optical isolation or perhaps with some of the newer magnetic isolation ICs developed for EV vehicles.

Prototyping:

I built the prototype on a protoboard so I could easily substitute components and make changes. I did not build the entire circuit at once and "swing for the fences", e.g. powering it on and hoping I didn't release any smoke. I built it up in stages, testing as it was built.

1. I built the resistor divider/Zener circuit and tested the resulting voltage level.

2. I added the unity gain amplifier which provides isolation and the voltage reference to the 2nd op-amp.

3. I added the output stage with the voltage divider feedback circuit.