Updated, functional design at the bottom of this question
I wish to detect when a low-impedance signal is below or above 60VDC (±3V). When the signal is above 60V, I'd like to indicate this by illuminating an LED and opening a relay. Else, illuminate another LED and close the relay.
To do this I propose to use a the following comparator circuit. This is my best attempt - I've made functional blocks and connected them together as best as I can see. Is it a reasonable design?
Here is my justification for each part of the circuit:
HV is the a low-impedance 0-200VDC signal which is divided by 20, so that when HV=60V, the inverting input at the comparator should be approx 3V. Multiple resistors are used within the voltage divider to ensure manageable voltages across each resistor. A reference voltage of 3.0V is applied to the noninverting input.
When HV < 60V, the comparator output should be high, which drives the relay through Q1, and illuminates D2. When HV > 60V, the comparator output should be low, which de-energises the relay and illuminates D4.
The comparator output is an open-collector, so R8 acts as a pullup.
The 12V zener at the inverting input is included to provide transient protection.
I have not included input-offset adjustment because this circuit should not be user-adjustable. I've picked a comparator with acceptably low input offset voltage and high accuracy is not a concern.
R6, R7 and C3 seem to be best-practice inclusions - I haven't done this before but I think they eliminate oscillation of the output caused by capacitive coupling between output and input. I've included these components provisionally but may decide against them.
I've avoided adding positive feedback for hysteresis because I'm having trouble including it without shifting the switching thresholds (drastically). Since this circuit is for a visual indicator only, I'm not overly concerned if the output chatters about the switching threshold - HV should never be steady close to 60V. If there is a painless way to add a few tens-of-millivolts hysteresis, I'd love to hear it. I've considered just adding some small capacitance to the output.
Side note: I'm acutely aware there's probably a very elegant means of doing this with a high-voltage linear regulator referenced by zener diode - a huge advantage there would be the ability to change the trigger voltage by changing only the zener - and doing away with voltage divider and comparator entirely. I don't have the analog-confidence to venture into that realm, but if you have any thoughts I'd love to hear them.
EDIT - an updated design
Thanks for your feedback everyone. Based off feedback from commentors I arrived with the following design. It performs acceptably when constructed and tested - VHI:62V, VLO:59V. The circuit is basically the same as the first proposed, but with a modified output-circuit. I'm more comfortable with voltage controlled devices like MOSFETs so they were used instead of BJTs. A comparator with reasonably low input-offset was selected to make this a build-and-forget design. It should not be necessary to tune the circuit.
There are plenty of undesirable failure modes - I'm aware. Read the full writeup on the readme.