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Will this work (optocoupler over shunt)?

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ArthurDent:
I would, as I said, follow the datasheet/application sheet information and, if included, would put the hysteresis resistor between pin1 (output of LM393) and pin3 (+ input) and ground the unused comparator inputs. Next I would replace the BC337 with a MOSFET and eliminate your concern of the base-emitter clamping although a resistor in series with the base would work as well.

If the collector of Q2 was connected to a resistive load I wouldn't be concerned but the relay more or less requires the added 2 diodes to eat up spikes (sorry for the technical jargon  :) ). Granted the circuit will rarely be triggered judging by the life of most lamps but I think using an inductor for the feedback point isn't wise.

We are talking in somewhat general terms here and if an actual circuit were to be built and tested I'm sure we'd find a number of other mods that would make sense.   

Zero999:

--- Quote from: ArthurDent on December 07, 2018, 01:44:43 am ---I would, as I said, follow the datasheet/application sheet information and, if included, would put the hysteresis resistor between pin1 (output of LM393) and pin3 (+ input) and ground the unused comparator inputs. Next I would replace the BC337 with a MOSFET and eliminate your concern of the base-emitter clamping although a resistor in series with the base would work as well.

If the collector of Q2 was connected to a resistive load I wouldn't be concerned but the relay more or less requires the added 2 diodes to eat up spikes (sorry for the technical jargon  :) ). Granted the circuit will rarely be triggered judging by the life of most lamps but I think using an inductor for the feedback point isn't wise.

We are talking in somewhat general terms here and if an actual circuit were to be built and tested I'm sure we'd find a number of other mods that would make sense.   

--- End quote ---
Ideally I'd try to find a relay with a lower current draw <50mA, replace the LM393 with the LM311 and drive the relay directly, but a MOSFET is a good suggestion.

How is taking the feedback from the inductive relay coil, rather than a resistor a problem? When Q2 turns on, the collector voltage instantly falls near 0V and when it turns off, the collector voltage rises to a diode drop above the power supply voltage, until the flux in the inductor is depleted. Also only one diode is required. D7 is superfluous and will never conduct.

ArthurDent:
Here is my final variation of the detector that may address some concerns and, like I said, if any of these various circuits were actually built and tested there could be minor or major changes that might come to mind. I think stenespen now has enough information so they can pick and choose what they think might be best for them and try that circuit. 

Zero999:

--- Quote from: ArthurDent on December 07, 2018, 06:45:10 pm ---Here is my final variation of the detector that may address some concerns and, like I said, if any of these various circuits were actually built and tested there could be minor or major changes that might come to mind. I think stenespen now has enough information so they can pick and choose what they think might be best for them and try that circuit.

--- End quote ---
That will not overcome the issue of the transistor clipping the comparator's output.

If you want to use the comparator's non-inverting input for positive feedback and don't want to use a MOSFET, then use a PNP BJT, but the logic will be reversed.

You seem not to believe me when I said the relay coil's inductance doesn't generate any noise. Where did you get that idea from? It's compete nonsense!

Here's a simulation showing an inductor being pulsed on and off with a transistor. Note that the waveform on the collector is a fairly decent squarewave. The overshoot beyond the supply voltage, when it turns off, is no problem.

Still don't believe me? Try building the circuit.

spec:
Below is the schematic for a precision version, with hysteresis for H99, and an  NMOSFET for AD.

The trouble is that it uses non jelly-bean components, which may not suit the OP.

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