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    Trigger 12v relay with 3v source

    Easiest way I can think of would be put an opto input straight across the existing LED(4N25 or similar), drive a relay with the open collector output. https://www.vishay.com/docs/83725/4n25.pdf Diagram above showing a 1k5 base resistor would be way too high a value, more like maybe 220R.
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    Help to modify push button (reed switch) power circuit

    We used very similar arrangement on RC aircraft. Operation was, magnet in .....(in a brass tube with a red cloth marker)....switch off. That way one couldn't loose the magnet in flight and have it switching off. Below is the circuit we used and the pcb layout. Note:- in the schematic below...
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    Fuel level switch

    Most fuel guages are a current meter driven via a variable resistance ( fuel tank sensor) via a fixed voltage regulator.
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    1000VA UPS

    No idea how you arrived at the idea that the fuses are too large. I don't see any reference to size on the schematic. Being a 1000VA unit, one would expect to see fuses of this size on the DC input anyhow. I'd have to say the burnt pins would be your first problem. The rest of comment #2 is...
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    Laser retro reflector for non-contact odometers.

    Requirement was 20cm , or 200mm :)
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    Laser retro reflector for non-contact odometers.

    ...........really..????? I think not.
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    Replacing solar module with DC output

    If it's supplied with 1.2v now.....3v will cook it. No idea what the waffle about 3v alternating is all about. Modes are set in the unit , on at dusk is a simple ldr switcher , or may simply monitor solar panel voltage and switch from there, so it depends how you connect to the new supply. Draw...
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    Laser retro reflector for non-contact odometers.

    Modulated IR module would do the same job. Visibility, use your phone camera. Or phototransistor as used in RC tachos if you have a daylight source.
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    Where to add a potentiometer to vary input voltage?

    Oh snowflake, how would one who has no idea come to that conclusion? You'll search my history but won't get off your butt and do some searching on the tips already given. Good luck with your future. To allay any doubts, I do know what I'm talking about, thankyou. You've been told the answer...
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    Where to add a potentiometer to vary input voltage?

    So, redesign the comparitors to suit your needs. You have 2 voltage dividers, change whatever to achieve what you want....simple ohms law.
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    Where to add a potentiometer to vary input voltage?

    Your words, not mine. If your need is to change the complete circuit to suit different thresholds, you should simply say so, rather than offer your version of a fix in your query.
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    Nixie Tube Driver Question (w/ Schematic)

    Overkill .......... https://gra-afch.com/how-it-works/how-to-drive-a-nixie-tubes/ Or if you must....... https://hackaday.com/2023/05/02/modern-components-enable-cheap-and-compact-nixie-driver-circuit/
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    LED Photoelectric switch

    New one to me....never had a problem running a 20mA LED on 5mA ever before.
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    Where to add a potentiometer to vary input voltage?

    That might be as it is now but the resistance is really unimportant....the voltage from the divider is.
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    help with a 12v control circuit

    Perhaps like in the previous suggestions above, you think..............? :rolleyes:
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    Where to add a potentiometer to vary input voltage?

    10k across 5v supply, centre to same input as the v_divider now.(remove existing) You will need to establish the mechanical constraints to give you the limit to the range you require.
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    Part identification help?

    Bin it. When the memory battery such as this goes south as dramatically as this did, one can be certain it would have taken the flash memory with it and possibly even the micro itself.
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    Why Atex gland is different ?

    Someone left the gate open again.......... :)
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    Stuck! I changed the belt on the dymo disc painter and put it back together, it turns on but won’t move

    Warning to anyone trying to watch this video.....can be dangerous if you are subject to massive movements without focus.. When you post these things, do you people ever play it back before hand just to see how ridiculous the playback is....shows bugger all. The drive with the "threaded rod" is a...
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    LED Photoelectric switch

    One does not need a lot of light at night ........even a single LED running @16mA as shown above would more than likely be too much. It would certainly drain the 9v battery (assume 216 type) in no time flat. Better to run off small plug pack. Start out running the LED around 5mA and see......not...
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