Hi all I have a problem I need to fix.
I have a temperature gauge that works on 5v using a thermistor that grounds the 5v on a sliding scale of 0-190 ohms
The thermistor however reads/uses 0-2900 ohms to ground and I can’t swop the thermistor for another one.
I’m assuming it works as a voltage regulator of between 0 and 5v (empty and full)
So I need to be able to send 5 v out to thermistor and read how much grounds I think? Then use that information to ground the actual digital signal from the gauge the right amount so the gauge reads correctly?
I hope that makes sense
Richard
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That is a bald assumption.
The easiest solution is make a transistor act as a variable resistor, by driving it with a voltage divider consisting of the other thermistor, and a few more resistors and trim-pots, but it may not be sufficient.
I have dealt with fuel-gauges, and matched those to different senders, and i ended up using Op-amps to convert the the input voltage to a suitable output voltage to drive the guage. Main thing to sort out first is how the gauge 'actually' behaves. Can you control it with an NPN instead of the thermistor, or can you manage with a PNP instead ?
Hi thanks for getting back to me.
I’m not sure on the couple of things you just mentioned as I don’t know what they are(at least in initials)
I can make you gauge move up and down with a potentiometer if that helps?
In that case i don't think i will be able to help you.
I mean what’s an NPM and the PNP
I’ve just looked those up and understand now!
Google NPN transistor & PNP transistor. A project like this requires basic electronics education. Knowledge of transistors and Op-Amps in particular. Without that i don't see much point in trying to coach you into a solution.
OK do you know what an Op-Amp is and how they can be used ?
I know my electronic knowledge is poor!
I’ll get there in the end
Thanks anyway
I sor5 of understand I have a circuit diagram that should in theory do the I’m wanting to use a nano or similar to cut out so much wiring.
Anyway I want to do that circuit using a nano and software if it’s even possible which for someone who isn’t as stupid as me🥲
So you want to use a digital signal to reduce the wiring of an analog circuit, controlling an analog signal. Good luck with that. It's not a bad circuit that one, Mine has about the same level of complexity. Less capacitors and zener diodes, but using 3 Op-Amps.
I am not saying your stupid, but just ambitious, and not going about it in the right direction.
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The schematic has an anomaly though. The (-) on the LM358 is connected to GND, from the OP-Amp Symbol, Now the LM358 doesn't have a (-) pin, but just a GND pin, so even though, if you connect it like that it will work as intended, but the symbol used is incorrect. (-) on an Op-Amp symbol refers to a negative Voltage.
Anyway, you have a schematic, I would just order the parts and built it. It is not all that much wiring and using an Arduino is not going to simplify things.
No I know what you mean. I know my limitations!
But I am keen to learn and I do get there in the end on these projects!
I’m with you I have it built. My soldering isn’t brilliant either. I’m trying to get it working on the arduino simply because I will be building this circuit at least 3 times a week and if I can do it with software then that would hopefully be quicker! Maybe that’s not the case. This is me starting research into the future of making things easier.
Yes that was my thought. Obviously it doesn’t work like that! No matter back to the drawing board. I think I’ll make a pcb instead. Then it will just be adding the components each time. I can manage that quickly enough. I have these odd ideas from time to time. Thanks for your help anyway it’s appreciated
I don't know if you are stupid or not, I suspect not. Presumably lacking knowledge and experience 
I've got about 48 years of electronic design experience and the more I've learnt makes me realise how much more I have to learn 
Yes lacking both unfortunately!
But I’ve been thinking overnight.
The thermistor has two wires that work like a simple lightbulb circuit as in can be wired either way.
I’m thinking of sending the 5v down one wire and reading what comes back on the other.
With that information I should be able to get some code to work out how many vi need to send to the gauge? I should also be able to make a warning light come on when it’s getting too hot. Well I think so anyway.
I’ve not given up yet
I would take a 1K resistor and connect it to the thermistor, connect the resistor end to VCC and the thermistor end to GND and connect the point in between them to an analog input pin. That way you can use analogRead() to get a reading. That said, for the output side of things, you will still need a fair bit of circuitry. An Arduino does not really have an analog output, just PWM which would need to be filtered to drive a transistor to replace the thermistor that belongs to the gauge.
So this is why i keep saying. The path you want to take is not easier, and will not have less circuitry.