Variable resister/fuel level sensor - fuel gauge - change resistance range

Thread Starter

nicktruman

Joined Feb 4, 2019
73
What type of PCB kit?
Likely better to just build it on a small strip-board as Alec suggested.
Hi, this type. I could do a strip board but dying to use this :)
Is there a way to make the simulated resistance variable? I don’t know, but it is possible that the resistances are reversed, so 350 empty and 50 full.
until I hook up the adapter I won’t know!
 

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Thread Starter

nicktruman

Joined Feb 4, 2019
73
Doing something like this isn't as easy as you might think. The original sender might not be linear due to the nature of the fuel tank construction.

Do this mind exercise. If the tank was like a pyramid (base down), having the float at the halfway mark wouldn't mean the tank was half full. Now consider the pyramid standing on its tip. Same situation, but in the first, you had more than half of the fuel left; in the latter, you have less than half left.

Or the sender could be linear, but there's a look up table to map position to remaining volume.
The sender is linear because I can see the code in the dashboards hex code
 

dl324

Joined Mar 30, 2015
15,471
The sender is linear because I can see the code in the dashboards hex code
Your still SOL. Your sender is 0-100 ohms and you need to map it to 50-350. You could add 50 ohms to make it 50-150, but getting from 150 to 350 in a linear way won't be straightforward.
 

Alec_t

Joined Sep 17, 2013
13,232
I could do a strip board but dying to use this
From the pic inside the kit it looks like it's for etching a copper-clad board.
Is there a way to make the simulated resistance variable?
Vary R1 and/or R2.
it is possible that the resistances are reversed, so 350 empty and 50 full
My circuit doesn't know or care what empty and full mean :).
Try the gauge with the 'gauge' terminal shorted to ground. What does it read? And what does it read with that terminal unconnected?
 

Alec_t

Joined Sep 17, 2013
13,232
An electronics hobbyist local to you could build it. But whoever builds it you won't know its effect until you actually connect the circuit to your dashboard. The internal circuitry of your gauge is unknown. Answers to my questions in post #24 would give us a clue.
 

Thread Starter

nicktruman

Joined Feb 4, 2019
73
An electronics hobbyist local to you could build it. But whoever builds it you won't know its effect until you actually connect the circuit to your dashboard. The internal circuitry of your gauge is unknown. Answers to my questions in post #24 would give us a clue.
Thank you, I've just ordered the strip board, transistor, diode, capacitor and power amp, I'm struggling with the resistors, i only need a couple not 100s :)
 

crutschow

Joined Mar 14, 2008
31,138
I am worried about connecting my circuit board to my car.
The circuit will emulate a 50-350 ohm variation of the fuel gauge so it is unlikely to cause any problems if the dashboard expects a 50-350 ohm resistance to ground, and if the circuit is wired correctly.
We would need to know more about the dashboard circuit to determine that with a higher degree of certainty.
 

Thread Starter

nicktruman

Joined Feb 4, 2019
73
Hi Everyone, I have ordered the parts to make this adapter, but the op amp they sent me was so tiny it wont / cant fit onto the board. I have found another Op amp, ua741cn in my box of bits, will that work?
secondly I cant find anywhere that will sell me the 5 resistors I need; RS was the closest but they wanted a £12 handling charge.
Any ideas?
kind regards
Nick
 

Thread Starter

nicktruman

Joined Feb 4, 2019
73
Be sure and order the op amp in a DIP through-hole package.
CPC do not do a DIP op amp, and the 49.9k resistor is proving allusive! Why cant things be simple..
I just connected my laptop to the car and read the diagnostics from the dash board, it can read the resistance of the fuel sender, 0 - 100ohms, its such a shame i can set that as the range for the gauge
 

Alec_t

Joined Sep 17, 2013
13,232
the 49.9k resistor is proving elusive!
I used 47k in my simulation, a standard value. (49.9k was Crutschow's tweak to get closer matching to the theoretical emulated resistance value, and is within the common 5% tolerance range of the standard value). Almost any opamp with a common-mode input range down to ground will suffice (e.g. LM358, LM324, LT1006). NOT a '741 !
 
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