hassmaschine wrote:I talked to an EE friend, he says the senders the new gauge uses are basically rheostats, which is why I get the behavior I'm seeing.
Mee_n_Mac wrote:hassmaschine wrote:I talked to an EE friend, he says the senders the new gauge uses are basically rheostats, which is why I get the behavior I'm seeing.
A rheostat is just a variable resistor. You use one to make a varying voltage by using it in a voltage divider. In your case it's a variable R2 in the diagram below. R1 would be the internal resistor I'm referring to. And Vout (below) would be the pin on the gauge that's connected to all 3; the internal resistor, the sender (rheostat/R2) and the A/D input.
So when R2 is a large value, say a really really really large value, like an open circuit, you get the Vref (Vin on pic above) you've measured at the gauge pin. With any lower resistance you get a voltage less than Vref per the equations in the wiki. When you measure a voltage that's equal to 1/2 of the open circuit voltage, that resistance value (of R2) is equal to the internal resistance (R1) value.
lyndon wrote:Slightly OT, but do you have a mfr/part number for either of those sensors? I have a project where I need to add a gauge to a gas tank and I'm still at the build vs. buy decision point.
hassmaschine wrote:they're BMW parts so you probably don't want to spend coin on them. I think the ones in my car are around $100 each? they last forever which is a bonus but I wouldn't want to buy new ones if I didn't have to.
Mee_n_Mac wrote:That's the data needed ! So let me be sure I understand. You have the old/stock gauge and sender and came up with the (12V) readings above ? Then you used the new gauge and new sender (?) to come up with the (5V) readings above ? When you say "In order to keep other systems on my car happy, I need the 1st curve to stay the same, and convert it before it goes into the new gauge", what exactly do you mean ? Does the old gas tank voltages have to go to some other gauge in addition to the new gauge (that runs off 5V) ? Or do you only use the "converted" voltages ? I had a simple 1 transistor,3 or so resistor circuit to do the old conversion, I'll see if that can easily be adapted to the "3 full and 75 empty" curve but it would simply output the new voltages. There would be no output that's the same as the stock gauge&sender to send to any other instrument. If needed that can be done; to send out 2 voltages, the "5V curve" to new gauge (to 2 places to simulate 2 senders) and the old "12 V curve" to XYZ instrument.
ps - Are you now sure the old/stock sender (when wired in series) goes 3 - 75 ohms ? At one point you had to verify that. And that each unit in the new sender (that you're not going to use) would go 200 - 700 ohms ?
I take the project is to retrofit a new instrument cluster in an old Bimmer ?
I think you're focusing on the potential difference with 5v too much. You should care about what the potential difference with ground is, which is equal to the pontential. You're trying to make 5v the ground, which it's not. Ground is the ground...
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