I have a motorcycle with a gas gauge. The sending unit for that gauge reads 10 ohms when the tank is full and 215 ohms when the tank is empty. OK so far.
I want to add a second gauge (2 total gauges). The second gauge requires a sending unit that reads 10 ohms when full but only 70 ohms when empty.
So, if I simply parallel the two gauges, the additional one will read empty long before the tank gets empty (70 ohms vs 215 ohms). Also, I suspect that paralleling two gauges will make the original gauge read wrong, too. That really complicates the problem.
I can't install an additional sending unit in a motorcycle tank. No way to do that.
I measured with the second gauge since it's on my workbench. Here's what I got. I connected the 12 volt pin to a 12 volt gel cell battery that measured 12.8 volts. The sensor pin was connected to a 10turn 100 ohm pot. The other end of the pot was connected to the battery's negative terminal. I then measured the voltage across the resistor (with a DVM).
Voltage across the resistor Voltage across the gauge
10 ohms  1.06 v = 106 ma 11.74 v
20 ohms  1.82v = 91 ma 10.98 v
30 ohms  2.36 v = 78.6 ma 10.44 v
40 ohms  2.80 v = 70 ma 10.0 v
50 ohms  3.12 v = 62 9.68 v
60 ohms  3.41 v = 56.8 ma 9.39 v
70 ohms  3.63 v = 51.7 ma 9.17 v
You can see it's nonlinear, but not too far off.
I haven't measured the motorcycle's gauge. I'm going to try that ASAP. Then Ill know the ratio between the two gauges.
Attached is an opamp circuit that I think will work. The opamp will input the voltage across Gas Gauge 1 and amplify it to the correct level for Gas gauge 2. Right now I don't know exactly what voltage will be across Gas Gauge 1 since I havent measured it yet, but I do know the voltage required for Gas Gauge 2 is approximately 9.2 to 11.74 volts based on V1 being 12.8 volts. Of course V1 will be higher when the motorcycle is running, but I think the ratio will be close enough. After all, its just a gas gauge. Anybody ever seen an accurate gas gauge?
I doubt that the two gauges need the same voltage so that would mean I need a nonlinear differential amplifier(?) since both gauges need 10 ohms to read full, but they need totally different resistances to read empty (215 vs 70 ohms).
Also, the opamp will need to be able to source 100 ma or so.
Anyone know how to do that?
Jack
I want to add a second gauge (2 total gauges). The second gauge requires a sending unit that reads 10 ohms when full but only 70 ohms when empty.
So, if I simply parallel the two gauges, the additional one will read empty long before the tank gets empty (70 ohms vs 215 ohms). Also, I suspect that paralleling two gauges will make the original gauge read wrong, too. That really complicates the problem.
I can't install an additional sending unit in a motorcycle tank. No way to do that.
I measured with the second gauge since it's on my workbench. Here's what I got. I connected the 12 volt pin to a 12 volt gel cell battery that measured 12.8 volts. The sensor pin was connected to a 10turn 100 ohm pot. The other end of the pot was connected to the battery's negative terminal. I then measured the voltage across the resistor (with a DVM).
Voltage across the resistor Voltage across the gauge
10 ohms  1.06 v = 106 ma 11.74 v
20 ohms  1.82v = 91 ma 10.98 v
30 ohms  2.36 v = 78.6 ma 10.44 v
40 ohms  2.80 v = 70 ma 10.0 v
50 ohms  3.12 v = 62 9.68 v
60 ohms  3.41 v = 56.8 ma 9.39 v
70 ohms  3.63 v = 51.7 ma 9.17 v
You can see it's nonlinear, but not too far off.
I haven't measured the motorcycle's gauge. I'm going to try that ASAP. Then Ill know the ratio between the two gauges.
Attached is an opamp circuit that I think will work. The opamp will input the voltage across Gas Gauge 1 and amplify it to the correct level for Gas gauge 2. Right now I don't know exactly what voltage will be across Gas Gauge 1 since I havent measured it yet, but I do know the voltage required for Gas Gauge 2 is approximately 9.2 to 11.74 volts based on V1 being 12.8 volts. Of course V1 will be higher when the motorcycle is running, but I think the ratio will be close enough. After all, its just a gas gauge. Anybody ever seen an accurate gas gauge?
I doubt that the two gauges need the same voltage so that would mean I need a nonlinear differential amplifier(?) since both gauges need 10 ohms to read full, but they need totally different resistances to read empty (215 vs 70 ohms).
Also, the opamp will need to be able to source 100 ma or so.
Anyone know how to do that?
Jack
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