# Converting GND switch states to two resistance values to GND

#### marcus905

Joined Aug 20, 2018
2
Hello everyone,
I'm trying to connect a universal gauge to a motorcycle. This gauge includes a fuel indicator that basically shoes a bar display measuring a resistance to ground, with 10ohms for a full tank to 90ohms for an empty tank.
The fuel tank itself though has a much simpler design: if the fuel is low it will short a signal to ground, if it's not low the signal is left floating.

Basically I'm trying to devise a simple circuit that will convert a floating input to a 10ohms resistance to ground and a grounded input to a 90ohms resistance to ground.

I have no clue how to do this, if you have an idea or can point me in the right direction I'd be very grateful.

Thank you very much!

#### crutschow

Joined Mar 14, 2008
25,269
Here's a simple circuit using one transistor and three resistors to do what you want.

When the switch is open, the transistor is on, connecting R2 to ground, and the resistance value seen by the gauge to ground is R2 and R3 in parallel, which is 10.05 ohms.

When the switch is closed, the transistor is off, so the gauge sees only R3 or 90.9 ohms.

Sound okay?

#### marcus905

Joined Aug 20, 2018
2
Hello!

Yes, it's perfect! I will replace the R2 resistor with a series of a 10 and a 1.2 ohm and the R3 resistor with a series of a 68 and a 22 ohm to achieve roughly the same effect.

Thank you very much both for the circuit and for a new found interest in electronics.

I will try to implement the circuit and I'll let you know!

Thanks again!

#### crutschow

Joined Mar 14, 2008
25,269
I will try to implement the circuit and I'll let you know!
Yes, please let us know how it works.
I had to guess at the base resistor value, since I didn't know how much current the gauge takes.

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