I have some old sensors in my car that drive aftermarket gauges for oil pressure and temperature, which I want to read using an Arduino instead.
The sensors are a simple variable resistor but they're very low impedence, with a range from 18-180 ohms. With a voltage divider, is this safe to measure using an Arduino's analog pin? Most voltage divider examples I've found use resistors in the kilo-ohm range to prevent overloading the analog pins, so I wanted to check if there's a safe way to implement this?
You used the word "instead". OK, so no connection to any other part of the car's wiring.
In one way that is reasonable because you just supply them from your 5 V supply (derived from the 12 V car battery using a "buck" converter) via a 120 Ohm resistor. The resistor will dissipate no more than a quarter watt. You connect the common point of resistor and sensor to the Arduino via a 4k7 resistor to provide some protective isolation.
Alternatively, you could just as well leave them connected to the aftermarket gauges and monitor the voltage via a voltage divider of 22k in series and 10k to ground, to the analog inputs. You need then to compensate for the car battery voltage which you could do by another voltage divider to an analog input, monitoring the battery voltage at the gauges.
Thanks for confirming, and yes, "instead" means that there isn't a 12V connection to the sensor, although I need to connect the arduino ground to the car to complete the circuit.
If I use a 120ohm resistor and the arduino's 5V, then I'll get a reading of nearly 4.4v at the sensor's maximum value (18 ohms - resistance drops as the temp or pressure goes up). Is it safe to sink that much through 2 or 3 Analog pins, or would it be better to drop the fixed resistor to 12 Ohms to get smaller voltages on the inputs?
You could drive them with a constant current source and measure the voltage at the sensor with an analog input .
Use the internal reference and the current will be much lower than that normally passed by the sensor .