Digital potentiometer or similar???

Using Arduino Sensors
#1

I use my arduino mega 1280 to run my bus simulator's dashboard, easy enough for the warning lights, and the tachograph is fairly easy needing just a pulsed signal (i use the tone output command)

But the 3 gauges are another story,

These are standard automotive air core style gauges (fuel, water temp and oil pressure) they have 3 terminals, power, earth and sender,

Power is easy... 24 volts DC, earth... well earth or 0 volts from the psu,

The sender terminal expects to be connected to a variable resistance to earth, 10 - 180 ohms.

I've tried before with other gauges using a PWM signal from the arduino (via a transistor, and without powering the gauges with 5 volts) but i could never get the full scale movement, and each gauge acted differently despite being the same model type.

So i was wondering about a way to feed these gauges a real-ish variable resistance,
i've heard of digital potentiometers, but was wondering if there was anything else i could try,
The gauges do not need to be that accurate, as long as they sort of track the on screen virtual gauges, so having steps between resistances is ok if they are small enough,

i had thought about driving a potentiometer with a servo, but do not want the noise servo's make if it can be helped.

#2

you could use an R-2R ladder dac

its just a handful of resistors to change a series of bits into an analog value

http://www.scienceprog.com/avr-dds-signal-generator-v10/

#3

Not sure how many steps i'd need to make the pointer move smoothly, i'd guess 255 steps would be more than enough, but that's a lot of resistors?

Other thing is i need to keep the 24 volts away from the arduino, i'm wondering about something like an opto coupler type thing....

I am wondering if a photo sensor or LDR can be had that can do 10 to 180 ohms, and handle 24 volts at 80 Ma that the gauge seems to pull,
put the LDR or similar in a dark tube one end, other end an led, control the led's brightness via PWM from the arduino, and get analog variable resistance from the LDR,

But i imagine it wouldent be as simple as that.

#4