It sounds like what you are doing on the arduino is
analogRead(), converting the range from 0-1024 to 0-255 then using
What you are doing is converting a analog signal to a PWM signal. The
analogWrite() function is badly named in the arduino sdk. It does not write an analog signal to a pin. Instead it sends a series of pulses, with varying width between them where 0 is fully off and 1 is full on. In some devices (such as leds) this mimics the effect of changing the voltage. Other devices such as servos and motor controllers actually expect a PWM signal to function correctly. There is no way without addition hardware for an arduino UNO to generate a true analog signal.
Now, in electronics analog to digital converters and digital to analog converters convert a signal from a min to a max voltage in so many steps, 1024 in the case of the arduino ADC. Even if analogWrite produced a true analog signal, its range is 0-255, but the min and max voltage would still likely be the same - just with less resolution so all you are doing is lowering the resolution of the signal rather then reducing its voltage.
Now, the pi has no ADC built into it so you need some sort of external ACD to read an analog signal, there are many such chips about and you can skip the arduino UNO entirely by using one, without having to worry about coding it at all - you can just worry about coding on the pi.
Assuming you still want to use the arduino as a ADC you are going to need to talk to the pi over a protocol it understands. There are many of them to pick from: serial (UART), i2c or spi.
WARNING: the arduino UNO is a 5v device dy default, its pins will read/write signale at 5v. The raspberry pi can only accept 3.3v MAX on a pin. Sending a 5v signal to it will damage the pi. You can use a logic level converter to shift from 5v to 3.3v. Alternatively, you can run the arduino off 3.3v (by supplying 3.3v to the pin label
5v (and bypassing the 5v reg onboard) or by replacing the voltage reg with a 3.3v reg, at which point the pins will read/write 3.3v max.
Serial is probably the easiest to get started with. You will want to change your sketch basically;
value = andlogRead() then
Serial.println(value) and finally a small
delay(500) to stop it overwhelming the serial line.
From the pi you can read these values with any serial reading application (picocom, screen, minicom) on the
/dev/ttyAMA0 device file:
picocom -b 9600 /dev/ttyAMA0
Ensuring the baud rate matches the rate set in your sketch (9600 is a good value to start with). Use
ctrl-x to exit. This is useful for debugging the output from the arduino.
You can then use pythons pyserial module to open the serial port read a line and do what ever you want with the value.