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You can use the gpi bits to do A->D and D->A, you could use a USB sound device as analog input (but you are constrained in sample rate choices), in some cases if you strip the input capacitors you'll have level reading capability.

Or you could do something like this:


The cheapest way to get A/D is to add a bunch of resistors via a summing network to one input of an op-amp and the other input to the signal you want to measure. The output of the op-amp goes into one of the digital inputs. Binary search across the output bit combinations to find the point where the input bit swings. This you can do for pennies.

Lots of options to add analog without having to add an arduino.

Sounds complicated. Why not use one of the hundreds of I2C and SPI ADC options available on digikey?

A simple PCB in the picrust form factor could implement a high quality multichannel ADC IC, with no microcontroller programming required.

And as a bonus, you'd be able to design it with sane connectorization for your actual application. (eg, appropriate scaling resistor your sensor, tidy two-wire plug or screw terminal, etc)

I'm very pleased that the RPi has stayed focused on not being everything to everyone, and has left these specialized applications to be implemented as peripherals.

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