This page lists some examples of analog front end designs that you might find useful. As much as possible the designs will adhere to the Scoppy ethos of keeping the price as low as possible.
Please note that these designs are definitely NOT appropriate for measuring high voltages. If you choose to use any of these designs you are doing so at your own risk. The author cannot be held responsible for any damage caused by the use of these designs.
For each design an indicative cost of the components is given. This doesn't include the cost of the Pico, Android device, prototyping supplies or shipping costs. Prices are based on the prevailing prices at Digikey and Mouser
Design 1
Single-channel, single voltage-range (sensitivity) of more than 3.3V
Cost: less than $2
Design 2
Single-channel, single voltage-range (sensitivity) of less than 3.3V
Cost: less than $2
Design 3
Single-channel, single voltage range with over-voltage and under-voltage protection
Cost: less than $2
Design 4
Single-channel, multiple voltage ranges and over-voltage protection
Cost: less than $4