Either one of these sensor types can be used if you choose to flash the code from my github repository (with sensor output connected to ADC0/GP26, Pin 31):
- capacitive type sensor HW-390
- resistive type sensor ME-110
What you see in the photo are the first prototypes with custom made PCB (Version 1.0):
The algorithm is capable to detect the connected sensor type correctly during an initialisation procedure.
You'll find more descriptions and explanations in the code comments.
Further HW/SW improvements might or should include:
- "hiding" the Pico under the PCB (new design) and getting rid of all the green/white wiring (that just maintains some flexibility with the PCB prototype and is used for fault-injection test, but is otherwise obsolete)
- reducing the overall geometrical size of the device
- getting rid of the CD4511BE ICs, the Pico drives the segments directly (the advantage with the ICs is however that you use just 8 Pico ports instead of 14 and the Pico GPIOs are not "stressed" with high current for the segments; with VBUS you can get easily 30mA for one individual segment (brightness), while a GPIO can provide max. 14mA)
- getting rid of 5 resistors and 1 CD4511BE IC by implementing a multiplexing logic with 2 transistors (however, this measure reduces costs not significantly if you use only two displays)
- running several sensors simultaneously on one single PCB (with switching every few seconds between the sensor values on the 7-Segment-Display)
- alternative power supply via e.g. solar cells
- wireless sensor (might abandon this idea completely in order to keep costs low)
- downsizing to a cheaper microcontroller
Further ideas appreciated.