Next on the list was wiring up and soldering the temperature sensors (thermistors) to the heated bed PCB. Since the heated bed covers a fairly large area, I’ve used a combination of 4 to provide a mean value for the temperature across the bed. By wiring up two thermistors pairs in series and then in parallel as shown in the drawing and pictures, the overall resistance, and in effect temperature, provides a form of “averaged” value. If all thermistors register the exact same temperature and have the same resistance, the result  is the same at the output of the  whole circuit.

Layout diagram for the four thermistors wired in series and parallel. The thermistors’ total output resistance results in an averaged temperature reading.

Top view of all four thermistors wired.

Central thermistor. Heat-resistive Kapton tape holds down the thermistors on the heated bed.

Corner thermistor.

I put the thermistors roughly in place first using normal tape, in order to measure wire lengths and solder with ease.  The soldered joints should be fine as the heated bed has to reach around 110°C for ABS plastic and 60°C for PLA plastic, whereas solder melts at around 180°C. The thermistors were finally to the heated bed with heat-resistive Kapton tape.

Next, I should have the remaining parts needed to continue building the frame!