PIC Microcontroller based resistor decade box

I was thinking about making a resistor decade box, but I found the rotary switches that are used in these most of the time rather expensive and a bit old fashioned. That’s why I decided to use a rotary encoder and select the resistors with a microcontroller.

PIC Microcontroller based resistor decade box

Now I only had to find a way to “make” the resistance. Switching resistors with transistors or mosfets gives a high switch resistance and can’t handle AC voltages. That’s why I opted to use relays. The currents that need to be switched are rather small, therefore a small relay is sufficient. The relay board is used 2 times with different resistor values.

Part list:
PCB low resistance:

3 1K R22, R23, R24
1 1K8 R26
3 1R R1, R2, R3
1 1R8 R5
1 2K2 R25
1 2R2 R4
1 3K3 R28
1 3R3 R7
1 4K7 R27
1 4R7 R6
3 10R R8, R9, R10
1 18R R12
1 22R R11
1 33R R14
1 47R R13
3 100R R15, R16, R17
1 180R R19
1 220R R18
1 330R R21
1 470R R20
2 100nF C1, C2
2 74HCT595 IC1, IC3
16 TQ2-24V K1, K2, K3, K4, K5, K6, K7, K8, K9,
K10, K11, K12, K13, K14, K15, K16


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