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.
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 |
For more detail: PIC Microcontroller based resistor decade box
