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.
Relay PCB:
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 |
| 2 | ULN2803 | IC2, IC4 |
| 16 | TQ2-24V | K1, K2, K3, K4, K5, K6, K7, K8, K9, K10, K11, K12, K13, K14, K15, K16 |
PCB high resistance:
| 3 | 10K | R1, R2, R3 |
| 1 | 18K | R5 |
| 1 | 22K | R4 |
| 1 | 33K | R7 |
| 1 | 47K | R6 |
| 3 | 100K | R8, R9, R10 |
| 1 | 180K | R12 |
| 1 | 220K | R11 |
| 1 | 330K | R14 |
| 1 | 470K | R13 |
| 3 | 1M | R15, R16, R17 |
| 1 | 1M8 | R19 |
| 1 | 2M2 | R18 |
| 1 | 3M3 | R21 |
| 1 | 4M7 | R20 |
| 2 | 100nF | C1, C2 |
| 2 | 74HCT595 | IC1, IC3 |
| 2 | ULN2803 | IC2, IC4 |
| 12 | TQ2-24V | K1, K2, K3, K4, K5, K6, K7, K8, K9, K10, K11, K12 |
Control PCB:
Part list:
| 2 | 1uF | C3, C4 |
| 1 | 10K | R2 |
| 1 | 10K | R1 |
| 1 | 16F648A (Programmed) | IC1 |
| 2 | 100nF | C1, C2 |
| 1 | Push button | S1 |
| 1 | Rotary encoder | S1 |
| 1 | REG1117-5V | IC2 |
With frontpanel:
Power supply
This part is made on experiment board, therefore there are no circuit drawings available. The pcb exists of a 18V transformer; a rectifier and a capacitor.
Everything mounted in the enclosure:
Ready:
Code in the microcontroller:
Device = 16F648A Config FOSC_INTOSCIO, WDTE_OFF, PWRTE_ON, MCLRE_OFF, BOREN_OFF, LVP_OFF, CPD_OFF, CP_OFF All_Digital = true Xtal = 4 Declare PortB_Pullups On Declare LCD_DTPin PORTB.4 Declare LCD_ENPin PORTB.2 Declare LCD_RSPin PORTB.3 Declare LCD_Interface 4 Declare LCD_Lines 2 Symbol RotEncoderA = PORTB.1 Symbol RotEncoderB = PORTB.0 Symbol RotSwitch = PORTA.2 Symbol PushSwitch = PORTA.4 Dim PushSwitchBit As Bit TRISB = %00000011 TRISA = %01111100 OPTION_REG = %01000000 INTCON = %10010000 Symbol INTF = INTCON.1 Symbol klok = PORTA.0 Symbol Data_Pin = PORTA.7 Symbol ZetVast = PORTA.1 Dim DispWaarde[7] As Byte Dim SetWaarde[7] As Byte Dim DispTeller As Byte Dim CursorTeller As Byte Dim UitHex As Byte Dim UitDig As Byte Dim UitCount As Byte Dim ArrayTeller As Byte Dim Warning As Bit 'dim teller as byte On_Hardware_Interrupt GoTo IrInterrupt GoTo Init '**************************************************************** IrInterrupt: Context Save If INTF = 1 Then 'Flaggen resetten INTF = 0 If RotEncoderA = 1 Then 'Aftellen If DispWaarde[6 - DispTeller] > 0 Then Dec DispWaarde[6 - DispTeller] EndIf Else 'Optellen If DispWaarde[6 - DispTeller] < 9 Then Inc DispWaarde[6 - DispTeller] EndIf EndIf EndIf Context Restore '**************************************************************** Init: Cls UitDig = DispWaarde[3] UitHex = UitDig << 4 UitDig = DispWaarde[2] UitHex = UitHex + UitDig UitHex = ~UitHex SHOut Data_Pin, klok, MsbFirst, [UitHex \ 8] UitDig = DispWaarde[1] UitHex = UitDig << 4 UitDig = DispWaarde[0] UitHex = UitHex + UitDig UitHex = ~UitHex SHOut Data_Pin, klok, MsbFirst, [UitHex \ 8] UitHex = 0 UitDig = DispWaarde[6] UitHex = UitHex + UitDig UitHex = ~UitHex SHOut Data_Pin, klok, MsbFirst, [UitHex \ 8] UitDig = DispWaarde[5] UitHex = UitDig << 4 UitDig = DispWaarde[4] UitHex = UitHex + UitDig UitHex = ~UitHex SHOut Data_Pin, klok, MsbFirst, [UitHex \ 8] High ZetVast DelayUS 20 Low ZetVast '12345678 Print At 1, 1, "E-Stynus" Print At 2, 1, " 2014 " DelayMS 750 Cls Print At 1,1, "Set:" DispTeller = 0 'Ohm teken in lcd steken Print $FE,$40,$00,$0E,$11,$11,$11,$0A,$1B,$00 '**************************************************************** Main: While 1 = 1 'Relais aansturen bij rotary encoder button druk If RotSwitch = 0 Then For ArrayTeller = 0 To 6 SetWaarde[ArrayTeller] = DispWaarde[ArrayTeller] If SetWaarde[ArrayTeller] > 9 Then SetWaarde[ArrayTeller] = 0 DispWaarde[ArrayTeller] = 0 EndIf Next UitDig = DispWaarde[3] UitHex = UitDig << 4 UitDig = DispWaarde[2] UitHex = UitHex + UitDig UitHex = ~UitHex SHOut Data_Pin, klok, MsbFirst, [UitHex \ 8] UitDig = DispWaarde[1] UitHex = UitDig << 4 UitDig = DispWaarde[0] UitHex = UitHex + UitDig UitHex = ~UitHex SHOut Data_Pin, klok, MsbFirst, [UitHex \ 8] UitHex = 0 UitDig = DispWaarde[6] UitHex = UitHex + UitDig UitHex = ~UitHex SHOut Data_Pin, klok, MsbFirst, [UitHex \ 8] UitDig = DispWaarde[5] UitHex = UitDig << 4 UitDig = DispWaarde[4] UitHex = UitHex + UitDig UitHex = ~UitHex SHOut Data_Pin, klok, MsbFirst, [UitHex \ 8] High ZetVast DelayUS 20 Low ZetVast DelayMS 100 While RotSwitch = 0 DelayMS 100 Wend EndIf 'Warning teken Warning = 0 For ArrayTeller = 0 To 6 If DispWaarde[ArrayTeller] <> SetWaarde[ArrayTeller] Then Warning = 1 EndIf Next If Warning = 1 Then Print At 1, 8, "!" Else Print At 1, 8, " " EndIf 'Decade positie If PushSwitch = 0 And PushSwitchBit = 0 Then PushSwitchBit = 1 DelayMS 100 If DispTeller < 6 Then Inc DispTeller Else DispTeller = 0 EndIf EndIf If PushSwitch = 1 Then PushSwitchBit = 0 EndIf For ArrayTeller = 0 To 6 If DispWaarde[ArrayTeller] > 9 Then DispWaarde[ArrayTeller] = 0 EndIf Print At 2, 7 - ArrayTeller, Dec1 DispWaarde[ArrayTeller] Next Print At 2,8, 0 '(Print At 1,4, Dec DispTeller Print $FE, $0E 'Underline cursor on Print $FE, $C0 'Move cursor to beginning of second line CursorTeller = 0 While CursorTeller < DispTeller Print $FE, $14 'Move cursor right one position Inc CursorTeller Wend DelayMS 100 Wend End
Hex file: link
Files:
- Code file: Download
- Hex file: Download
- Front PCB: Silkscreen, top, bottom
- Relay PCB: Silkscreen, bottom
Dutch 
