Keep in mind that the MD-2 program is easily changeable, and can be adapted to many different purposes. In my organ I have two swell shoes, which will use two MD-2 boards. At this point I am planning to use one for each keyboard, but I may at some point decide to use one of these as a crescendo pedal.

In the original Schober organs, the swell shoe controlled a potentiometer ("pot") through a rack-and-pinion gear arrangement. In early organs this pot directly controlled the volume, but after some time this developed a problem --- the pot would become noisy and introduce crackling into the audio. Schober thus redesigned the circuit: the pot would control the intensity of a light bulb, the bulb would shine on a cadmium sulfide cell, the cell's resistance would change with the light intensity, and this would be used to control the volume. The advantage of this system is that the thermal inertia of the lamp causes just enough delay to smooth out any irregularities.
 

I've replaced the potentiometer with a digital rotary encoder, shown in the photo.  The encoder is a three-terminal device which acts like a pair of switches that alternately open and close as the encoder shaft is turned. As you turn the shaft, the switches operate like this:

            Switch B      Switch A
                open            open
                open            close
                close            close
                close            open
                open            open
                        etc. etc.
In other words, both switches start open. After a tiny turn, switch A closes. A slight turn further and B closes. Another slight turn and A opens, and then a bit later B also opens. This repeats, over and over, a total of 24 times over one 360-degree revolution. (For digital freaks, this is called a 2-bit Gray code.) If the encoder shaft turns in the opposite direction, the sequence is reversed.

The advantage of this scheme is that if the switches age and become intermittently noisy, the digital effect is minimal. The encoders I chose cost about $2 each and are guaranteed for 10,000 revolutions (they are part number 318-ENC160-24P from Mouser Electronics, a Taiwan Alpha Rotary Encoder model RE160-40E3-20A-24P). If that isn't good enough, I can always replace them with $70 encoders which use optics instead of physical switches and which are guaranteed for something like 10 million revolutions. (There are several other mechanical encoders in the Mouser catalog as well.)

But there is also a disadvantage. Since the two-bit code repeats over and over, the system can recognize tiny swell shoe movement, but it can't be sure exactly how far the swell shoe pedal is depressed. My software will require a learning sequence - when you first turn the organ on, regardless of where the swell shoe is actually positioned, the MD-2 board will assume that it is at minimum volume. The organist will have to do one complete top-to-bottom-to-top movement so that the MD-2 software can learn what the maximum and minimum settings are. After that, it will figure out the current position by keeping track of the history of up and down motions.

Swell shoe sensor position is monitored by a 68HC11 microprocessor; its circuitry is very similar to the MD-1 board, and is explained later. The processor keeps track of swell shoe position and does three things each time it senses that the swell shoe has been moved:

1. It outputs a MIDI controller message via its 5-pin MIDI OUT connector. The channel number is set by a four-pole DIP switch, similar to that on the MD-1 board.

2. It outputs a 7-bit code out the 9-pin serial connector. A serial cable sends this code to the MD-1 board. The MD-1 program accepts this number and uses it as the velocity byte in current note-on messages.

3. The MD-2 board also has space for a PGA2310PA audio integrated circuit. This is a dual volume control (for stereo applications) which is controlled digitally by the microprocessor. This allows me to directly control audio volume for other signal sources.

My main organ tone generator is either the Hauptwerk program, or the Kloria MyOrgan program; both of these use samples of real organ pipes. These two programs both accept MIDI controller messages, but Hauptwerk ignores velocity commands. (I don't yet know about MyOrgan.)

My secondary tone source is a Roland CM32P. This is a MIDI tone module which uses short samples to play sounds. It has a number of sounds, but I anticipate using only the piano in conjunction with Hauptwerk's theatre organ samples. This module uses the velocity code in note-on messages.

At this point I don't see the need for an audio volume control so, although the MD-2 board has space for the PGA2310PA audio volume control, that part of the boards is not populated, nor have I written the software for that function.

The MD-2 board is divided into two parts --- the digital side on the left, and the analog side on the right. To prevent the digital control signals or noise from getting into the analog circuits, the two sides use separate grounds and separate power supplies for  isolation. The digital side is shown in Figure 1, while the analog side is in Figure 2.

Digital circuitry

The digital circuit in Figure 1 is actually very similar to the MD-1 board.

Let's explain the various parts, starting at the top left corner of the 68HC11.

(a) Top left is the 8 MHz crystal and attached circuitry, which sets the speed of the processor. The 68HC11 internally divides this by 4, so the main clock frequency of the processor is 2 MHz.

(b) Several resistors under that (and resistors elsewhere) connect various pins of the HC11 to +5-volt Vcc, to provide a pullup function - that is, to hold these pins at +5 volts when they are not used for other functions.

(c) Jumper P1 allows bootstrapping the HC11. In normal mode, this jumper would be open. When pins 1 and 2 of the jumper are shorted together, the processor goes into a bootstrap mode, where it can be loaded with a program through the serial port. This is the method that the EEPROM software would initially be loaded.

(d) The DS1233-10 IC, U2, is a reset circuit. It monitors the Vcc power, and automatically resets the processor -- i.e., restarts it -- when the power goes below about 4.5 volts.

(e) The 68HC11 has a built-in serial I/O port which can run at the standard baud rates, as well as at 31.25 kHz, the MIDI rate. It uses pins 20 and 21. This serial port connects both to 9-pin RS-232 connector J2, and also to MIDI OUT jack J1.

Like the MD-1, the MD-2 board has two serial ports, though only one of them can be used at a time:

There is 9-pin DB9F connector, which can be connected to the comm port of a PC. It is does two jobs: (1) During program development, it is used for loading the software into the processor, and also for program testing. (2) During operation, it outputs velocity information which is sent to the corresponding MD-1 board. The MAX232 IC, U3, is used as the converter between the low-level TTL voltages used by the HC11, and the positive and negative larger voltages used by typical RS-232 ports at the DB9F connector. It includes a charge pump which provides the necessary +10 and -10 power for output).

There is also a 5-pin MIDI connector, which is the main MIDI output.

(f) P3 is the input connection for the swell shoe sensor. IC U4b is a buffer which isolates the sensor inputs from the microprocessor to prevent damage to it, but it also converts the Gray code output of the sensor into a binary output to the processor. 
This function could just as well be done in software, but it seemed like a good use for the spare half of U4, and it also provides some buffering and isolation. The encoder wiring is shown here (it is labelled C4 in the block diagram).

(f) Jumpers P2 and P4 are left open in organ applications, but they allow this board to run with the Motorola Buffalo version of the 68HC11 for debugging purposes. P2 selects whether the Buffalo program will run, or whether the HC11 will instead jump to location B600 on bootup. P4 is used to enable tracing and breakpoints. As mentioned, these two jumper headers are here strictly for debugging and other applications.

(g) Power supply circuitry at the bottom includes a 7805 regulator and filtering, so the board can be fed from an unregulated DC source, or even from a "wall wart" converter. If you use a +5-volt supply, then the 7805 can be left out and its terminals 1 and 3 shorted together.

(h) Switch S2 is a four-pole DIP switch which sets the channel number for the MIDI output. MIDI signals can be assigned a channel from 1 to 16.

(i) Connector pins P7 provide four auxiliary inputs "just in case".

Figure 2 shows the analog portion of the board.

The heart of the board is the PGA2310 IC, which is a stereo volume control chip. It receives digital control signals (on the three lines coming in from the left) from pins 23, 24, and 25 of the processor. The right side of the IC is strictly analog, and has two inputs (coming from J3 and J4) and two outputs (going out to J5 and J6). Each of these is buffered by an NE5532 op-amp. A separate power supply at the bottom provides +12 and -12 volts to the analog circuits. Resistor R11 is just a jumper which connects the analog and digital grounds together at one specific point.

The following figures show the MD-2 PC board layout. The board is a two-sided board with plated through holes, and the size is 3.5 by 6 inches. While I do not sell any blank PC boards, kits, or completed boards, you can make your own boards from the layouts shown below, or there are a number of PC board manufacturers who can make a board for you if you send them the Gerber files available  here.

The top solder side:
 

The bottom solder side (as viewed from the top, through the board):

The top silk screen, which shows the location of components:

The completed PC board:

The 68HC11A1 has 512 bytes of EEPROM and 256 bytes of RAM, and only about 50% of each is used in the program. Here is the listing for version 1.0.

0001                         * MD-2 Swell Shoe program
0002                         * Copyright (C) 2006 by Peter A. Stark
0003
0004                         * Version 1.0 8/29/2006 - original
0005                         * THIS VERSION ONLY OUTPUTS SERIAL OUTPUT ON RS-232 AND MIDI
0006                         * AUDIO VOLUME CONTROL IS NOT IMPLEMENTED
0007
0008                         * I/O bits:
0009                         *   Inputs
0010                         * PD0: Serial RS232 input from J2
0011                         * PC3-0: Channel DIP switch S2
0012                         * PC7-4: extra inputs on P7
0013                         * PE0: Jumper to ground for Buffalo P2
0014                         * PE7-4: Rotary encoder signal from P3
0015                         * MODB: Bootstrap mode jumper P1
0016                         * XIRQ': Trace enable from P4
0017
0018                         *   Outputs
0019                         * PA7: H=RS232 (default), L=MIDI out
0020                         * PD1: Serial out to RS232 or MIDI to J2
0021                         * PD5-3: SS', SCK, MOSI to PGA2310
0022
0023
0024                         **************************
0025                         *   GENERAL SYSTEM EQUATES
0026                         **************************
0027
0028 0000                    RAM     EQU   $0000     START OF RAM
0029 1000                    REG     EQU   $1000     START OF REGISTERS
0030 b600                    EEPROM  EQU   $B600     START OF EEPROM
0031 b7ff                    ENDEEP  EQU   $B7FF     END OF EEPROM
0032 1000                    PORTA   EQU   $1000     PORT A DATA & TIMER - B7=BI, B6-3=O,B2-1=I
0033 1026                    PACTL   EQU   $1026     PORT A CONTROL
0034 1004                    PORTB   EQU   $1004     PORT B DATA - OUTPUT ONLY
0035 1003                    PORTC   EQU   $1003     PORT C DATA - BI
0036 1005                    DDRC    EQU   $1005     PORT C DIRECTION
0037 1008                    PORTD   EQU   $1008     PORT D DATA - 6 BITS BI & SCI/SPI
0038 1009                    DDRD    EQU   $1009     PORT D DIRECTION
0039 100a                    PORTE   EQU   $100A     PORT E - INPUT ONLY & A/D
0040 102b                    BAUD    EQU   $102B     SCI BAUD REG
0041 102c                    SCCR1   EQU   $102C     SCI CONTROL 1 REG
0042 102d                    SCCR2   EQU   $102D     SCI CONTROL 2 REG
0043 102e                    SCSR    EQU   $102E     SCI STATUS REG
0044 102f                    SCDAT   EQU   $102F     SCI DATA REG
0045 1028                    SPCR    EQU   $1028     SPI CONTROL REG
0046 1029                    SPSR    EQU   $1029     SPI STATUS REG
0047 102a                    SPDR    EQU   $102A     SPI DATA REG
0048 103a                    COPRST  EQU   $103A     COP RESET REG
0049 000b                    BXCONT  EQU   11        CONTROLLER CHANNEL = 11
0050
0051                         *****************
0052                         *   RAM LOCATIONS
0053                         *****************
0054
0055 0000                            ORG   RAM
0056
0057                         * SET UP A CIRCULAR SERIAL OUTPUT BUFFER
0058                         * CAUTION - BUFFER MUST START AT $0000 !
0059                         * EMPTY IF BUFIN=BUFOUT, DON'T WORRY ABOUT FULL -
0060                         * HOPEFULLY WILL NEVER HAPPEN
0061 0000                    BUFFER  RMB   64   SERIAL OUTPUT BUFFER
0062 0040                    BUFIN   RMB   2    PUT NEXT BYTE HERE POINTER
0063 0042                    BUFOUT  RMB   2    NEXT BYTE TO OUTPUT POINTER
0064
0065 0044                    SWSHOE  RMB   1    CURRENT SW SHOE POSITION
0066 0045                    STAPOS  RMB   1    SHOE STARTING POSITION
0067 0046                    PREVSH  RMB   1    PREVIOUS POSITION
0068 0047                    VOLUME  RMB   1    CURRENT VOLUME
0069 0048                    TIVOL   RMB   1    VOLUME DOCTORED UP FOR TI CHIP
0070 0049                    RSVOL   RMB   1    VOLUME DOCTORED UP FOR RS-232 OUTPUT
0071 004a                    BXVOL   RMB   1    VOLUME FOR BX CONTROL CHGE MSG
0072 004b                    BXCHAN  RMB   1    BX + CHANNEL NUMBER
0073
0074 007f                    STACK   EQU   $007F
0075
0076                         ****************
0077                         * Start and Initialize ports
0078                         ****************
0079
0080 b600                            ORG   EEPROM
0081
0082 b600 8e 00 7f           COLDST  LDS   #STACK
0083 b603 7f 10 04                   CLR   PORTB     OUTPUT - NOT USED
0084 b606 7f 10 05                   CLR   DDRC      PORT C IS ALL INPUT
0085 b609 86 38                      LDAA  #$38      PORT D IS INPUT EXC SERIAL...
0086 b60b b7 10 09                   STAA  DDRD      ...AND SPI OUTPUT
0087 b60e 7f 10 0a                   CLR   PORTE     PORT E IS ALL INPUT
0088
0089                         **********
0090                         *   Initialize the SCI serial port
0091                         *        use $30 for 9600 baud during testing
0092                         *        use $20 for 31.25K baud for final version
0093                         **********
0094 b611 ce 10 00                   LDX   #REG      POINT TO REGISTERS
0095 b614 1c 00 80                   BSET  PORTA-$1000,X $80 PA7=H FOR RS232
0096 b617 1c 26 80                   BSET  PACTL-$1000,X $80 ... AND OUTPUT
0097 b61a 86 20                      LDAA  #$20      SET 9600 BAUD TEMPORARY
0098 b61c b7 10 2b                   STAA  BAUD      BAUD REGISTER
0099 b61f 86 00                      LDAA  #$00      SET 8X1, NO WAKEUP
0100 b621 b7 10 2c                   STAA  SCCR1
0101 b624 86 0c                      LDAA  #$0C
0102 b626 b7 10 2d                   STAA  SCCR2     ENABLE
0103
0104                         **********
0105                         *   Initialize SPI port not needed until we add audio control
0106                         **********
0107
0108
0109                         **********
0110                         *  WARMST - WARM START
0111                         **********
0112
0113 b629 b6 10 03           WARMST  LDAA  PORTC     GET CHANNEL NUMBER
0114 b62c 84 0f                      ANDA  #$0F      MASK IT
0115 b62e 8b b0                      ADDA  #$B0      MAKE INTO CTRL CHG COMMAND
0116 b630 97 4b                      STAA  BXCHAN
0117 b632 7f 00 47                   CLR   VOLUME    INITIAL = 0
0118 b635 7f 00 45                   CLR   STAPOS    STARTING POSITION = 0
0119 b638 86 04                      LDAA  #4
0120 b63a 97 46                      STAA  PREVSH    PREV POSITION = 4
0121 b63c 4f                         CLRA
0122 b63d 5f                         CLRB
0123 b63e dd 40                      STD   BUFIN     BUFFER IS EMPTY
0124 b640 dd 42                      STD   BUFOUT
0125
0126 b642 86 55              MAINLP  LDAA  #$55
0127 b644 b7 10 3a                   STAA  COPRST    RESET COP TIMER
0128 b647 86 aa                      LDAA  #$AA      JUST IN CASE
0129 b649 b7 10 3a                   STAA  COPRST
0130
0131 b64c b6 10 0a                   LDAA PORTE      GET DATA
0132 b64f 43                         COMA
0133 b650 44                         LSRA
0134 b651 44                         LSRA
0135 b652 44                         LSRA
0136 b653 44                         LSRA
0137 b654 97 44                      STAA SWSHOE
0138 b656 90 46                      SUBA PREVSH     SUBTRACT PREVIOUS
0139 b658 27 6a                      BEQ  OUTPUT     NO CHANGE
0140
0141                         * FIGURE OUT IF UP OR DOWN
0142 b65a 81 07                      CMPA #$7        DOWN CUR=8 PREV=1
0143 b65c 27 0e                      BEQ DOWN
0144 b65e 81 f9                      CMPA #$F9       UP CUR=1 PREV=8
0145 b660 27 05                      BEQ UP
0146 b662 4d                         TSTA
0147 b663 2a 02                      BPL UP
0148 b665 2b 05                      BMI DOWN
0149
0150 b667 7c 00 45           UP      INC STAPOS
0151 b66a 20 05                      BRA OK
0152
0153 b66c 7a 00 45           DOWN    DEC STAPOS
0154 b66f 20 00                      BRA OK
0155
0156 b671 96 44              OK      LDAA SWSHOE     GET CURRENT POSITION
0157 b673 97 46                      STAA PREVSH     SAVE AS PREVIOUS
0158 b675 81 04                      CMPA #$04       ON INDENT?
0159 b677 26 4b                      BNE OUTPUT      NO, SO CONTINUE
0160
0161                         * SWELL SHOE IS BACK ON A CLICK, SO SEE WHICH WAY IT WENT
0162                         * AND ADJUST THE VOLUME ACCORDINGLY
0163
0164 b679 96 45                      LDAA STAPOS
0165 b67b 27 47                      BEQ  OUTPUT     JUST IGNORE IF NO CHANGE
0166 b67d 7f 00 45                   CLR  STAPOS     CLEAR IT FOR NEXT
0167 b680 4d                         TSTA
0168 b681 2a 08                      BPL  INCVOL     INCREASE VOLUME IF +
0169
0170                         * DECREASE VOLUME IF -, BUT NOT BELOW 0
0171 b683 96 47              DECVOL  LDAA VOLUME
0172 b685 4a                         DECA
0173 b686 2a 0c                      BPL  VOLOK
0174 b688 4f                         CLRA
0175 b689 20 09                      BRA VOLOK
0176
0177                         * INCREASE VOLUME IF +, BUT NOT ABOVE 20
0178 b68b 96 47              INCVOL  LDAA VOLUME
0179 b68d 4c                         INCA
0180 b68e 81 14                      CMPA #20
0181 b690 23 02                      BLS  VOLOK
0182 b692 86 14                      LDAA #20
0183 b694 97 47              VOLOK   STAA VOLUME
0184
0185                         * NOW DOCTOR UP THE VOLUME TO DESIRED RANGE ~~
0186
0187 b696 96 47                      LDAA VOLUME     VALUE = 0 TO 20
0188 b698 48                         LSLA
0189 b699 48                         LSLA            VALUE = 0 TO 80
0190 b69a 8b 1e                      ADDA #30
0191 b69c 97 49                      STAA RSVOL      VALUE = 30 TO 110
0192
0193                         * ALSO SET TIVOL AND BXVOL ~~
0194
0195 b69e 97 4a                      STAA BXVOL      FOR NOW USE THE SAME
0196
0197                         * NEXT OUTPUT VIA SPI PORT TO TI CHIP - USE TIVOL ~~
0198
0199                         * PUT RSVOL STUFF INTO CIRC BUFFER
0200
0201 b6a0 de 40                      LDX   BUFIN
0202 b6a2 86 80                      LDAA  #$80   OUTPUT VIA RS232
0203 b6a4 d6 49                      LDAB  RSVOL  VOLUME
0204 b6a6 ed 00                      STD   0,X    PUT BOTH INTO BUFFER
0205 b6a8 8d 3c                      BSR   INCIN  BUMP POINTER BY 2
0206
0207                         * PUT BXVOL STUFF INTO CIRC BUFFER
0208
0209 b6aa de 40                      LDX   BUFIN
0210 b6ac 86 00                      LDAA  #$00    OUTPUT VIA MIDI
0211 b6ae d6 4b                      LDAB  BXCHAN  $B AND CHANNEL...
0212 b6b0 ed 00                      STD   0,X     ...BOTH INTO BUFFER
0213 b6b2 8d 32                      BSR   INCIN   BUMP POINTER BY 2
0214 b6b4 de 40                      LDX   BUFIN
0215 b6b6 c6 0b                      LDAB  #BXCONT CONTROLLER NUMBER...
0216 b6b8 ed 00                      STD   0,X     ...INTO BUFFER
0217 b6ba 8d 2a                      BSR   INCIN   BUMP POINTER BY 2
0218 b6bc de 40                      LDX   BUFIN
0219 b6be d6 4a                      LDAB  BXVOL   AND VOLUME...
0220 b6c0 ed 00                      STD   0,X     ...INTO BUFFER
0221 b6c2 8d 22                      BSR   INCIN   BUMP POINTER BY 2
0222
0223                         * NOW OUTPUT FROM BUFFER IF NEEDED
0224
0225 b6c4 96 41              OUTPUT  LDAA  BUFIN+1
0226 b6c6 91 43                      CMPA  BUFOUT+1
0227 b6c8 26 03                      BNE   SKIP1   CONT IF NOT EMPTY
0228 b6ca 7e b6 42                   JMP   MAINLP  ELSE REPEAT LOOP
0229
0230 b6cd f6 10 2e           SKIP1   LDAB  SCSR    READ STATUS
0231 b6d0 c4 40                      ANDB  #$40    TRANSMIT COMPLETE FLAG
0232 b6d2 26 03                      BNE   OUT2    OK IF XMTR FINISHED, ELSE...
0233 b6d4 7e b6 42                   JMP   MAINLP  ...XMTR BUSY, SO REPEAT LOOP
0234
0235 b6d7 de 42              OUT2    LDX   BUFOUT
0236 b6d9 ec 00                      LDD   0,X     GET TWO BYTES FROM BUFFER
0237 b6db b7 10 00                   STAA  PORTA   CHOOSE RS232 IF H OR MIDI IF L
0238 b6de f7 10 2f                   STAB  SCDAT   SEND CHARACTER
0239 b6e1 8d 0c                      BSR   INCOUT  BUMP POINTER BY 2
0240
0241                         * AND FINALLY RETURN INTO LOOP
0242
0243 b6e3 7e b6 42                   JMP  MAINLP
0244
0245                         * INCREMENT BUFIN POINTER BY 2
0246 b6e6 d6 41              INCIN   LDAB BUFIN+1
0247 b6e8 5c                         INCB
0248 b6e9 5c                         INCB
0249 b6ea c4 3f                      ANDB #$3F     BUFFER SIZE = 64
0250 b6ec d7 41                      STAB BUFIN+1
0251 b6ee 39                         RTS
0252
0253                         * INCREMENT BUFOUT POINTER BY 2
0254 b6ef d6 43              INCOUT  LDAB BUFOUT+1
0255 b6f1 5c                         INCB
0256 b6f2 5c                         INCB
0257 b6f3 c4 3f                      ANDB #$3F     BUFFER SIZE = 64
0258 b6f5 d7 43                      STAB BUFOUT+1
0259 b6f7 39                         RTS
0260
0261
0262 b6f8 43 4f 50 59 52 49  COPRIT  FCC 'COPYRIGHT (c) 2006 BY '
     47 48 54 20 28 63
     29 20 32 30 30 36
     20 42 59 20
0263 b70e 50 45 54 45 52 20          FCC 'PETER A. STARK'
     41 2e 20 53 54 41
     52 4b
0264
0265
0266                                 END

The above program is subject to change. Aside from the lack of PGA2310PA audio support -- which may not ever really be needed -- there may be some slight adjustments needed to provide the correct scaling for the MIDI controller output and velocity output. Right now, both of these settings range from a minimum of 30 to a maximum of 110, but this may need to be changed. See the sections identified with two tildes (~~) in the above listing.
 

Jumpers: There are several header strips for jumpers. When using a 68HC11A1 that has Motorola's Buffalo debugger programmed into its ROM, P1, P2, and P4 determine the operating mode for the software, as follows:

1. To run Buffalo, place a shorting jumper on P2 and P4. This allows you to do various debugging functions via the serial port; if you have programmed the MD-1 software into the chip, you can do a manual jump to it at location B600.
2. To have Buffalo start, but then right away automatically jump to the MD-1 software at location B600, remove all jumpers. This is the normal operating mode once you have programmed the chip and installed the board into your organ.
3. To go into bootstrap mode, so you can program the chip via the serial port, place a shorting jumper on P1.

Location P7, right next to the DIP switch, is "for future use". It is an extra set of four inputs that can be used for other applications, if needed, but not used in this application. For example, the 4-pole DIP switch could be replaced with an 8-pole switch, with the extra four poles used for other input information.