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More-MIDI A low-cost MIDI expander box Want to drive more synthesisers or instruments from your MIDI sequencer or computer sound card and MIDI-Mate combo? Here’s a simple little expander box that takes one MIDI signal and lets you feed it to four instrument inputs. It’s low in cost and can be assembled in just an hour or two. By JIM ROWE I F YOU’RE REALLY INTO electronic music, odds are that you now have quite an array of synthesisers, MIDI instruments, se­quencers, keyboards and other controllers. And that probably means swags of MIDI cables, daisy-chaining around from this box to that box to the other box, and so on. 84  Silicon Chip That’s OK, but daisy-chaining introduces cumulative delays into the MIDI system and sooner or later those delays can become audible and irritating. The simplest solution is an expander box like More-MIDI. It “pumps up” the number of MIDI outputs from your sequencer or com­puter and lets you run more signals directly out to the instru­ ments, in “star” fashion. You still need just as many cables but at least the instruments are all driven with just one short delay – ie, the minuscule 5µs or so introduced by the circuitry in More-MIDI itself. That’s really not significant in MIDI terms. Best of all, More-MIDI is cheap and very easy to build. It uses just three low-cost ICs plus a handful of passive components and literally everything mounts on a small PC board which fits in a compact low-profile instrument case. Power comes from a stan­dard 9-12V DC plugpack. Circuit description Fig.1 shows the circuit diagram and, as you can see, More­-MIDI is very straightforward in electronic terms. It’s basically just a MIDI input stage, Fig.1: the circuit uses OPTO1 to isolate the input stage plus a 74HC04 hex inverter (IC1) to drive the four output stages and the signal indicator LED. buffered correctly using the usual optocoupler, which then drives four identical MIDI output stages. The input stage is based on OPTO1, a 6N138 fast optocou­ pler. A 220Ω series resistor sets the correct current level through the optocoupler’s input LED, while the 1N4148 diode protects it from possible damage due to reversed-polarity inputs. Don’t be tempt­ed to substitute another optocoupler for the 6N138, by the way. Its speed is necessary for handling MIDI signals correctly. The output from IC1 (at pin 6) is effectively an inverted version of the incoming MIDI logic signal and inverter IC2f (74HC04) is used to restore its polarity. The output from IC2f is then used to drive the four MIDI output stages via inverters IC2a, IC2b, IC2d & IC2e. As you can see, each of these drives one of the four MIDI outputs, with a pair of 220Ω series resistors to set the correct 5mA output current level in each case. The sixth and final inverter inside IC2 is used to drive indicator LED1, so that it blinks to show when MIDI signals are passing through More-MI- DI. This sort of indication can be very handy when you are trying to sort out cable problems! Power supply Power for More-MIDI’s circuitry can come from any convenient source of 9-12V DC, such as a small plugpack supply. The current drain is less than 50mA, allowing the use of a simple voltage regulator system based on IC3 – a standard 7805 3-terminal regu­lator. A 78L05 in the smaller TO92 case could be used if you prefer, although they’re not much cheaper. Diode D1 (1N4004) is connected in series with the DC input to prevent damage to the 7805 or 220µF input filter capaci­ tor in the event of the polarity being accidentally re­versed. Construction Assembling More-MIDI is very easy, as everything fits on a PC board. This measures 117 x 112mm and is coded 01103011. As you can see from the internal photo and parts layout diagram (Fig.2), even the DIN sockets and power connector mount directly on the board, so there’s no off-board wiring at all. There are no wire links on the board itself, either. After checking the board for possible solder bridges and other defects, begin the assembly by fitting the five DIN sockets, as these can be a little tricky. Once fitted, all seven of their mounting pins should be soldered to the board pads, to make sure each one is solidly attached. You might like to fit the 2.5mm DC input connector at the same time, as this too can be a bit fiddly. Ideally, the board should be provided with small slots for its 3mm-wide mounting lugs but if not, you can elongate the holes with a jeweller’s rat-tail file or fine holesaw. The resistors and capacitors can be fitted next, taking care with the polarity of the two electrolytics. After that, you can fit the diodes, ICs and LED. These also need to be fitted with the correct orientation, as shown in the board diagram. Note that the LED is fitted with the flat side of the collar on its plastic body towards CON4, while the longer anode lead goes towards CON5. Before mounting it, bend its leads down at right angles about 6mm from the bottom of the body so that it faces forwards correctly. The leads are solMARCH 2001  85 Parts List 1 PC board, code 01103011, 117 x 112mm 1 low-profile instrument case, 141 x 111 x 35mm 5 5-pin DIN sockets, 90° PCmount 1 2.5mm PC-mount DC power connector 1 10mm x M3 machine screw with M3 nut 4 small self-tapping screws, 6mm long Semiconductors 1 6N138 fast optocoupler (OPTO1) 1 74HC04 hex CMOS inverter (IC1) 1 7805 5V regulator (REG1) 1 3mm red LED (LED1) 1 1N4004 1A diode (D1) 1 1N4148 or 1N914 switching diode (D2) Fig.2: follow this parts layout diagram to build the PC board. Capacitors 1 220µF 25VW PC-mount electrolytic 1 100µF 16VW PC-mount electrolytic 1 0.1µF monolithic Resistors (0.25W 1%) 9 220Ω 2 330Ω around” and also provides a tiny amount of heatsinking. The heat­ sinking is not really needed here but it sure doesn’t do any harm. Final assembly Fig.3: check your PC board against this full-size etching pattern. dered to the board pads so the body axis is about 11mm above the top of the board, ready to line up with the corresponding hole in the front panel. 86  Silicon Chip If you wish, the voltage regulator IC3 can be secured to the PC board using an M3 screw and nut, as in the prototype. This stops it “flapping Once the board is completed, it’s mounted inside the case using four small self-tapping screws (6mm long), which mate with some of the pillars moulded into the bottom half of the case. If you don’t have pre-punched front and rear panels, you can drill (or punch) the required holes using photocopies of the front and rear panels as templates. The hole for the LED is a whisker over 3mm diameter; that for the DC input connector is 8mm dia­meter; and those for the DIN sockets are 15mm or 16mm in diameter. On the prototype, the latter were punched using a 16mm screwtype hole punch, after first drilling suitable guide holes and enlarging them as required with a hand reamer. The labels can now be affixed to All the parts, including the MIDI sockets and the DC power socket, are mounted on the PC board, so there is no internal wiring to be done. Make sure that all polarised parts are correctly orientated. the panels, after which they can be slid into the case slots and the LED pushed through its matching hole in the front panel. Finally, the lid can be fitted to the base and secured using the two screws provided. Getting it going There are no setting-up or other adjustments for More-MIDI and it should work correctly as soon as you apply DC power. Note that the LED will only glow when MIDI signals are actually pass­ing through the unit – after all, it’s an activity indicator rather than a pilot light. What if More-MIDI doesn’t work? Well, there’s only a small number of possible reasons for this, so it shouldn’t take long to track down the cause of the problem. For example, you might have fitted the input protection diode (D1) incorrectly, which will stop the circuit from working at all. Check the +5V rail to confirm that everything is OK here. Another possibility is that one of the two electrolytic capacitors is the wrong way around and drawing Fig.4: these full-size panel artworks can be used as drilling templates. heavy leakage current. Alternatively, you might have IC1 or IC2 around the wrong way, which would again prevent normal operation. If all these things check out correctly, perhaps you have the 1N4148 diode (D2) around the wrong way. This would effective­ly “short circuit” the MIDI input, preventing the input signal from getting any further. Finally, there’s one more possible error. If the circuit does seem to be working in terms of distributing MIDI signals but the LED stubbornly refuses to blink, guess what? You’ve almost certainly fitted the LED itself around the wrong way! But if you haven’t made any of these mistakes, congratula­tions. Your More-MIDI should spring to life and be ready to expand your MIDI capaSC bilities. Happy music making! MARCH 2001  87