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I designed two circuit boards using optical interrupters to replace the contact in Wurlitzer, Rodgers and Gulbransen keyboards. I did this for some Garden State Theatre Organ Society instruments and my own instruments. The circuit boards have eight interrupters each, so a 61-note keyboard requires eight boards, one of which has to be cut down to 5 circuits. One of the circuit boards has the optical slot parallel to the keys and the other is perpendicular. each note has an optional LED indicator to make it easy to adjust the trigger point. On the Gulbransen, I mounted the boards at the back end of the keyboard and cut thin brass strips to act as flags that block the light path. On the Wurlitzer keyboard, I mounted the boards under the keyboard, using the original Wurlitzer contact bars as the optical flag. So far they have proved to be highly reliable. The cost is about one dollar in parts for each note plus the cost of the circuit board. The circuit can use anything from 5 volts up to power the optical isolators and LEDs.

This has other applications. If you have tuned percussions, such as chimes, a xylophone or chrysoglott, you need a similar function. Untuned traps, such as drums, cymbals, etc. also need a driver like this.

You have some excellent points Lyle. I have considered polyfuses, even as replacements for the one-shot circuit, but time to reset would have to be considered. As protection alone, they would be fine. The Artisan MicroMidi system has the ability to perform the one-shot function as well as re-it. I'm using that system in two pipe organs that I maintain. This, however, is for my own electronic instruments where experimentation is most of the fun. I am looking at the source code for the MIOS output functions and I am tempted to try some rewrites to get the functionality that I am looking for. The problem is that I am not a very proficient code writer. This post was to make sure that I am not re-inventing the wheel, or pulse output. George

Hi, There is nothing inherently wrong with using software any more than using a hardware solution. 555s, transistors, capacitors can fail, so can code. The coils could be protected by fuses. As far as the current and voltage requirements, the ULN2803 can handle 50V at 1/2 amp. The ULN2823 can handle 95V at 1/2 amp. The data sheet says, and from personal experience I know that the outputs can be used in parallel to increase the current capability. The interface to the hardware is simple if the pulse comes from software. The hardware solution would be a fabrication nightmare.

Hi All, Is there a facility in MIOS for driving a DOUT output pin with a fixed length pulse? This would be useful in driving a solenoid-based percussion instrument where the solenoid is not rated for continuous duty. I am looking at a glockenspeil on my Rodgers organ that uses thirty-volt solenoids. They would melt with anything longer than a 500ms pulse. I need to rebuild it so that it is driven by a midi signal. I need to drive the output high for a fixed time period when the core receives a note-on message with velocity > 0. Any other message would be ignored. There is the hardware option of using a 555 on each output, but that would be messy. The pulse period would also not be easily changeable if I guessed wrong on the pulse length. A variable pulse width would also be useful to change the 'volume' of the instrument. Suggestions? Thanks, George

Using the ULN2803 outputs in parallel is not be a problem and requires no external diodes, BUT make absolutely sure that the +12 volt supply for the coils is tied back to pin 10 on the chip and the negative side of the 12 volt supply is connected to the negative side of the 5 volt logic supply. If the grounds of the two supplies are not tied closely to the ULN2803's ground and pin 10, the chip will literally blow it's top. I've had the chip tops pinging around the chamber. By closely tied, I mean the driver board containing the ULN2803s and the 12 volt brute supply should be located close together (within a few feet) and 18 gauge or better wire should tie the grounds of the 12 volt brute supply and 5 volt logic supply. Do not rely on power cord grounds. It's a good idea to plug the two supplies into the same power strip. Especially in old buildings (theatres) you can find a significant voltage between the ground connection on outlets on different circuits. I've measured ten volts or more. I had carefully tied the 12 volt supply tied to pin 10 and ASSUMED that the 110v power plug grounded them together. Result: blown chips. I have had no problems since I connected the negative sides of both power supplies together. George