First post!

I’m retrofitting a piano to be MIDI controlled and have limited electrical circuit knowledge a but willingness to learn and scour the internet for info.

I bought a MIDI processor that contains eleven (11) 18 pin ULN2803As to drive 88 separate solenoids for each piano key. The solenoids have a 44Ω coil and perform the required task at 12-15Vs. The MIDI processor has two voltage inputs, 5V to run the MIDI decoder and another one for a higher voltage of the users choice to push the solenoids.

From what I can gather the ULN2803A can handle 500ma for one line but using 12 -15V and pushing 250 - 350ma through 4,5,or 6 lines at one time will fry it.

Sooo…I decided to push 5V through the ULN2803A and let a TIP122 do the heavy lifting to the higher voltage.

Please comment if any of the above content requires feedback so far.

The problem is that the MIDI processor appears to be routing the -5V through the switched transistor array and the last pin is the common positive. All of the wiring diagrams I’ve come across show it opposite.

My web search skills have taken as far as they can and now I am reaching out for help. Attached below is my incomplete schematic for someone smarter than me to provide the solution. Please take a look and provide feedback.

 

Your schematic should work fine although I would like clarification on a few points. Where does the -5V come from? -5V will never turn thos TIP122 transistors on. You need to ground the emitter of each of those transistors. You also would need a pull-up resistor from the base to +15V or +5V or what ever your general supply is. that way when the ULN2803 output is off the base will be pulled high and the transistor will be turned on operation the solenoid. If this operation is backward you would need to swap those NPN transistors with PNP. Also the diode on the solenoids I beleive are around the wrong way.

 

Thank you windoze killa. The MIDI board comes set up with individaully switched -Vs and a common +V to drive the solenoids, I chose to switch the TIP122 with a 5V supply to keep the voltage going through the ULN2803 low.

You lost me when you replied: "You need to ground the emitter of each of those transistors. You also would need a pull-up resistor from the base to +15V or +5V or what ever your general supply is." This is what I'm having trouble with understanding. Can you mark up my schematic to graphically show this?

I need for the ULN2803 when activated to turn on the solenoid, If I understand you correctly I need a PNP rather than a NPN.

I'll check the diode direction.

 

Windoze Killa,

I changed the NPNs to PNPs and it makes more sense now except for the your refence to a "pull-up resisitor" for for the +5V to the +15V.

Excuse my newbness, but can you comment on the following:

1. Concept of the "pull-up resisitor".

2. I'm showing a TO220, please confrim this will work.

3. Do I need to do anything with the common positive coming from the 9th pin on the ULN2803A?

4. Double check my revised shematic for flaws like missing or missized resistors.

Thank you for your feedback