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Optocoupler output too low?

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Newbie level 1
Jul 5, 2011
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Hey, I'm trying to make a MIDI/joystick port cable, with this schematic > **broken link removed**

I can send MIDI events to my keyboard just fine, but I can't receive. I didn't have the optocoupler in the schematic, so I pulled an LTV4N35 off of an old pci modem to use instead. What do I need to do to get this working?

Did you change the original diagram?
Have you check de polarity for D1 diode and the pinout for LTV4N35 in the circuit ?


Did you verify current is flowing through the internal LED? Averaging 5-10 mA?


Does your device need the same value, or a different value resistor at the transistor collector? A higher value is worth a try. Just in case there's reduced efficiency of operation in the optocoupler.

You need to verify that pin 6 (or your device's corresponding output pin) is alternating between hi and low V level.


I see the schematic shows a pin (#7) connected to the base of the output transistor. It looks like a convenient place to apply a test signal to the base?

Does your device have any such pin? That would let you confirm operation in some way.

For instance, send positive supply V through a 3300 ohm resistor to the test pin. That should turn on the transistor sufficiently to pull the output low at pin 6 (or your optocoupler's corresponding output pin).


Is there a chance your pin 15 (on the DB9) needs to be pulled high instead of low, and vice-versa? In other words, to reverse the phase?

I didn't have the optocoupler in the schematic, so I pulled an LTV4N35 off of an old pci modem to use instead.
There is a thing called datasheet. You'll realize, that 4N35 is specified with CTR > 100%, which is quite reasonable for a standard opto coupler, the original type is however a darlington OC which a much higher CTR. Unfortunately the circuit requires a high CTR of e.g. > 300 %. You possibly can fix the problem by changing R1 to 680R - 1K, but it also depends on the circuit behind the 15-pin MIDI port.

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