MIDI Out/Thru Jack
An LED between pins 4 and 5 on a MIDI Out or Thru
jack will flash when data passes. Fig. A: When wiring a
receptacle remember that the pins are reversed. You can
also connect directly to a captive MIDI cable.
Fig. B: You can wire a 220-ohm resistor in series with
the LED to protect it, but for quick checks, it’s perfectly
safe without one.
5-pin DIN jack or MIDI cable
Insulated wire or wire wrap
(Optional: 220 resistor, LED socket)
Soldering iron or wirewrap tool
How It Works
Only the three center pins in a MIDI Out jack
are connected. Pin 2 is ground, pin 4 is +5VDC,
and pin 5 is normally at +5VDC, but it drops to
0VDC when MIDI data goes out. So if you connect
the anode (long leg) of an LED to pin 4 and the
cathode to pin 5, the LED will flash when MIDI
data goes out (see Figure B). Note that MIDI pins
are numbered non-consecutively, with pin 2 in the
middle, between pins 4 and 5.
You can also use this device as a visual metro-
nome. Just send it a control-change message
your instrument is generating MIDI messages for every quarter note, with a burst of messages to
both raw notes and any additional twiddling. If mark the downbeat. (If sending from a sequencer,
the detector shows that the MIDI is coming out of you may need to filter out MIDI Clock data, which
your instrument, you know you have to look else- pulses 24 times per quarter note.)
where for the reason you’re not hearing anything.
To determine if my keyboard was successfully
generating data, I built this simple MIDI detector
and packaged it in a small Japanese monster toy.
Besides looking cool, the toy — a Bandai Mechanikong — pulled apart easily to grant access to its
insides. It also had a flat spot on the back where I David Battino (
batmosphere.com) edits the O’Reilly Digital
mounted the MIDI jack. I’ve since built three more Media site (
digitalmedia.oreilly.com) and co-wrote The Art
monster MIDI detectors (see previous page). of Digital Music.
See a movie of the Mechanikong MIDI detector
in action at