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MIDI Monitor
Stand-alone and portable
The MIDI-Monitor is a handy MIDI device that shows incoming MIDI-messages on an LCD screen. It can be used to troubleshoot MIDI systems or debug a MIDI device that your making yourself. Or just to see what your MIDI controller is sending. The types of messages it displays are fully configurable. This enables you to check if specific messages are sent and if messages are sent on a specific MIDI channel.
Simple but effective
The unit has a hardware MIDI THRU connection, which transmits all incoming MIDI messages without any modification whatsoever. You can simply connect it in series with your transmitting device (for example a MIDI keyboard or PC with a USB-MIDI interface) and your receiving device (for example a synthesizer or a MIDI module).
Power can be delivered by either a single 9V battery or a 9V DC power supply, the same as used for guitar pedals. It has standard 5-PIN MIDI connectors so normal MIDI cables can used.
Origin and build
The idea for this device came when I was designing the keyboard scanning circuitry for the QUARTZ-8 polyphonic synthesizer. I felt the need for a small and portable device that could show if and which MIDI messages are sent.
The heart of the system is an Atmega 328P microcontroller. It receives incoming MIDI messages via the optocoupler. The incoming MIDI messages are buffered and sent out via MIDI Thru. There’s a (blue) LED that blinks when MIDI signals are received. A red LED will blink MIDI Clock messages are being received. The microcontroller reads the encoder and buttons and controls the LCD.
The case is made with a 3D printer and has a separate compartment for holding the 9V battery. The top panel is an aluminium plate, covered with a printed sticker.
Resources
I’ve written a user manual for this device, which describes the operation of the unit in full detail. You can download it here:
User Manual (PDF) 
And for those who want to go even deeper into the nitty gritty details, here are the schematics: