As seen on LittleBits: https://classroom.littlebits.com/inventions/littlebits-arduino-midi-master-clock-with-tap-tempo

You can also do this with a regular Arduino, but you'll have some extra work soldering the button(s)/dimmer

See the excellent tutorial at this link: http://www.instructables.com/id/Send-and-Receive-MIDI-with-Arduino/step3/Send-MIDI-Messages-with-Arduino-Hardware/ .

Or check the electrical spec here: https://www.midi.org/articles/midi-electrical-specifications

• Connect MIDI ground to Arduino ground
• Connect MIDI 5V to Arduino 5V with a 220 Ohm resistor in between
• Connect MIDI signal line to Arduino serial input pin (D1)

Connect a button to D2

• Upon startup, Arduino will start sending 100 BPM MIDI clock signal (or last saved BPM value if available).
• Tap the tempo (minimum 3 times)
• After the last tap, clock tempo will be updated and MIDI clock signal will send new BPM

• MIDI clock output on pin D1 TX
• Tap tempo input
• Dimmer input when a dimmer is connected to A0 to set the tempo by twisting the knob!
• Tempo blinking LED on pin 5
• Sync signal on pin 9 (for example to sync with Korg Monotribe...)
• MIDI real-time start/stop is sent when button press is detected on A1 port
• BPM storage in EEPROM and restores it on power up
• MIDI forwarding if a MIDI input is present on pin D0 RX
• Output of the BPM to a TM1637 LED display

• master: Code for the Arduino Leonardo (or LittleBits Arduino)
• arduino-uno: Code for Arduino Uno and compatible devices (D0/D1 RX/TX pins are used for MIDI in/out, so no debug console!)
• olimex-midi-shield: Based on the arduino-uno branch, but with some tweaks to make the code work better with the Olimex MIDI shield (https://www.olimex.com/Products/Duino/Shields/SHIELD-MIDI/open-source-hardware) (Work in progress!)