Vision

To create a metronome where the tempo is set by tapping your foot.

Rationale

I run the sound desk and play guitar in one of the bands at my local church. During one practice, one of the drummers asked if I was aware of a metronome that you could set by tapping your foot. Typically, larger bands would run to a click-track, that is a track that only the band can hear that keeps tempo and signals when moving to the bridge or chorus. This would be run by a member of the band who also has the role of musical director. In our instance, the worship leader wants to be able to set the tempo and communicate this to the drummer. Typically this takes the form of them tapping their hand on their leg. There are many solutions to this problem, ranging in complexity. This could include a midi-controller footswitch connected to a laptop, or a guitar pedal with metronome for practicing. But in every instance the solution seemed overly complicated or expensive.

Features

• Footswitch - for controlling the tempo
• XLR connection - for connecting to the sound-desk or stagebox
• Phantom powered - to remove cabling or reliance on batteries

Realisation

For me, phantom powering the device made perfect sense. Most consumer-grade audio equipment use single-ended signals for audio. This means that the audio signal is referenced to ground. A pair of headphones includes 3 pins; LEFT, RIGHT, and GROUND. Professional-grade audio equipment uses differential signaling instead. This means the audio is sent on two wires, instead of one, but at opposite polarity. This is used so that any noise that gets coupled onto the wires will be cancelled out. Phantom power uses this idea to inject 48V onto each of the two signal wires while retaining ground as a reference. The standard defining phantom power (IEC 61938) sets out a current limit of 7mA, which translates to over 3.3W, sufficient for powering a simple microcontroller.

A microcontroller with a 12bit DAC is used to generate a tone. A transformer is included to convert between the single ended signal of the DAC and the differential signal required by the XLR connector, and to protect the electronics from the 48V of phantom power. The rate at which the tone is played is dependent on the time measured between 2 taps on the footswitch. An LED is included to provide visual feedback of the tempo.

Future

A simple 8x2 LED display could have been used to provide the user with the actual tempo