MIDI Vibraphone

This design is based around the MIDI Keyboard Encoder and external Pre-amplifier for Electronic switches, both circuits are required. The electronic switches are type 4066. The switches are controlled by piezo transducers which produce an electronic voltage when hit. These can be mounted under pieces of timber which produce the vibraphone.

Each 4066 contains 4 electronic switches which can be used in place of the momentary action, push to make, single pole single throw (SPST), switches. The electronic  switches replace the mechanical switches, which are wired via IN4148 diodes. The LM324 contains 4 pre-amplifiers. Each one provides enough gain to produce the required output voltage to switch on the 4066 switches, when a piezo transducer is hit.

The vibraphone can have from 1 up to 128 sensors to produce the equivalent MIDI note-on/note-off data commands. 

The MIDI Vibraphone Facilities are:

• The transmit MIDI channel (1-16) can be selected by a 4-way DIP switch. 
• The velocity byte is preset to half of the maximum value.
• The keyboard encoder is 32-note polyphonic.
• The switches, 0 to 127, are defined in the MIDI Note Number to Frequency Conversion Chart.
• A wiring layout for the circuit is also available.

The MIDI Vibraphone Unit utilizes:

• A Printed Circuit Board (PCB), 
• A pre-programmed 16F877 PIC,
• The 4-way DIP switch which selects the MIDI transmit channel via DIP switches 1-4 (See Table 1) 
• A 9 volt battery connector
• A MIDI 5-pin DIN output socket
• A screw terminal connector for an external 9-12 volts DC supply and associated power-on LED,
• Three 8-way screw terminal connectors for connecting the external switches, see layout diagram

The  MIDI Vibraphone Unit requires:

• a 9v battery or equivalent DC power source
• Required external pre-amplifiers, 4066 switches and associated piezo transducers, see Pre-amplifier for Electronic switches

Explanation of pre-amplifier and electronic switches circuit:

Each 4066 contains 4 electronic switches which can be used in place of the momentary action, push to make, single pole single throw (SPST), switches. The electronic  switches replace the mechanical switches, which are wired via IN4148 diodes. 

Each LM324 contains 4 pre-amplifiers. Each one provides enough gain to produce the required output voltage to switch on the 4066 switches, when a piezo transducer is hit. The gain is set to 10. However the variable pre-set resistors (VR1-4) can be used, if required, to adjust the sensitivity so that adjacent transducers do not pick-up false triggers.

The diodes (D1-4) rectify the signal and the capacitors (C1-4) act as a sample and hold circuit.