8-Way MIDI DRUM PADS

The circuit consists of 8 inputs to which 1 to 8 ceramic piezo transducers can be connected. Note that unused inputs can be grounded to prevent spurious noise pickup. The signal from the piezo transducers is applied between the  8 analogue inputs, AN0 to AN7, and ground.  To allow for different ceramic piezo transducers sensitivity an 8-way pre-amplifier with a gain of 50, can be used to increase the output voltage.

The analogue inputs to Port A are scanned and when an input occurs above a certain threshold level when any of the piezo transducers are hit, then the corresponding analogue input is read and converted to an equivalent MIDI velocity byte.

The velocity sensitivity is dependent on the particular piezo device used. 

Power Supply:

The power supply consists of a linear regulator, type 7805, which provides a constant 5 volt supply when fed from a DC source of 9 to 12 volts. Decoupling of the power supply is provided by capacitors C1 and C2. The LED D9 is fed via R17 and provides power indication when the on/off switch SW2 is closed.

Liquid Crystal Display (LCD):

The 16x2 LCD displays the various menus. The screen contrast can be adjusted using the pre-set variable resistor VR1. The diode D1 can be used, if required, to reduce the voltage to the LCD back-lit LED to 4.3 Volts. 

Piezo Transducers:

The piezo transducers can be mounted internally in diecast aluminium boxes which can be hit directly with drum-sticks. Rubber padding can be glued and screwed to the box to provide some 'bounce'. The circuit is designed that a reasonable hit is required before the drums will trigger. This prevents spurious intermittent triggers from adjacent diecast drum boxes. The variable potentiometers feeding the 8-way preamplifier can be included in the diecast metal box to provide adjustable sensitivity. This allows adjustment so that adjacent piezo sensors do not interfere with each other.