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Piezo Switch:

The Stainless Steel, non-magnetic Piezo Switch is ideal to use in conjunction with a TaskLED Piezotg board. The combination of the two units provides a water proof solution for underwater dive lights or other products that are used in harsh environments.

The following picture shows the key dimensions of the Piezo Switch (dimensions in millimetres). A 12.0mm drill hole will allow the switch to comfortable fit. The switch is supplied with an o-ring to assist in creating a water proof finish. The 5.8mm dimension is the maximum recommended bulkhead thickness that still allows full thread engagement of the supplied nut.

The Piezo Switch as supplied.

This schematic shows how to wire the Piezo Switch to the Piezotg board.

The Piezo Switch is a momentary action device and even if pressed down for a long period of time can only generate a maximum pulse (active low while pressed) of about 200 - 400 milliseconds.

Tapping the front of the Piezo Switch can create short duration pulses that need to be filtered in hardware or software to prevent spurious switching action of the circuit. The Piezotg board performs this filtering in hardware.

The following oscilloscope capture shows the filtering action of the Piezotg with the Piezo Switch connected. Top trace is the Piezo switch (high is no pressure on the switch, low is with the switch pressed). The lower trace is the output of the Piezotg board driving a single white LED (high is LED off, low is LED on). Note the 112ms delay between the switch first being pressed and the LED being turned on (at the output of the Piezotg board).

The following oscilloscope capture shows the Piezo Switch being pressed again to turn off the Piezotg output. Note the 112ms delay between the switch first being pressed and the LED being turned off (at the output of the Piezotg board).

As can be seen from the above two oscilloscope captures, it takes about 112ms for the output to switch after the Piezo Switch is pressed. This delay is the hardware 'filter' on the Piezotg board ignoring any switch event that lasts less than 112ms to prevent noise from toggling the on/off state of the output.

The following oscilloscope capture shows the Piezotg board ignoring 2 short pulses from the Piezo Switch. These pulses were generated by 'tapping' the front of the switch in the same manner that would happen if the switch momentarily was activated due to a shock (dropping the light) or other similar 'accident'. Note the output of the Piezotg board does not change since it ignores these short duration 'glitches'.

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