PCB mount piezo push button? - Page 1
Forum section: electronics / projects

author: DBecker
author_rank: Frequent Contributor
timestamp: July 17, 2017, 05:12:40 am
content: I just came across an unusual design, and am looking for some background.I have long intended to modify my hot tub to have remote monitoring and control. The final impetus when a critical button on the topside control panel went from flaky to failed.I had assumed the switch a capacitive, membrane or dome type. Instead it appears to be a 20mm piezo disk -- visually similar to the disk on a piezo buzzer. On the failed switch about 90% of the ceramic element had shattered and delaminated. On several others sections had chipped off and were suspended in the soft sealing gel. That explained the difference in sensitivity and failure over time.Thinking about the requirements, I understand the advantages of this type of switch. They require very little movement, thus can work under a robust polycarbonate label and panel. The trivial movement of the button inserts is unlikely to force much moisture into the gel-sealed assembly. The element is inheriently sealed, which works well with gel potting. And they might be run unbiased, reducing the corrision risk (although there was a lot of really annoying potting gel.. did I mention the gel?)Of course the disadvantage is obviously that they fail. Perhaps it was just this design, which ran a thick springy contact wire in an 'S' over the surface, soldered only at the end.. That likely applied uneven force, cracking the ceramic.Once I figured out what they were, I looked for a replacement. There were many disks intended as buzzers, cheap speakers, microphones, and musical instrument pickups. But none were sold as pushbuttons.Is this really a unique use, or am I just uninformed? If I ever wanted to use this in my own design, is there a part a datasheet?The basic specs of the existing part are 19.75mm diameter stainless disk with three sharp dimples on the perimeter to hold it off of the PCB surface, 14mm element, capacitance about 10000pf / 10nf.

author: thm_w
author_rank: Super Contributor
timestamp: July 17, 2017, 11:20:23 pm
content: Yes for a buzzer, instrument pickup, button, the piezo disk is essentially the same.It is unique, in some ways (I have not seen disks with dimples you mention). Also I have not seen it used with acrylic, usually it will be used with aluminum or steel to provide a vandalism resistant keypad/button. With lots of water splashing, capacitive touch might have been hard to get working well.Sealed example:https://www.digikey.ca/product-detail/en/schurter-inc/1241.5003/486-1565-ND/2538221Here is a disk similar to your specifications:https://www.digikey.ca/product-detail/en/murata-electronics-north-america/7BB-20-6/490-7711-ND/4358151Just watch out for the high voltage output spikes if you use as a button.

author: DBecker
author_rank: Frequent Contributor
timestamp: July 18, 2017, 02:27:28 am
content: The Murata datasheet was interesting. It clearly isn't the product used. It had warnings about corrosion, while the disks used appear to be stainless steel. It also had repeated warnings against applying any force (albeit the disks used did crack in service).I have already order a few different types of disks to experiment with. I expect to break a few, so I ordered cheaper ones.I'm wondering the bounds of the delivered energy. They seem to be wired directly to the pins of a Microchip PIC, with no (obvious) protection.

author: Unknown
author_rank: Unknown
timestamp: July 18, 2017, 04:13:22 am
content: Strange; I would think something like a"Metal Over Capacitive"touch sensor would be easier to implement? It would also remove the issues with wet touch sensors.

author: DBecker
author_rank: Frequent Contributor
timestamp: August 03, 2017, 06:06:30 am
content: Now that a few piezo element have arrived, I had another round of examining the control panel.The button input circuit was surprisingly simple. The piezo metal disks were connected to ground through the three support dimples. Each piezo element output was connected to a 1Mohm resistor to ground (which 'resets' the piezo charge to 0V over time) and a 1Mohm resistor to a 74HC input. Pressing the button generated as positive charge, which reads as a logic '1' for a short period.As a quick sanity check, the RC constant appears to be about 10 nF * 1 Mohm = 10 milliseconds. That's a reasonable human scale. It supports quick button presses, while avoiding bounces. You won't be able to detect press-and-hold, but a piezo element isn't good at that anyway.The specific part used was a 74HC165D parallel-to-serial shift register, which was clocked into the microcontroller. It appears that the 74HC part was used because it has a high input impedance and handles a +- 20mA input current injection. The PIC might have handled this directly. Perhaps the PIC didn't have enough pins, but I suspect it's really because you don't want to be injecting unpredictable pizeo-generated power into your microcontroller pins.