Piezo Sounders - mounting under a decal?

[max_torque]

I have a project where we would like a haptic feedback beyond that provided by the actual buttons and indicator leds on the HMI.  An existing product uses pcb mounted buzzers, pointing out sideways, through gore vents, to provide a small range of "beeps and chimes" to indicate operational events.

What i'd like to do is to simplify (and potentially reduce cost too) that solution, ie to provide a relatively loud (>85dB) sound indicator to the user, but the solution must be waterproof and pass some stringent EMC requirements too (hence the previous buzzers being inside the box).  To add a further complication, ideally i'd like a solution with the lowest "stack" height possible!  (the buzzer solution, vent and hole is "tall")

My current best solution, but with completely unproven provenance is to use one (or more) flat piezo sounders like this:




Which i plan to mount in a shallow recess machined into the top of the alluminium case of the HMI, and to cover that sounder with the decal that acts as the waterproofing agent for the LEDs/ Buttons.  This leads to two main questions:

1) How much damping, and hence attenuation in volume will the decal introduce?   (the answer of course is "what's the decal like?"    )  There maybe ways to reduce damping through thickness / mass optimisation.  The Sounders are typically at around 1khz, so the mass of the decal could well be significant?

2) How to electrically connect to the Piezo?  I want to avoid soldering, wires or connectors.  Current plan is to use two spring contacts, soldered to the pcb which sits under the top of the HMI and holds the switches / LEDs. Those contacts could push up through the lid, and make contact with the back of the sounder?

something like these:







Production volumes are low (say 1k units PA) but it needs to be easy and simple to assemble.


Anyone have any relevant experience or knowledge in this area?   

[Richard Crowley]

We have seen teardowns on EEV and other YouTube channels of gadgets with piezo sounders where the sounder was glued into the case and contact was made from the PC board with simple gold-plated solid spring-wire bent into the appropriate shape. You don't need much current-carrying capacity, so those spring terminals in your photo seem like huge overkill.

[Berni]

These piezo discs are actually designed to be glued onto something.

Often mounting them tightly to a thin material makes them louder because the flat object it is mounted to starts to flex along with it, this produces sound from a larger surface area.

[Wendy_Preston]

Hi all, just to confirm that the picture is of our Spring Contacts here at Harwin. These ones are our Antenna contacts - they might look big in the picture, but they are pretty small in reality. The heights range from 1.23 to 2.00mm high for the upper row, and 2mm to 5mm for the lower row.

You can see the whole range of spring contacts in our Catalog section: https://cdn.harwin.com/pdfs/Harwin_PC_EMC_Shielding_Spring_Contacts.pdf

I'm not familiar enough with your application to confirm they'd be the exact solution you require, but if you need miniature spring contacts, these are a good choice. Stocked in depth in our authorised distributor network, and you can also request samples from our website https://www.harwin.com.

Need any help, feel free to contact me on technical@harwin.com.

[Berni]

Ah didn't know Harwin had a applications engineer on here.

Some might say this is spam, but i think this is a great way for manufacturers to interact on the forums. A user has a need for a thing and they suggest there solution. Rather than buying ad space or starting threads filled with just pure marketing bullshit.

[Zero999]

Ah didn't know Harwin had a applications engineer on here.

Some might say this is spam, but i think this is a great way for manufacturers to interact on the forums. A user has a need for a thing and they suggest there solution. Rather than buying ad space or starting threads filled with just pure marketing bullshit.

There have been a few from different companies and I believe it doesn't break the rules, so long as they don't push their products, when it's inappropriate or have more than one account.

[Simon]

Manufacturers are welcomed here if they are providing support and advice on their products. I think a number of test gear manufacturers are on here but general component manufacturers there are less of.

As Wendy only has 4 posts at this time she has obviously only just joined and I expect ran a search on "Harwin" to see if their products are being mentioned and if she can offer any help.

[JVR]

provide a relatively loud (>85dB) sound indicator to the user

I've done a product where we integrated the piezo into the layers of the membrane keypad. This allowed for the connection to be via the normal ribbon connector. Audio quality was great, but volume was lacking. We were also going for 85dB.
In the end we changed the housing mould to have a resonant chamber at the most common frequency, and laser drilled holes through the membrane to allow sound to get out. The laser drills do not allow moisture to pass, but more than enough audio energy makes it out.

This needs a good amount of research, as different piezo elements have different volumes at different driving frequencies and voltages. We ended up sourcing multiple samples from Wurth to test with, and used them in production. IIRC to date around 10k of these are in the field, worn by personnel in truly shitty environments for electronics.

[Benta]

If you can make the adhesive side of the decal conductive, you can kill several birds with one stone:
You'll have a common ground connection for all the piezos, so you can eliminate the black wire except at one point.
You'll have an electrical shielding that could help your EMI requirements.
You can pre-mount the piezos to the decal, making assembly quicker and easier.

You'll of course need the spring contacts for the red connection.

Talk to your decal manufacturer.

[Simon]

Some how I think a decal like that will be expensive unless made in serious volumes and large batches at a time.

[Benta]

Some how I think a decal like that will be expensive unless made in serious volumes and large batches at a time.

I don't think so. Decals are made like PCBs at the same cost point.  And conductive adhesive is no magic nowadays. For 1 kpcs. it'll be reasonable.

But let's see what max_torque finds.

[Wendy_Preston]

As Wendy only has 4 posts at this time she has obviously only just joined and I expect ran a search on "Harwin" to see if their products are being mentioned and if she can offer any help.

You got it in one Not here to push Harwin specifically - although I will mention our products if I think they might help with a solution. Doesn't mean you have to buy it - it might just get someone thinking on an alternative track. Other manufacturer's products are available.    But definitely want to help if someone mentions issues with Harwin products.

I promise to be helpful and not pushy - I'm an engineer too, didn't do too well when I tried Sales    We just thought in the Marketing Team here that I might be able to contribute, rather than just wait for support requests to come in my email and Live Chat.

[JVR]

Some how I think a decal like that will be expensive unless made in serious volumes and large batches at a time.

We got the above solution built at ~$12 ea IIRC, with CMYK printing, 1 button and a few RGB LED's all integrated into the decal.

[Simon]

but are you talking the type of decal that has buttons and OLED lights in it? they are not too expensive but there is tooling.

[Psi]

It's notoriously difficult to model the behavior of a mounted piezo.
Often the math says X should be loud but in reality it's not.
And a random mounting idea turns out to be amazing.

Ya really just have to resort to trial and error to see what works and what doesn't.

Also, pro tip for using piezos,
Always put a cap in series if your piezo will otherwise have DC on it. You can spec the cap to be low resistance at your drive frequency.
Usually you drive piezos differentially, alternating the polarity around to make the freq you want, but in a two state system this means it has voltage across it when not in use.
Piezos will often partially fail and go from >1Mohms to as low as 2k.
Depending how you have it wired up this can cause idle current flow and be really bad in a battery powered device.

I recommend using a piezo drive IC, you can get them that run from 3.3V and do a capacitive voltage boost to 10V to drive the piezo. This gives you 20V of deflection so really good volume. The chips also ususally have some way to enable/disable each voltage boost stage for simple GPIO 3level volume control

[JVR]

but are you talking the type of decal that has buttons and OLED lights in it? they are not too expensive but there is tooling.

Jip, called membranes, like so:

It's notoriously difficult to model the behavior of a mounted piezo.
Often the math says X should be loud but in reality it's not.
And a random mounting idea turns out to be amazing.

Seconded. We did all of the math, in the end we autotuned them by feeding a dB meter back to the MCU while it stepped frequencies. Driver was a JRC part IIRC.

[Berni]

Yes as PSi said. How you drive a piezzo makes a massive difference in output power. They love big sharp AC voltage swings.

A quick hack for driving one can be a RS232 driver chip. They generate the voltage and provide a fairly low impedance AC drive to it all in one chip.