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Topic: Hacking a sound pressure level meter. (Read 880 times) previous topic - next topic

RIN67630

I have bought a couple of cheap digital sound pressure level meters and tried to hack them to provide a linear dB output, since they pretty all have ATMELS inside as well.
They are all astonishly complex.

I am not a rookie in electronics, but I did not succeed to find any usable DC signal in any of them.

The main problem is that the signals are floating, and they are NOT linear. Nowhere. Not even close to be DC.

Even a device, that provided a DC jack, turned out to supply a linear signal until a certain level that "geared down" several times without notice when the sound pressure increased, even when selecting fixed range.

A real linear output seems to be provided only by expensive devices, or by exotic devices that are not easy to get on the market.
Currently the best deal is using old SPLs that have a linear galvanometer-display (not just a logarithmic scale).

Those are easy to hack but are not easy to find on the market in quantities.

Has someone succeded and can recommend a cheap SPL that will do?


 


RIN67630

#1
Aug 11, 2017, 07:59 am Last Edit: Aug 11, 2017, 08:05 am by RIN67630
Finally, that one did the job:

http://www.ebay.com/itm/Digital-Noise-Pressure-tester-Level-Meter-30-130dB-Decibel-Sound-Measurement-XP-/112336473823
Be aware that a similar looking model exists with USB connection, which has internally a completely different design, and is unusable for our purpose.
So pay attention on the exact keys arrangement.


That sound pressure level meter delivers an analog output 0-1,1V proportional to dB (10mV/db) , that is bounded to ground, which make it easy to share the power supply with an Arduino.

It requires theoretically a 6-9V power supply and must be started by a pushbutton.

With the following hack, you may however use a plain 5 V supply and the meter will start automatically.



The green wire is soldered to the right inside contact of the DC jack.

The blue wire goes to ground, the red one goes to 5V, the green one to A0 of the Arduino.


Used with analogReferenc(INTERNAL) you get directly a 0,1 dB resolution for your sketch.


You now have got for less than 20 dollar the only perfect Arduino sound level pressure meter available on the market.

Enjoy!

Phin

Thank you! I was looking for weeks to get a somewhat reliable SPL-meter working, even teared down two other cheap ones... sadly the smaller version of this one doesn't seem to have any analog output.

Can you clarify what the actual "Hack" is to get the thing starting without a button-push? I thought closing the switch-pad area with solder on the PCB would do that trick? Anyway, does it also have the auto-standby mode?
The smaller one turns of after like 10-15 mins. A transistor could be used connecting the switch-pads and controlled by a digital-pin on the Arduino.... atleast that was my idea to avoid the standby.

RIN67630

Thank you! I was looking for weeks to get a somewhat reliable SPL-meter working, even teared down two other cheap ones... sadly the smaller version of this one doesn't seem to have any analog output.

Can you clarify what the actual "Hack" is to get the thing starting without a button-push? I thought closing the switch-pad area with solder on the PCB would do that trick? Anyway, does it also have the auto-standby mode?
The smaller one turns of after like 10-15 mins. A transistor could be used connecting the switch-pads and controlled by a digital-pin on the Arduino.... at least that was my idea to avoid the standby.
I have been looking for years ;-) and have tested more than 20 cheap SPLs...

If you use the model given (~25$) and solder on the pads indicated on the picture you are bypassing the complete power circuitry: the SPL starts automatically and stays on forever. It draws 10.5 mA on 5V.

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