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I want to implement a sound meter on a very low power wireless system but I've not had to deal with electret microphones before. The system right now draws around 5uA when in sleep mode, and wakes up occasionally to process some data from sensors. My aim is to keep the average current drain below 70uA so that the system can last ~5 years with a 3000mAh battery.

My goal is to be able to determine the sound level after feeding the signal into the microcontroller's (nRF51822) ADC/Low power comparator, which would be set to record the sound if it was above a certain level. I need assistance with finding a low power microphone/analogue front end solution.

What would be the lowest power way to implement a sound measuring feature into my system? Is what I'm looking for even possible at low power?

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  • \$\begingroup\$ "Measure sound" how? Voltage from the electret mic? Decibels? \$\endgroup\$ – DerStrom8 Jul 19 '16 at 15:03
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    \$\begingroup\$ What are you trying to achieve? \$\endgroup\$ – Eugene Sh. Jul 19 '16 at 15:03
  • \$\begingroup\$ Update the post. Essentially I want to detect sounds past a certain level using my microcontroller's ADC and an input from a microphone. \$\endgroup\$ – Sensors Jul 19 '16 at 15:09
  • \$\begingroup\$ Level of what? Energy? Decibels? Frequency? \$\endgroup\$ – Eugene Sh. Jul 19 '16 at 15:11
  • \$\begingroup\$ Amplitude of sound, so dB. \$\endgroup\$ – Sensors Jul 19 '16 at 15:16
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The input part

Electret microphone usually require a current from 100µA to 800µA (given in their specifications). This is low power, but not low power enough for your application, it seems.

MEMS microphones seem to require about the same amount of current. I found a few, however, that need low current and seem to be relatively easily available:

Another option would be to use a piezo element. Their frequency response is poor, and I am not sure about the sensitivity you'd have, but here, the advantage is obvious: they don't need power at all (it is even used to harvest energy - well, you'd need to knock the device to a table to get some useable power, though).

An option that could be considered, if the sound level to detect is constant (when present), and if you can tolerate some latency when the sound comes, would be to power the microphone bias only 1/10th of a second every second, for example. That would divide the overall consumption by 10. Of course, this depends on your application, and this may affect the mic performance.

The amplification part is less challenging, there are plenty of ultra low power opamps available.

The MCU part

To keep the power usage as low as possible when there is no sound (which is, I suppose, the case most of the time), instead of checking the sound level from the MCU software, I would use a hardware comparator.

Checking the sound level from the MCU means regularily sampling the signal through the ADC, which means the MCU must be awoken, and the ADC powered. This will use significant power, especially considering the fact the sound must be sampled at a relatively high frequency (a few kHz, depending on the maximum sound frequency you want to be able to detect).

On the other hand, a comparator can use very low power (there are some cheap ones that consume only a few µA of current) and will be constantly monitoring the sound level, triggering an interrupt and awaking the MCU only when the required sound level is actually reached. Then, from the MCU, you can sample the waveform through the ADC. So the mic needs to feed both the comparator and the MCU ADC.

In your specific case, you're using nRF51822 which has an internal low-power comparator (LPCOMP). You can use it instead of an external, additional chip. Moreover, you can choose to feed the reference input of the LPCOMP with any multiple of Vdd/8 (up to Vdd*7/8), which is very handy here, if your supply voltage is fixed.

Note that there is no need to rectify+filter the signal. It doesn't matter if you're missing each lower half of the signal wave. If the interrupt can be correctly triggered from the temporary event raised by the comparator when the upper half of the input signal exceeds the reference, it's enough.

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  • \$\begingroup\$ I believe the microcontroller I'm using has a built-in low power comparator, so I think I would probably use this to handle the wake up interrupt rather than use an additional component. How about the microphone itself though, would it draw much current? \$\endgroup\$ – Sensors Jul 19 '16 at 15:30
  • \$\begingroup\$ @Sensors: Which MC are you using? \$\endgroup\$ – zeffur Jul 19 '16 at 15:34
  • \$\begingroup\$ nRF51822, BLE SoC from Nordic Semiconductor \$\endgroup\$ – Sensors Jul 19 '16 at 15:38
  • \$\begingroup\$ @Sensors OK, I thought you already chose the microphone. I'll edit the post later to make some suggestions regarding this part. \$\endgroup\$ – dim Jul 19 '16 at 15:42
  • \$\begingroup\$ Yeah, I just realised I maybe wasn't very clear. It's the microphone side of things that I need to develop and keep low power as possible. \$\endgroup\$ – Sensors Jul 19 '16 at 16:23
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I have been involved in a project where ultrasonic impact events are measured and as it is on harvested energy, the power budget is a bit challenging.

The entire analogue front end (some signal conditioning needs to be done) consumes < 8mW and that is reduced (if you can live with the startup latency) by using an amplifier with shutdown.

The microcontroller has autonomous peripherals, so it can sense events, run the ADC and DMA the data to memory all without processor interference, so the processor only comes out of sleep to actually do something with the data, which keeps the power down.

The architecture is a (set of) low power comparators and signal conditioning chains which is fed to the microntroller ADC inputs. The comparator starts things when a signal above a predefined threshold is detected.

The average power of this system is tiny (which is just as well as it is powered from harvested energy).

Note that ST and others have jumped on the autonomous peripheral bandwagon, so there are plenty of choices out there.

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  • \$\begingroup\$ I have already selected a microcontroller for the project, which does have a low power comparator built in. The part that I'm unfamiliar with is the low-power microphone input/analogue front end. \$\endgroup\$ – Sensors Jul 19 '16 at 16:22
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Electret microphones typically draw VERY LOW power. We can easily get 1 year or more out of an ordinary AA cell. It should be pretty easy to actually measure the DC current being drawn by an electret condenser microphone capsule. IF you want the average (or peak?) Sound Pressure Level (SPL), then you will need to rectify and filter the audio waveform into a DC signal that represents the sound loudness.

There are many circuits available that show how to do that. It should not be difficult to create (or modify) a circuit to operate at low-power. Do you have a way of switching the power on/off for external circuitry (as will be required for your SPL detection?)

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  • \$\begingroup\$ Considering an AA battery can provide roughly 2400mAh, 1 year of life from that isn't nearly as low power as I need unfortunately. \$\endgroup\$ – Sensors Jul 20 '16 at 9:14

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