microPAM-T4 hardware#

Bill of Materials#

As of 24-Feb-2023 the microPAM-V1 consist of the following components

Component

Source

Product

Price

Config

MCU: Teensy 4.1

PJRC

Teensy 4.1_NE

29.60 USD

1+2+3

Microphone: I2S MEMS

Adafruit

Product ID: 3421

6.95 USD

1+2+3

Battery: 2200 mA LiIon

Adafruit

Product ID: 1781

9.95 USD

1

LiIon charger

Adafruit

Product ID: 2124

4.95 USD

1

USB Powerpack

TBD

TBD

TBD

1+2

3V Coin cell holder

Adafruit

PRODUCT ID: 4856

0.95 USD

2+3

5V DC/DC converter

Pololu

DV24V10F5

9.95 USD

3

Wiring#

Thre are different hardware setup possibles, depending on availability of power sources. They differ only by the way power is provided.

1) Reference setup#

The reference setup is a self-contained PAM system, the endurance of which can be extended woth an external 5V power source, e.g. a 5V powerbank.

microPAM

The image shows how the Teensy4.1 is connected to the I2S microphone to the right (shown upside down to see the pin labels), to the LiIon charger to the left and a LiIon battery on the top. Not shown is the USB cable that connects to the Teensy on the left and the uSD card that inserts to the Teensy SD card holder on the right.

The LiIon charger gets the the 5V from the Teensy USB pin and provides 3.7 to 4.1 V to the Vin pin of the Teensy 4.1. For this to work, the trace between the VUSB and the Vin Pin (on the backside of the Board) must be cut as shown in next image.

Cut

As soon as a USB cable is connected to a PC or other external 5V source (e.g. Powerbank) the internal LiIon battery will be charged.

When a UBS cable is connected to a PC it is further possible to interact with the acquisition program, listen to the microphone, and offload acquired data.

The I2S MEMS microphone has a selection (SEL) pin that can be used to decide if microphone is connected to the left or right channel. In the above image, the (SEL) pin is connected to ground (GND) an the microphone shows up on the left channel. To connect the microphone to the right channel, the (SEL) pin must be connected to (3V). When the (SEL) pin is not connected the microphone will appear on the left channel, that is, in the above image, the connection (SEL) to (GND) can be left out.

To record sound in Stereo, two I2S MEMs can be used, with the (SEL) pin of on microphone connected to (3V) and the (SEL) pin of the other microphone connected to ground (GND) (or left unconnected).

2) Minimalistic setup#

While the above reference configuration provides a complete PAM sytem, even without external 5V Powerbank, the following wiring describes the minimalistic setup

microPAM_minimal

The absence of the LiIon battery requires an 3V Coin cell attached to VBAT to provide power to the Real Time Clock (RTC) when the external 5V Powerbank is removed. The absence of the LiIon battery requires also that the VUSB - VIN trace is NOT cut as described above, but should be kept connected.

When using the minimalistic setup, then everytime the 5V power is removed, the Teensy looses power and stops working and when 5V power is applied again, the Teensy boots and starts recording. This type of hehaviour must be handled in the software.

3) Arbitrary power source#

In case that no external 5V USB power source is available, but say a 6 to 24 V battery (e.g. descrete D-cells or multiple LiIon 18650 batteries) then the following wiring may be possible.

microPAM_external

The additional component is a DC/DC converter with 5V output, that is connected to the Teensy (Vin) pin. As there is voltage provided to (Vin) and via USB cable, the trace Vin - VUSB on the backside of the Teensy must be cut as shown above for configuration #1.

Implementations#

In the following are some example implementations

microPAM_components

and

microPAM_tube

The tubes are 25 mm electro installation tubes about 21 cm long. The left and right tube isolation caps are still missing.

Comments on I2S Microphones#

So far the microPAM harware uses the Adafruit I2S Mems microphone as it is reasonable priced and available from a respected Source. However, this model uses a Knowles SPH0645LM4H microphone that has some ‘features’.

First, while advertised as a 24 bit device, the lower 6 bits are set to zero, limiting the dynamic range. As we expect that the lower 4 to 6 bits are anyway noise, this may for most application not an issue.

Second, while the data should be signed (2’s compliment) there seems to be a significant negative offset (top 5 bits of 24 were set to 1)

Alternative#

A well known alternative the the Knowles microphone is the Invensense ICS-43434 I2S Digital Microphone, which does not show the ‘features’ of the Knowles device.

https://www.tindie.com/products/onehorse/ics43434-i2s-digital-microphone/ sells this type of I2S microphone for $ 11.95.

Example of a real implementation#

The following picture shows an implementation that was fielded succesfully by Katrina Wilcox, Master’s student at University of Manitoba to record sea bird chicks in the wild.

microPAM_real

The minimalistic setup consists of Teensy 4.1, a MEMS microphone, a RTC battery and a powerbank, all contained in a small plastic food container with air/water tight snap-lock lids. This example shows that the recorder can be implemented ad-hoc by a scientist with little or basic soldering skills.