The hardware in detail
Microcontroller: Teensy 4.1 with micro SD card
The Teensy 4.1 microcontroller has an ARM Cortex chip with a clock frequency of 600 MHz. We expect our program to run very quickly, as measurement time and its processing is the maxim for this project. The Teensy board also has a directly integrated micro SD card slot. Ideal for Igor and his data when measurement results need to be processed quickly. The dev board is plugged directly into our circuit board via a socket connector, so that in the event of a defect, you can simply replace the microcontroller.
Light sensor: PT334-6C phototransistor with 100K pull-down resistor
We use a phototransistor to detect the brightness on the screen (black / white). This looks like a white 5mm LED. My first thought was a standard LDR (photo resistor), but unfortunately this reacts far too slowly (25ms – 100ms), which is far too slow for our purposes. The magic component in this case is called a phototransistor! This only needs 15 microseconds to detect a difference in brightness and is therefore much better suited to our time-critical project. We don’t want the microcontroller to interpret voltage fluctuations and any interspersed EMC voltages as false brightness detection. We therefore connect the emitter of the phototransistor to GND with a 100k resistor.
Sound detection: MEMS microphone ICS 43434
A MEMS microphone (Micro Electro Mechanical System) is very small, which makes its mode of operation all the more impressive: The sound is picked up by a tiny silicon diaphragm and thus changes the distance to the “backplate”. This principle changes the capacitance and the sound can be interpreted and converted into an electrical signal. This then goes to the ASIC, which is also located inside the housing of the microphone chip, and converts this analog signal into a digital one. At first I thought that it would be better to use an analog microphone directly because of the latency. But this is not the case, as the analog-to-digital converter (ADC) would be slower than sending the signal directly via I2S. “Ei square Es” was developed by Philips to send serial and digital signals within the audio equipment. The interface itself is unidirectional, i.e. it can only be used in one signal direction.
Screen: OLED display I2C 0.96 inch
A small OLED display with a diagonal of almost one inch is relatively unspectacular and technically less impressive. Sure, it shows us information and measurement data, but apart from the operating voltage and the connection via I2C (similar to I2S but not to be confused with it!) there is not much to say here.
Visual feedback: LEDs 5mm green and red, plus 2x 220 Ohm resistors
I installed the two LEDs with series resistors so that Igor always knows whether the microphone or the button has “triggered” when measuring. The LEDs should not shine ultra-brightly, but merely give a little more than a hint. That’s why I generously chose 220 Ohm as the series resistor for both colors.
Voltage stabilization: 220µF electrolytic capacitors
Also known as “fear capacitors” among electrical engineers, I use these here for both main voltage supplies, namely 3.3V and 5V. Under unfavorable environmental variables, it could be that all sensors and actuators including the microcontroller exceed the 500mA minimum USB specification at the same time during the switch-on phase. So: Better to have than to need!
Potentiometer: 10 kiloohm linear resistor
To adjust the trigger sensitivity of the microphone, we use a simple potentiometer with a linear resistance curve. with an operating voltage of 3.3V, a resistance of 10 kiloohms is sufficient, so that we get a fairly high resolution and Igor can adjust everything sensitively.
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