Circuit diagram and circuit board
The power stages at the top of the circuit diagram are actually only there to prevent voltage dips when the measurement technology is switched on. This is achieved with two 220µF electrolytic capacitors in parallel to the main voltages, here 3.3V and 5V. Strictly speaking, the 5V is not needed except for the USB bus, but I still like to carry the voltage on solder pads so that the board can be expanded later. The image of the Teensy 4.1 microcontroller (ARM Cortex M7) is prominently displayed in the center. The controller is already shown here as a finished developer board and all pins are easily accessible. At the top left we see the connection of the phototransistor. This is supplied with 3.3V at the collector and its output (emitter) is pulled to GND (low) by a 100k resistor in the non-conductive state. If the phototransistor then becomes conductive (change in brightness), the emitter passes the supply voltage on the same line to pin 14 of the controller. Below this is the potentiometer for adjusting the sensitivity of the MEMS microphone. As this type of microphone is very sensitive, a measure must be implemented to eliminate background noise or interference. The set voltage divider (0V – 3.3V) outputs a measurement voltage to pin 16.
The OLED display underneath is simply supplied with 3.3V and GND. The control is uncomplicated and fast via I2C and outputs the calculated latency values and more during operation. (Pin 18 & 19) At the bottom left is the microphone. This already sends native digital signals to the microcontroller and is also supplied with 3.3 V. A further GND line on pin 2 of the microphone indicates mono operation. DOUT_SD carries the serial data, BCLK_SCK provides the clock signal and LRCLK_WS selects the data by channel (L or R). (pins 8, 20 and 21) On the right-hand side of the microcontroller there are unspectacular features: the pushbutton for triggering the measurement calibration and two status LEDs with series resistors. These indicate whether the microphone or the button has been triggered.
After drawing the circuit diagram and then exporting it to the circuit board editor, the following airwires are created after positioning the components for the first time:
Airwires show logical links between physical components based on the circuit diagram. All yellow lines must therefore be connected and we use a simple two-layer PCB for this. If you want to see the layouting process, you can do so on Igor’s Youtube channel, I recorded it. Basically, all components should be arranged in this phase so that connections are as short and efficient as possible. Attention must also be paid to the PCB shape, electrotechnical features and the intended use. After I have laid out the PCB, the following picture emerges:
If we look at the layout, it is noticeable that there are large red and blue areas. The red area is the top copper layer (TOP), the blue area is the bottom copper layer (BOT). In this view, you can imagine that you are looking through the PCB from above. Green holes are vias and at the same time soldering areas. No solder resist has been applied here so that it can be soldered on later. I worked with THT components for this small project as I don’t have any equipment for SMD reflow soldering at the moment.
All areas, whether TOP or BOT, that do not carry signals were covered with a ground plane over the entire size of the board. This is for EMC, electrical safety and also makes PCB production slightly faster, as copper that is erased has to be etched away. Directional changes of twice 45 degrees in the cables prevent signal reflections and make it easier to trace the paths. I have routed main voltage supplies such as 3.3 V and 5 V with 0.508 mm, signal lines with 0.254 mm. With data signals, especially if they are very fast, you should pay attention to signal lines of the same length and crosstalk, but with I²C and I²S this is negligible.
I like to choose a slightly larger width for power supplies so that I can immediately identify whether they are signal lines and power supplies when I look at the layout. At the top right of the PCB we find 5V, 3.3V and GND as solder pads. If we need supply voltages and ground later on, we can easily tap them here. When the board was delivered, it looked like this:
I opted for a PCB manufacturer from the Netherlands and we can see that the work here was extremely clean. Of course, you can find the green solder resist boring, but I personally am always happy when I can physically hold a PCB that I have laid out in my hand. The workmanship is flawless, all holes, spacings and residual rings are exactly where they should be and even the small dedication to Igor at the top center was implemented very precisely. I am thrilled!
After assembling and soldering the circuit board and components on the underside, our prototype looked like this:
In the center sits our ARM Cortex and on the top from left to right we see: The 6-pin connector for the MEMS microphone, the two capacitors, the 4-pin connector for the OLED display, another 4-pin connector for the potentiometer and on the far right the solder pads for 3.3V, 5V and GND. The bottom side houses on the left: a connection for the phototransistor, all resistors and on the right the connectors for the LEDs and the calibration button.
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