Nominal Animal

(I did create my own single and dual 12V PWM fan controllers using ATtiny85 AVR MCUs (ATTINY85-20PU). Both the schematics and the board files are in Public Domain / CC0-1.0, but I haven't written the firmware yet; I'll use the dual one for my DIY...

The PWM signal is open drain/collector; I recommend using a BSS138 or NX138AKR N-channel enhancement mode logic level MOSFET, with source to ground, drain to the fourth pin of the 4-pin fan connector (MOLEX 47053-1000), and gate to a 100Ω – 220Ω...

There is no signed hexadecimal format support that I know of. However, it is simple to implement: if ((int16_t)value < 0) { display.print("-"); display.print((uint16_t)(-(int16_t)value), HEX); } else {...

The while (!Serial) ; line in your setup() function waits until an application opens the USB serial port. Since wall warts provide only power, your Teensy will wait for that to happen forever. So, just remove that line.

I like to use I²C or SPI digital-to-analog converters myself. For example, MCP4725 is a single 12-bit I²C ratiometric DAC (output depending on the supply voltage, no separate reference voltage) in SOT23-6, small but hand-solderable, with one I²C...

I agree with MarkT. If you use the larger packages like TO-220-3 with a suitable heatsink (but consider what the potential on the tab is, and whether you need an isolating "silpad" or mica sheet in between; Mouser/Digikey/etc. sell these and...

But the TLV75733P on the Teensy has an absolute maximum input voltage of only 6V, so it is that linear regulator that may go bork if a voltage spike occurs. Let's say you have buck regulator outputting 6V, with at least 4.7µF capacitance on the...

I would use a buck to 7V-9V, and a robust linear regulator down to 5V, with datasheet-suggested capacitors before and after. DC-DC converters can have issues like overshoot at very low loads or overshoot spikes at certain fault conditions...