A common use in the system on a chip (SoC) era is to obtain an microcontroller (MCU) on a pre-assembled printed circuit board (PCB) which exposes an array of input/output (IO) pins in a header suitable to plug into a breadboard, and then to prototype a circuit which exploits one or more of the MCU's peripherals, such as general-purpose input/output (GPIO), UART/USART serial transceivers, analog-to-digital converter (ADC), digital-to-analog converter (DAC), pulse-width modulation (PWM; used in motor control), Serial Peripheral Interface (SPI), or I²C.

Firmware is then developed for the MCU to test, debug, and interact with the circuit prototype. High frequency operation is then largely confined to the SoC's PCB. In the case of high speed interconnects such as SPI and I²C, these can be debugged at a lower speed and later rewired using a different circuit assembly methodology to exploit full-speed operation. A single small SoC often provides most of these electrical interface options in a form factor barely larger than a large postage stamp, available in the American hobby market (and elsewhere) for a few dollars, allowing fairly sophisticated breadboard projects to be created at modest expense.

Transistor

First-principles (polarity given for npn transistor, reverse for pnp).

1. Polarity: The colelctor must be more positive than the emitter
2. Junctions: The base-emitter and base-collector circuits behave like diodes. A small current applied to the basecontrols a much larger current flowing between the collector and emitter.
3. Maximum ratings: Any given transistor has maximum $I_\text{C}$, $I_\text{B}$, $V_\text{CE}$ that cannot be exceeded without destroying the transistor. There are other limits including power dissipation $I_\text{C} V_\text{CE}$, temperature, and $V_\text{BE}$.
4. Current amplifier: $I_C$ is roughly proportional to $I_B$

$$I_C = h_\text{FE} I_B = \beta I_B$$

Where $\beta$ is the current gain, typically around 100. And may vary from 50 to 250.

Appendix: Resources

• Electronic components for download: https://componentsearchengine.com
• The component selection holy grail: https://pcb.mit.edu/lectures/ComponentSelection/

https://www.youtube.com/playlist?list=PLJpsOGUzkGzKMEPmRKWAJcLpKBvSGrfvR

References