A transistor lets a small input current control a much larger output current. By the end of the week you can build a circuit where a small "input" current (the kind a microcontroller pin can provide) switches a higher-current load (a buzzer or small motor).
This is the last pre-microcontroller week. Week 7 brings the Arduino R4.
Reading (~45 min)
- Horowitz and Hill, The Art of Electronics, Ch 2 §2.1 (transistors as switches). Skim the rest of Ch 2; come back to it as needed
- The kit's transistor datasheet (typically a 2N2222 or 2N3904 NPN). Find the typical Hfe (current gain) and the maximum collector current
Lecture (~1.5 hr)
- What a transistor is, simply. Three pins: base, collector, emitter. A small current into the base lets a larger current flow from collector to emitter. The ratio is the current gain (Hfe), typically 100-300 for hobby transistors
- NPN vs PNP. NPN is the default in HW-101; the conventional current flows from collector to emitter when the base is pulled high. PNP works inversely; ignore it for HW-101 unless you have a specific reason
- Why a microcontroller cannot drive a motor directly. A typical microcontroller pin can source ~20 mA. A small motor or buzzer needs hundreds of mA. The transistor bridges the gap
- The protective resistor at the base. Required. Limits the base current to a safe level. Typical value: 1 kΩ for a 5 V control signal driving a 2N2222
- The flyback diode. When you switch off a motor, the motor's coil tries to keep current flowing; that energy has to go somewhere. A reverse-biased diode across the motor protects the transistor by giving the energy a safe path. Required for inductive loads (motors, relays); not needed for resistive loads (buzzers, LEDs)
Lab exercises (~2 hr)
Lab 6.1: Transistor Switch. Build an NPN transistor switch that turns on the kit's small buzzer (or piezo) when a manual control pin is pulled high. Measure base current vs collector current; verify the gain. ~90 minutes.
Independent practice (~3 hr)
- Build a transistor-switch circuit with the buzzer replaced by the kit's small DC motor. Add the flyback diode. Verify the motor runs when controlled
- Try the same circuit without the flyback diode. Use a multimeter set to its highest voltage range to measure the spike at switch-off (it can exceed 20 V for a brief instant). This is why the flyback diode exists
- Read the 2N2222 datasheet's "saturation" specifications. Find Vce(sat), the voltage drop across the transistor when fully on. Notice it is small but nonzero; the transistor is not a perfect switch
Reflection prompts
- The transistor gives current gain: a small input current controls a larger output current. Where does the extra current come from?
- The flyback diode is required for inductive loads but not for resistive ones. Build the intuition: why does the inductor try to keep current flowing when you switch off, and why does the resistor not?
- You are about to start microcontroller work next week. List two specific things from weeks 1-6 you expect to use every week from now on
What's next
Week 7 is the bridge. You meet the Arduino R4. You write your first sketch. The built-in LED blinks. The course shifts from "build circuits with passive components" to "build circuits a small computer controls." Everything you learned in weeks 1-6 still applies.