Ready-to-teach lab
Transistor switch lab: an NPN and an LED
This transistor switch lab uses one 2N2222 NPN transistor to turn an LED on and off, so students can measure the small current that does the controlling and the larger one being controlled.
The lesson
Objective and parts
By the end, students can
- Name the three terminals of an NPN transistor and the job of each.
- Measure base current and collector current and compare them.
- Explain saturation: when the transistor is fully on, the load sets the current.
Parts, already on the bench
- A 5 V supply
- One 2N2222 NPN transistor
- One LED with a 220 ohm resistor, in the collector path
- One 1 k base resistor
- A slide switch
Nothing to hand out or collect: the circuit opens built and wired.
Steps
What students do
Each step is short enough to read out. Students work in the simulator beside this page.
- Open the lab, press Run and click the slide switch. The LED turns on and off with it.
- With the switch on, press Shift+O. The DC operating point labels every net with its voltage: about 0.7 V on the base.
- Work out the base current: the voltage across the 1 k resistor, divided by 1 k. It is about 4.3 mA. Click the transistor to check it in the properties panel.
- Read the collector current in the same panel. It is the LED current, about 13 mA.
- Change the base resistor to 100 k and run it again. The base current is now about 43 microamps.
- Compare the brightness of the LED with the two base resistors.
Questions
Questions for students
Hand these out as they are. The expected answers are under the list, for you.
- Why does the base stay near 0.7 V whatever the base resistor is?
- With the 1 k base resistor, collector current is only about three times the base current. Is the gain of the transistor 3?
- What happened to the LED with the 100 k base resistor, and why?
- Why use a transistor at all, when the switch could drive the LED directly?
Answer key for the teacher
- The base and emitter form a diode. Once it conducts, its voltage barely changes with current.
- No. The transistor is saturated: it is fully on, and the LED and its resistor limit the current. The gain only shows when the base current is small.
- It dims. 43 microamps of base current is no longer enough to turn the transistor fully on, so the collector current is about the base current times the gain.
- The same circuit lets a tiny signal, such as a microcontroller pin or a sensor, control a load that needs far more current, such as a motor or a relay.
Go further
For students who finish early
- Find the largest base resistor that still keeps the LED at full brightness.
- Replace the slide switch with a pin of an Arduino and blink the LED through the transistor.
Your class
Give this lab to every student
Assign to my class puts this circuit on each student’s class page as a starter. They change their own copy and hand it in, and the work comes back to you in one list.
The class tools are free for 30 days for one class of up to 20 students, with no card. Opening and running the lab needs no account and stays free.
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