Lab 1: Transistor Switching

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Before the lab

Purpose and workflow

Your objective

Use a BC547 NPN transistor as an inverting switch, measure its voltage-transfer characteristic (VTC), and measure its switching response time.

Build

Construct and check two transistor-switching circuits.

Measure

Capture static transfer and dynamic timing behaviour.

Explain

Connect cutoff, transition, and saturation to digital inversion.

Bring and locate

Components and leads	Bench equipment
Breadboard, jumper wires, banana leads, alligator clips, BNC-to-alligator lead, BNC-to-probe lead	ABRA AB-3300 DC supply, GW INSTEK GFG-8216A function generator, RIGOL DS1052E oscilloscope, BK Precision 2831D DMM
BC547 NPN transistor; one red and one green LED; two 390 Ω, one 220 Ω, and one 1 kΩ resistor	Instructor-approved component information and the circuit figures below

I have identified the required components and bench instruments.

Safety first

Make the bench safe before wiring

Power off before changes.

Disconnect the DC supply and function generator before inserting, removing, or moving any wire or component. Your instructor must check each newly built circuit before you apply power.

Wear closed-toe shoes and safety glasses; remove dangling metal or jewellery. Know the exits, emergency power shutoff, and extinguisher location.
Keep the bench clear. Do not open or relocate equipment. Do not use damaged leads or instruments; report them.
Check LED polarity and the exact BC547 pinout before insertion. Avoid touching component leads unnecessarily.
Use a common circuit reference: DC supply ground, function-generator ground, and oscilloscope probe grounds must connect to the circuit ground shown. Never clip a grounded scope lead to an arbitrary powered node.
Avoid shorts and stay within component ratings. If anything heats, smells, or smokes, turn off supplies, disconnect sources, and tell the instructor.

Need a refresher?
→ Basic Laboratory Safety

Configure sources while disconnected

DC supply

Use an adjustable protected output—not the fixed 5 V/3 A terminals. Set independent mode, 5 V, and a current limit near 100–200 mA. Begin at the lowest current and raise it only until constant-current indication stops. Verify voltage with the DMM.

Function generator

Select the required waveform, frequency range, amplitude, and offset while disconnected. Verify the actual signal on the oscilloscope before attaching it to the transistor circuit.

What is a short circuit, how can you prevent one here, and what should you do if one occurs? Power is off, source outputs are verified, grounds are planned, and I will request an instructor check before power-up.

Preparation

Identify parts before insertion

BC547 flat-face package drawing labelled collector, base, and emitter from left to right — BC547 transistor pin identification. Verify the marking and datasheet for your exact part.

LED drawing showing the longer anode lead and shorter cathode lead beside the flat package edge — LED polarity cues: longer lead is usually the anode; the flat edge is nearest the cathode.

Do not assume all NPN packages share a pin order.

Read the marking on your transistor, find the approved datasheet for that exact part, and confirm collector, base, and emitter. For the BC547 shown here, the flat face toward you is C–B–E from left to right; verify the installed part before wiring.

Part	Check before use
390 Ω ×2, 220 Ω, 1 kΩ	Read the colour code or measure resistance; use ¼ W parts and remain below their rating.
Red and green LEDs	Identify anode/cathode; a series resistor is mandatory.
BC547	Record the full marking and confirm C, B, E from approved component information.
Breadboard	Trace connected rows and separated power rails with power off.

Need refreshers?
→ Breadboard Fundamentals · → Convert resistor colour bands to a resistance value · → BC547 Reference

I verified transistor pinout, LED polarity, resistor values, and breadboard connectivity.

Experiment 1

LED transistor switching circuit

Generator settings

Set a sine wave at 5 V peak-to-peak, 0 V DC offset, and 1 Hz. Verify all three settings directly on the oscilloscope before connection.

Power off: build the circuit exactly as shown. Join all source and instrument grounds at circuit ground.
Connect oscilloscope Channel 1 to the function-generator input node and Channel 2 to the transistor output/collector node. Set each probe attenuation correctly in both probe and scope menus.
Configure and verify the generator while disconnected: sine, 5 Vpp, 0 V offset, 1 Hz.
Ask the instructor to inspect the wiring. Then apply 5 V DC and the verified input.
Display one or two complete cycles. Use Measure or the graticule to obtain input period and peak-to-peak voltage. Observe both LEDs over several cycles.

Need refreshers?
→ Using a Function Generator · → Using an Oscilloscope

Oscilloscope capture: input and outputNo file selected.Select a clear image/PDF showing both channel traces, time scale, voltage scales, and at least one complete cycle. Selected here for the local submission ZIP; it is not uploaded.

Measured input period (s)Measured input Vpp (V)Observed LED blink frequency (Hz)Relationship between LED states

Which single-input logic-gate behaviour does this circuit simulate? Briefly justify from your observation.

Experiment 2 · static

Measure the voltage-transfer characteristic

Common-emitter BC547 circuit with input through 1 kilohm base resistor, emitter grounded, and collector output pulled to 5 volts through 220 ohms — Simple transistor switching circuit used for the VTC and timing work.

Power off and replace the LED circuit with the simple circuit shown. Ask the instructor to check it before applying power.
Use the DMM to measure actual input voltage V_i and output voltage V_o.
Starting at 0.0 V, adjust the DC input through 5.0 V in 0.1 V setpoint steps. At every step, record the actual input and output. Approach each setting deliberately; do not rewire live.
Plot V_o versus V_i in Excel. Label axes and units and use a scatter plot that preserves numeric spacing.
From the plot, estimate V_OH≈V_max and V_OL≈V_min. Find V_M where the curve meets V_o=V_i; draw the tangent at that point and extend it to the high and low output levels to estimate V_IL and V_IH.

Need refreshers?
→ Using a Digital Multimeter · → Voltage Transfer Characteristic

Before sweeping, describe another simple method that could obtain the transfer curve; discuss it with your instructor.

VTC measurements

Sweep V_i from 0.0 V to 5.0 V in 0.1 V increments. At each step, record the actual V_i and measured V_o in volts.

Target input	Actual `V_i` (V)	`V_o` (V)

VTC plotNo file selected.Select the labelled Excel plot or exported image/PDF. Selected here for the local ZIP; it is not submitted to Avenue by this page.

V_OH (V)V_OL (V)V_M (V)V_IL (V)V_IH (V)

Experiment 2 · dynamic

Measure switching response time

Keep the simple transistor circuit. With its input disconnected, configure a square wave at 5 Vpp, 0 V offset, and 1 kHz; verify it on the oscilloscope.
Connect Channel 1 to V_i and Channel 2 to V_o. Use a shared ground and display both traces.
Increase frequency until the output rise and fall intervals are clearly visible. Adjust time/div and volts/div without hiding either transition.
Measure output rise time t_R, fall time t_F, high-to-low propagation delay t_PHL, and low-to-high propagation delay t_PLH using consistent threshold points.
Save a scope snapshot that shows both signals, scales, and the transitions used.

Timing diagram with input and inverted output waveforms, showing propagation delays plus output rise and fall intervals — Switching-time measurement definitions for propagation delay, rise time, and fall time.

Need a refresher?
→ Rise, Fall, and Propagation Time

Frequency where transitions were measured (Hz)

Measurement	Value	Unit
Rise time, t_R
Fall time, t_F
High-to-low delay, t_PHL
Low-to-high delay, t_PLH

Switching-timing oscilloscope captureNo file selected.Select a clear capture showing both traces, scales, frequency, and the transitions/cursors used. It remains local until included in the ZIP.

Analysis

Think it through

Answer from your own circuit, plots, and waveforms. Explain relationships rather than restating measurements.

1. Use the transistor operating state and measured input/output behaviour to explain why the LED circuit acts as an inverter. 2. What does the shape of your VTC reveal about the usable LOW, transition, and HIGH input regions, and why is the transition region undesirable for a digital input? 3. Your output did not switch instantaneously. Relate the measured rise, fall, and propagation times to what a following digital circuit would observe. Instructor interaction record

Completion

Review and create the local submission ZIP

Keep the actions distinct

Completing this manual saves responses in this browser. Selecting evidence adds live files to the next locally generated ZIP. Neither action uploads to Avenue. Follow your instructor’s current direction for submission.

Confirm the instructor checked circuits before power and discussed your alternate VTC method.
Confirm the three evidence files clearly show the requested milestone.
Download the ZIP in this same session; after reload, evidence files must be reselected.

I reviewed my measurements, analysis, evidence, and instructor interactions.

Submission package

Download your Lab 1 submission ZIP

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Submission Details

Enter all four required details before downloading: Student Names, Student Numbers, Lab Section, and Lab Instructor.

Student NamesStudent NumbersLab SectionLab Instructor

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