kicad-learning-path

NPN Transistor Switch

A PN2222A NPN transistor that lets a push-button switch loads up to 75 mA at 5 V, with an LED indicator, a flyback diode and test points on the base and collector. Project 4 of my KiCad learning path: the transistor as a switch, the base resistor, saturation and the flyback diode.

3D render of the finished board

Overview
Key specifications
Gallery
Switching waveform and saturation curve
How the circuit works
Design calculations (summary)
Purpose of each component
Bill of materials
Netlist
Repository structure
Getting started
Testing
Verification status
Future improvements
License

Overview

Project 1 let a button drive an LED directly. A button and its contacts cannot carry much current, and a microcontroller pin can supply only a few milliamps. A transistor solves this: a small base current controls a much larger collector current, so a weak signal can switch a real load.

Part of the circuit	What it demonstrates
`Q1` with base resistor `R1`	A transistor driven hard into saturation so it acts like a closed switch
`R2`	A pull-down resistor that keeps the transistor firmly off when the button is released
`D1` and `R3`	An indicator LED that shows the switch is on, and adds a second load
`J2` and `D2`	An external load connector, and a flyback diode that protects the transistor from inductive spikes
`TP3` and `TP4`	Test points to measure the base and collector voltages

New ideas compared with Project 3:

The NPN transistor, its three regions (cutoff, active, saturation) and its datasheet limits
Choosing a base resistor with a “forced gain” of 10 so saturation is guaranteed
Switching a load on its low side
Inductive loads and the flyback diode
A net class for a net that carries real current (COLL)
A switching waveform in the documentation

The full engineering write-up (theory, derivations, tolerance analysis, KiCad configuration, layout rules, test procedure) is in docs/DESIGN_NOTES.md.

Key specifications

Parameter	Value
Supply voltage	5 V DC via `J1`
Transistor	PN2222A (NPN, TO-92), 40 V, 600 mA
Switched load at `J2`	up to 75 mA at 5 V, low-side switched
Indicator LED current	8.5 mA
Base drive current	10.4 mA (forced gain 7.6 at the 68 Ω reference load, never above 8.75)
Saturation voltages	`VCE(sat)` about 0.2 V, `VBE` about 0.9 V
Flyback clamp voltage	about 5.9 V (supply plus one diode drop)
Supply current	about 0 mA released, 19 mA pressed with no external load, about 90 mA with the 68 Ω reference load
Board layers	2
Board thickness	1.6 mm
Copper weight	1 oz (35 µm)
Board size	60 × 40 mm
Components	13 (9 circuit parts and 4 test points)
Pads / nets	25 / 6
Design tool	KiCad 10.0.x
ERC / DRC	0 errors / 0 errors

Gallery

Schematic

Full-size PDF: docs/schematic.pdf

PCB layout

3D views

Top	Bottom

Switching waveform and saturation curve

Switching waveform

Time	Event	Base (`TP3`)	Collector (`TP4`)	Load current
Before press	Button open, `R2` holds the base low	0 V	5.0 V (coil connected)	0
Button pressed	`R1` supplies 10.4 mA of base current	about 0.9 V	falls to about 0.2 V	rises with τ = L/R
Pressed, settled	Transistor saturated	about 0.9 V	about 0.2 V	68.6 mA for a 70 Ω coil
Button released	Base current stops, the transistor turns off	0 V	jumps to 5.9 V, clamped by `D2`	decays through `D2` in about 2.6 ms
After decay	Coil current has reached zero	0 V	back to 5.0 V	0

The diagram assumes a coil of 70 Ω and 100 mH, which is an illustration. The red dashed line marks what would happen without D2: the collector voltage would be limited only by breakdown.

Saturation curve

The curve shows the collector voltage against base current for a 68 Ω load. With little base current the transistor is off (cutoff). As the current rises, the collector voltage falls along a slope set by the gain (active region). Beyond a small current it flattens at about 0.2 V (saturation). This circuit drives the base with 10.4 mA, 15 to 44 times the amount needed to just reach saturation, so the switch is firmly on whatever the gain of the individual transistor.

How the circuit works

 +5V ──┬── SW1 ── R1 390 Ω ──┬── Q1 base
       │                      R2 10k to GND
       ├── R3 330 Ω ── D1 LED ──┐
       ├── J2 pin 1 (load +) ─┬─┤
       └── D2 cathode         │ ├── Q1 collector (COLL) ── J2 pin 2 (load −)
                              └─┘     D2 anode
                          Q1 emitter ── GND

Released. R2 holds the base at 0 V. Without about 0.6 V between base and emitter the transistor conducts nothing, so the LED and the external load are off.
Pressed. SW1 connects +5V to R1. About 10.4 mA flows into the base. That small current allows a much larger collector current: the LED branch (8.5 mA) and the external load (up to 75 mA) pull the collector down to about 0.2 V, so the load sees almost the full 5 V.
Released again. The base current stops and the transistor switches off. If the load is a coil (relay, solenoid, motor), its current cannot stop at once and tries to push the collector voltage up. D2 conducts and gives that current a path back to the supply, clamping the collector at about 5.9 V.

The transistor sits on the low side of the load: the load connects between +5V and the collector, and the transistor connects the load to ground.

Design calculations (summary)

Base resistor. Choose a collector current and a forced gain of 10 (the condition under which the datasheet guarantees VCE(sat)):

\[I_B = \frac{I_C}{10} = \frac{100\ \text{mA}}{10} = 10\ \text{mA} \qquad R_1 = \frac{V_{CC} - V_{BE(sat)}}{I_B} = \frac{5.0\ \text{V} - 1.0\ \text{V}}{10\ \text{mA}} = 400\ \Omega \;\rightarrow\; 390\ \Omega\]

Actual base current:

\[I_{R1} = \frac{5.0\ \text{V} - 0.9\ \text{V}}{390\ \Omega} = 10.51\ \text{mA} \qquad I_B = I_{R1} - \frac{0.9\ \text{V}}{10\ \text{k}\Omega} = 10.42\ \text{mA}\]

Collector current (68 Ω reference load plus the LED branch):

\[I_L = \frac{5.0 - 0.2}{68\ \Omega} = 70.6\ \text{mA} \qquad I_{LED} = \frac{5.0 - 2.0 - 0.2}{330\ \Omega} = 8.5\ \text{mA} \qquad I_C = 79.1\ \text{mA}\]

Forced gain: $I_C / I_B = 79.1 / 10.42 = 7.6$, below 10, so the transistor is saturated.

Quantity	Value
Worst-case forced gain (tolerances, same supply voltage)	8.75 at 4.75 V, 8.54 at 5.25 V
Largest total collector current with guaranteed saturation	10 × 8.89 mA = 88.9 mA
`Q1` power	25.2 mW (38 mW worst case), against 625 mW
`R1` power	43.1 mW (56 mW worst case), against about 125 mW
Flyback clamp	5.0 V + 0.9 V = 5.9 V, far below the transistor’s 40 V rating

The full derivation, datasheet values, tolerance cases and the flyback analysis are in docs/DESIGN_NOTES.md.

Purpose of each component

Ref	Value	Purpose
`J1`	2-pin header	Brings in 5 V (pin 1) and ground (pin 2)
`SW1`	6 mm tactile push-button	The input: closing it sends current into the base
`R1`	390 Ω, 1 %	Sets the base current to 10.4 mA, enough to saturate `Q1` with margin
`R2`	10 kΩ, 1 %	Pull-down: keeps the base at 0 V when `SW1` is open, so stray pickup below 60 µA cannot turn `Q1` on
`Q1`	PN2222A	The switch. 40 V, 600 mA, gain 100 to 300 at 150 mA
`R3`	330 Ω, 1 %	Limits the indicator LED current to 8.5 mA
`D1`	Red LED, 0805	Indicator: lights when `Q1` is on
`J2`	2-pin header	Connector for an external load (up to 75 mA at 5 V). Pin 1 is `+5V`, pin 2 is the switched side
`D2`	1N4148	Flyback diode across `J2`: carries the coil current when `Q1` turns off, and protects `Q1`
`TP1`	Test point	`+5V`
`TP2`	Test point	`GND`, the reference for all measurements
`TP3`	Test point	`BASE`: measure `VBE` and the base current through `R1`
`TP4`	Test point	`COLL`: measure `VCE(sat)` and the flyback clamp

Bill of materials

Ref	Qty	Value	Footprint	Description
`J1`	1	Conn_01x02_Pin	`PinHeader_1x02_P2.54mm_Vertical`	2-pin header, 5 V in
`J2`	1	Conn_01x02_Pin	`PinHeader_1x02_P2.54mm_Vertical`	2-pin header, external load
`SW1`	1	SW_Push	`SW_PUSH_6mm`	6 mm tactile push-button
`R1`	1	390 Ω	`R_0805_2012Metric`	Base resistor, 1 %
`R2`	1	10 kΩ	`R_0805_2012Metric`	Base pull-down, 1 %
`R3`	1	330 Ω	`R_0805_2012Metric`	LED resistor, 1 %
`Q1`	1	PN2222A	`TO-92_Inline`	NPN transistor, pin order E-B-C
`D1`	1	RED	`LED_0805_2012Metric`	Red LED, 0805
`D2`	1	1N4148	`D_DO-35_SOD27_P7.62mm_Horizontal`	Small-signal diode, flyback
`TP1` to `TP4`	4	TestPoint	`TestPoint_THTPad_D2.0mm_Drill1.0mm`	Through-hole test points

The same list is available as CSV in fabrication/bom.csv.

Netlist

Net	Connections
`+5V`	`J1`-1, `SW1`-1, `R3`-1, `J2`-1, `D2` cathode, `TP1`
`GND`	`J1`-2, `Q1` emitter, `R2`-2, `TP2`
`BTN`	`SW1`-2, `R1`-1
`BASE`	`R1`-2, `R2`-1, `Q1` base, `TP3`
`COLL`	`Q1` collector, `J2`-2, `D2` anode, `D1` cathode, `TP4`
`LEDA`	`R3`-2, `D1` anode

Repository structure

This project lives in the 04-npn-transistor-switch/ folder of the kicad-learning-path repository.

04-npn-transistor-switch/
├── README.md
├── 04_npn_transistor_switch.kicad_pro     KiCad project file
├── 04_npn_transistor_switch.kicad_sch     Schematic
├── 04_npn_transistor_switch.kicad_pcb     PCB layout
├── fabrication/
│   ├── gerbers/                           Gerber and drill files
│   ├── 04_npn_transistor_switch_gerbers.zip
│   └── bom.csv
└── docs/
    ├── DESIGN_NOTES.md                    Full engineering documentation
    ├── schematic.pdf
    ├── images/                            Waveform, saturation curve, screenshots, renders
    └── reports/                           ERC and DRC reports

Getting started

Open the project

Install KiCad 10.
Clone or download this repository.
Open 04-npn-transistor-switch/04_npn_transistor_switch.kicad_pro in KiCad. The project uses only the standard KiCad libraries.

Rebuild the manufacturing files

Open the PCB editor and run Inspect → Design Rules Checker. It should report 0 errors.
Open File → Fabrication Outputs → Gerbers and Generate Drill Files. The exact settings are listed in docs/DESIGN_NOTES.md.
Zip the output folder.

Order the PCB

Upload fabrication/04_npn_transistor_switch_gerbers.zip to a PCB fabricator. Suggested options for a low-cost fab: 2 layers, 1.6 mm, 1 oz copper, HASL finish.

Testing

Step-by-step measurements, expected values and a troubleshooting table are in docs/DESIGN_NOTES.md. In short: apply 5 V and press SW1. The LED lights, TP3 reads about 0.9 V, and TP4 falls to a few tenths of a volt. Connect a 68 Ω, 1 W resistor to J2 as a test load and check that TP4 stays below 0.3 V.

Verification status

Check	Result
Electrical Rules Check (ERC)	0 errors
Design Rules Check (DRC)	0 errors, 0 unrouted nets
Gerber viewer check	Passed
Physical board built and tested	Not yet

Reports: docs/reports/

Future improvements

Replace SW1 with a logic input so a microcontroller can drive the base (a 3.3 V signal gives a base current of about 6 mA, so R1 would need to be smaller)
Switch the same load with an N-MOSFET and compare the results (Project 12)
Add a second stage to drive larger loads
Add mounting holes

License

Released under the MIT License.

Author: Nitish Ghadage · GitHub

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