Electronic Combination Lock
This electronic lock with a combination of seven digits may be readily hardwired to work with any combination that you want. The circuit consists of ten push button switches, an NPN trans…
Know the mission before touching a wire.
Understand what you are making, prepare the right tools, and make the workbench safe.
Project details
Electronic Combination Lock is a electronics project. This electronic lock with a combination of seven digits may be readily hardwired to work with any combination that you want. The circuit consists of ten push button switches, an NPN trans…
- Source pages
- 152-154
- Named parts
- 2
- Build goal
- Working, tested prototype
Tools you need
- Digital multimeter
- Wire stripper and side cutters
- Soldering iron with a fine tip
- Current-limited bench supply
Use eye protection, good lighting, and a clean insulated surface throughout the build.
Safety precautions
- Disconnect every power source before changing a connection.
- Check component polarity, pinout, and supply voltage twice.
- Use a current limit for the first power-up.
- Keep liquids, loose metal, and uninsulated wires away from the bench.
Gather, identify, and understand every part.
Use the standardized inventory, then open What's this? to learn each part's role, advantages, limitations, handling, and specifications.
What's this?Image, role, pros, cons, handling & specifications
Push button
A named project component whose exact role is defined by the source circuit and build guide.
What it does hereIt performs a documented electrical, control, interface, or construction function in this project.
Buy / compare this part ↗Advantages
- Selected for this project
- Can be checked independently
- Supports modular troubleshooting
Limitations
- Substitutes may differ
- Generic names can hide variants
- Pinouts and ratings vary
Handling
- Compare the received part with the source
- Keep it labelled
- Do not force connectors or adjusters
Specifications to verify
- Use the exact model, value, package, and rating listed for Push button; similar-looking parts are not always interchangeable.
- Confirm dimensions, ratings, connection method, polarity, and environmental limits.
What's this?Image, role, pros, cons, handling & specifications
T1, T2 - transistor stages identified in the circuit
A semiconductor stage performs switching, amplification, regulation, rectification, or logic.
What it does hereIt controls current or signal flow at a defined point in the circuit.
Buy / compare this part ↗Advantages
- Fast and efficient
- Compact
- Can control larger loads from smaller signals
Limitations
- Pin order varies
- Sensitive to overvoltage and reverse polarity
- May need cooling or bias components
Handling
- Verify the datasheet pinout
- Avoid static and soldering heat
- Check notch, stripe, or flat-face orientation
Specifications to verify
- Use the exact model, value, package, and rating listed for T1, T2 - transistor stages identified in the circuit; similar-looking parts are not always interchangeable.
- Confirm pinout, maximum voltage/current, dissipation, gain or forward voltage, and package.
Connect one verified path at a time.
Explore the named components and standardized signal flow, then use Source Check to verify exact physical pins, values, and topology before applying power.
Trace this circuit.
Electronic Combination Lock: interactive parts, standardized terminals, responsive anchored wires, student explanations, and the original circuit reference in one shared system.
Trace before wiring
Follow power, ground, inputs, processing, and outputs in that order. Never guess a pin from package shape alone.
Connect with power off
Make short, labelled connections and share a common ground only where the schematic requires it.
Inspect every joint
Check continuity, polarity, adjacent shorts, and loose connections before the first power-up.
Open all source diagrams and build views 1 visuals
Confirm the hardware-only control path.
This project does not include firmware in the source. The circuit itself provides the required behaviour.
How to connect
- Match every controller label to the circuit view and source pin map.
- Join grounds before signal wires when separate low-voltage supplies are used.
- Keep motors, relays, pumps, and other loads on a suitable driver and external supply.
Common mistakes
Reversed VCC/GND, board-label versus GPIO-number confusion, missing common ground, and charge-only USB cables.
Troubleshoot
Disconnect loads, continuity-test one path at a time, then test with a current limit.
Software preparation
No IDE, board package, library, or firmware upload is required for this project.
If you add a programmable controller as an extension, document its pin map separately.
How to upload code
The original design is implemented entirely in hardware, so proceed after verifying the circuit and supply.
Assemble, deploy, test, and troubleshoot.
Use the complete source notes in build order, then pass the final checks before calling the project finished.
Build in functional stages
- Power and regulation
- Controller or processing stage
- Inputs and sensors
- Outputs and loads
- Enclosure and strain relief
Power up safely
- Inspect unpowered continuity first
- Apply the lowest safe current limit
- Measure supply rails before signals
- Add one load at a time
- Record expected and actual results
Work from simple to complex
- Confirm power, ground, polarity, and orientation
- Compare each pin with the source
- Test inputs separately from outputs
- Replace only one variable at a time
- Power off before every correction
Follow the documented instructions.
These notes come from this project's source and remain in their original order.
Project overview
Project build noteThis electronic lock with a combination of seven digits may be readily hardwired to work with any combination that you want. The circuit consists of ten push button switches, an NPN transistor, and a Johnson counter with 4 bits and a divide-by-8 operation (IC1). When the power is turned "on," an 820-kilohm resistor causes the capacitor C2 that is attached to pin 15 of IC1 to charge to a high level. This maintains the counter in the reset state. Under these circumstances, only the output on pin 2 of the counter IC1 is high, while the other pins' outputs are all low.
Fig. Electronic combination lock circuit Circuit operation When the switch S2 is pressed, transistor T1 begins to current, and capacitor C2 begins to drain by the action of diode D1 and resistor R2. This frees up the reset input for the counter. T1 is turned off and its collector is pulled high when S2 is allowed to be released; this causes a rising edge to be produced on the input clock pin 14 of the counter. Switch contact bounce can cause numerous clock pulses to be generated on pin 14 of IC1, but a simple filter comprised of capacitor C1 and resistor R3 in the base circuit of transistor T1 can prevent this from happening. The clock pulse increments IC1’s count by one, so O0 goes low and O1 goes high. Therefore, the next step is to press switch S7, as it is connected to output O1. The greatest amount of time that can pass in between switches being pressed is equal to the amount of time required for capacitor C1 to charge to a logic high level. In that case, the counter will be started over. After all of the switches have been pressed in the appropriate order (S2-S7-S3-S4-S5-S2-S2 as illustrated), the output of the counter, pin 10, will be high for approximately ten seconds. This output is sent to the driver transistor T2, which then drives the solenoid valve, so unlocking the door.
Construction & Testing
Project build noteConstruct the circuit on a standard printed circuit board (PCB), and then encase it in a plastic enclosure. Make the connection between the circuit and the solenoid valve using a flexible wire. When you are soldering, you need to be careful not to cause a short. Utilize an IC base for simplicity's sake when troubleshooting. Establish the connections necessary to operate the switches located on the lid of the plastic enclosure.
You built Electronic Combination Lock.
You followed the full workflow from understanding the mission to testing the finished project. That is a real engineering achievement - well done.
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