Digital Audio/Video Input Selector
Do you want to hook up more than one audio/video (AV) source to your color TV? Here's an AV input expander for your TV, so don't worry. It is cheap and easy to put together. The way the c…
Know the mission before touching a wire.
Understand what you are making, prepare the right tools, and make the workbench safe.
Project details
Digital Audio/Video Input Selector is a electronics project. Do you want to hook up more than one audio/video (AV) source to your color TV? Here's an AV input expander for your TV, so don't worry. It is cheap and easy to put together. The way the c…
- Source pages
- 515-517
- 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
T1 - transistor stage 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 - transistor stage identified in the circuit; similar-looking parts are not always interchangeable.
- Confirm pinout, maximum voltage/current, dissipation, gain or forward voltage, and package.
What's this?Image, role, pros, cons, handling & specifications
LED1, LED2, LED3, LED4, LED5, LED6 - indicator LEDs identified in the circuit
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 LED1, LED2, LED3, LED4, LED5, LED6 - indicator LEDs identified in the circuit; similar-looking parts are not always interchangeable.
- Confirm dimensions, ratings, connection method, polarity, and environmental limits.
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.
Digital Audio/Video Input Selector: 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 noteDo you want to hook up more than one audio/video (AV) source to your color TV? Here's an AV input expander for your TV, so don't worry. It is cheap and easy to put together. The way the circuit works is simple and easy to understand. If 12V DC is put into the circuit, the power-on LED1 will light up. Now, press the switch S2 for a moment to make the Q0 output
of IC1 high and reset the decade counter. When LED2 lights up, it means the circuit is ready to go. With switch S1, you can choose a certain audio- video (AV) signal. Press switch S1 once to choose the first AV signal. Press switch S1 twice to choose the second AV signal. You can choose the other two signals in the same way. When switch S1 is pressed briefly once, the decade counter is started, and relay driver transistor T1 conducts to turn on relay RL1. Now, two- changeover relay RL1's normally open (N/O) contacts connect the TV's inputs to the first AV signal (marked as Video-In 1 and Audio-in 1). This is shown by the glow of LED3. When the switch S1 is pressed twice, IC1's Q2 output goes high. So, 2C/O relay RL2 (not shown in the circuit) is turned on, and the second AV signal is connected to the TV inputs (not shown in the figure). This is shown by the glow of LED4, which isn't shown in the figure. In the same way, when you press switch S1 three times, the Q3 output of IC1 goes high. So, 2C/O relay RL3 (which isn't shown in the figure) gets turned on, and the TV inputs are connected to the third AV signal source. This is shown by the glow of LED5, which is not shown in the picture.
Again, when you press switch S1 four times, IC1's Q4 output goes high. So, 2C/O relay RL4 is turned on, and the fourth AV signal source is connected to the TV inputs (marked as Video-in 4 and Audio-in 4). This is shown by the glow of LED6. When you press switch S1 again, the decade counter starts over and LED2 lights up again. After that, the cycle starts over again. The circuit is set up to accept four inputs, so the Q5 output of IC1 is connected to IC1's reset pin 15.
You built Digital Audio/Video Input Selector.
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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