HDD Selector Switch
This add-on circuit utilizes the computer's switch-mode power supply (SMPS) to allow for the selection of three separate hard disk drives (HDDs) and the guarantee that you’re secured HDD…
Know the mission before touching a wire.
Understand what you are making, prepare the right tools, and make the workbench safe.
Project details
HDD Selector Switch is a electronics project. This add-on circuit utilizes the computer's switch-mode power supply (SMPS) to allow for the selection of three separate hard disk drives (HDDs) and the guarantee that you’re secured HDD…
- Source pages
- 392-395
- Named parts
- 3
- 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
Power supply
A power component supplies, converts, stores, or regulates energy for the project.
What it does hereIt must provide the documented voltage, polarity, isolation, and sufficient current safely.
Buy / compare this part ↗Advantages
- Stable power improves reliability
- Current limiting protects first tests
- Regulation reduces resets and noise
Limitations
- Wrong polarity can cause immediate damage
- Underrated parts overheat
- Mains circuits require qualified supervision
Handling
- Measure output before connection
- Use a fuse or current limit
- Insulate exposed conductors
Specifications to verify
- Use the exact model, value, package, and rating listed for Power supply; similar-looking parts are not always interchangeable.
- Confirm input/output voltage, current, polarity, connector, isolation, and thermal rating.
What's this?Image, role, pros, cons, handling & specifications
CD4093
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 CD4093; 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, LED4 - 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, LED4 - 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.
HDD Selector Switch: 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 3 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 add-on circuit utilizes the computer's switch-mode power supply (SMPS) to allow for the selection of three separate hard disk drives (HDDs) and the guarantee that you’re secured HDD will remain secure at all times. It is a good way to keep hackers and spies from getting in. The block diagram of the HDD selector switch is shown in Figure 1. Within a certain amount of time (say, 20 seconds) after turning on the PC, you can choose between three different hard drives, such as "main," "net," and "backup." After the time has passed, you can't open your HDD without restarting your PC. Figure 2 shows that the HDD selector switch is made up of IC CD4093 (IC1) and a few other separate parts. The IC CD4093 has four Schmitt NAND gates with two inputs each. Pin 1 of gate N1 is connected to the positive supply through capacitor C1 and diode D1. It is also connected to the ground (GND) through resistor R1. Resistor R2 pulls pin 2 of gate N1
low. The master lock switch is also linked to pin 2 and +12V. Pin 3 of gate N1 is connected to the inputs of gates N3 and N4, which are pins 8–9 and 12–13, respectively. The outputs of gates N3 and N4 are sent to gate N2. A two-color LED (LED1) is connected to the output pins of N3 and N4 and the input pins of N2. Pins 1 and 2 make the LED light up green, while pin 4 makes it light up red. The point where pins 5, 6, 10, and 11 meet is linked to the point where switches S2 through S4 all meet. Fig. 1: Block diagram of hard disk drive selector (HDD) Through diodes and resistors, switches S2 through S4 are used to turn on the gates of SCR1 through SCR3 (for 12V supply) and SCR4 through SCR6 (for 5V supply). Three green LEDs (LED2 through LED4) are attached between the cathodes of SCR1 through SCR3 and grounds to indicate the selected HDD. When the appropriate switches (S2 through S4)
are pressed, the cathodes of SCR1 through SCR3 will get 12V and the cathodes of SCR4 through SCR6 will get 5V. How the circuit works is easy to understand. When the SMPS is first turned on and the lock switch S1 is closed, the bicolor LED1 lights up green for about 20 seconds. In this state, switches S2 through S4 can be used to connect any HDD to the power supply. For example, if you want to turn on the net HDD, you need to briefly press switch S3 within 20 seconds. This will give this HDD 5V and 12V power supplies. When LED1 turns red, switches S2 through S4 stop working, so you can't turn on any HDD. On the other hand, LED1 glows red when the SMPS is "on" and switch S1 is open. No hard drive (HDD) may be powered by connecting to the supply through switches S2–S4 in this configuration. Fig. 2: Circuit of HDD selector switch Put the circuit together on a PCB that can be used for many things and put it in a box. Since SCRs produce a lot of current, PCB tracks should be thick enough. The PCB and the wires that connect it should be made well. Install the components in the same manner as shown in the author's prototype, and
then mount the PCB on the spare drive slot cover of the CPU cabinet measuring 13.4 centimeters or 5.25 inches.
You built HDD Selector Switch.
You followed the full workflow from understanding the mission to testing the finished project. That is a real engineering achievement - well done.
Simple Key-Operated Gate Locking System
Ready to reuse what you learned in another project from the same track?
Start this project →