SmartCableTester

Smart Cable Tester is a modular device designed to test electrical cables. It supports testing of both short cables (both ends accessible) and long cables (with ends far apart). The device visually indicates conductor continuity using LEDs.

Table of Contents

• Overview
• Repository Structure
• Device Operation Manual
• Operating Principle
• Long Cable Tester
• Short Cable Tester
• Prototype Photos & Implementation Notes
• Possible Improvements & Future Extensions
• License

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Device Overview

The device tests cable continuity by sending signals from the Driver block to the cable conductors. LEDs in the Indicator block show which conductors are functional. For long cables, the system can be split into a remote unit and main unit to connect each end independently.

You can simulate the circuit using Falstad Circuit Simulator:

1. Open https://falstad.com/circuit/circuitjs.html
2. File -> Import From Text...
3. Paste the contents of Schematics/FalStad Scheme.txt
4. Run the simulation

Project Motivation

This project was created to explore alternative methods of cable continuity testing, especially for long or permanently installed cables (e.g. in-wall wiring), where connecting both ends to a single device is impractical.

Project Status

• Hardware: schematic-level design completed
• PCB: not yet designed
• Firmware: not required
• Testing: assembled and tested on real cables; circuit operation additionally verified using Falstad simulation

Supported Cable Types

• Multi-core cables
• In-wall cables
• Long cable runs
• Custom connectors
• Cables with more than 8 conductors (via daisy-chained shift registers)

Electrical Assumptions & Limitations

• Minimum number of conductors: 2
• One conductor is used as the return path
• High conductor count can increases current in the return line
• LED matching affects visual symmetry (optional dual-LED mode)

⚠ Safety Notes

• Do not test cables with unknown voltage present
• High conductor count may cause excessive current in the return line
• Use current-limiting resistors with appropriate power rating

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Repository Structure

├── Schematics/
│   ├── KiCad Schematics/        # KiCad project files
│   └── Falstad Scheme.txt       # Simulation for Falstad (https://falstad.com/circuit/circuitjs.html)
├── Demo/
│   ├── Driver.png               # Images of schematics for quick reference
│   ├── Indicator.png
│   ├── Root.png
│   ├── SingleIODriver.png
│   ├── Universal Cable Tester-Driver-Single IO driver.svg
│   ├── Universal Cable Tester-Driver.svg
│   ├── ...
└── README.md                   # This file

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Device Operation Manual

1) Testing Long Cables

1. Apply power to the main unit by switching the power toggle or connecting an external power supply.

2. Connect one end of the cable under test to the main unit and the other end to the removable (remote) unit.

3. Press and hold the button on the main unit enclosure (SW1 on the main Root schematic) until the first LED lights up, then release the button.
   The button may be held longer if needed; however, it is recommended to leave at least one line inactive, as it is used as the return path to the supply ground.

   Important:
   In this mode (when activating more than 4 lines at a time), the total current from all other conductors flows through the inactive line.
   Especialy when testing cables with a large number of conductors, this may cause damage to the corresponding line in the tester or in the cable itself.

4. Observe the LED switching:

   • if a specific LED does not light up, the corresponding conductor in the cable is broken;
   • if none of the LEDs light up, this means that fewer than two conductors are intact (0 or 1).

5. Repeat the test if necessary.

6. Turn off the device power.

Note:
The device can test cables containing at least two conductors.
For proper indication, at least two conductors must be intact:
one for the positive supply and one for the return path (ground).

2) Testing Cables with Both Ends Accessible Nearby

1. Apply power to the main unit by switching the power toggle or connecting an external power supply.
2. Connect both ends of the cable under test to the appropriate connectors on the device.
3. Immediately after connection, the LEDs indicating conductor continuity should light up.
4. After evaluating the results, disconnect the cable from the device.

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Operating Principle

1) Long Cable Tester

1. Low-Frequency Clock Generator

   

   The low-frequency generator based on the NE555 IC produces a clock signal used to sequentially shift bits in the shift registers;

2. Shift Register Block

   

   The 74HC595 shift registers block generates control signals for the driver stages that define the polarity of the voltage applied to the cable conductors.

   The clock signal is applied to the SRCLK and RCLK inputs from the low-frequency generator block.
   The SER input receives a signal generated by pressing the SW1 button, which is used to load bits into the shift registers.

   The outputs of the shift registers are connected to a BJT-based amplifying and inverting stage.

   When using more than one shift register, the QH' output of the previous register is connected to the SER input of the next one.
   In this configuration, the signal from the SW1 button is applied only to the SER input of the first register in the chain.

3. Driver Block

   

   The first transistor stage (Q5, Q8 in the schematic) controls the power-switching transistors (Q6, Q7), forming the output signal <Out>.

   • When a high level is present at the <In> input:

     • transistor Q5 turns on, pulling the base of the PNP transistor Q6 toward ground, which turns Q6 on;
     • simultaneously, transistor Q8 pulls the base of the NPN transistor Q7 toward ground, turning Q7 off.
       As a result, the positive supply voltage appears at the <Out> output.

   • When a low level is present at the <In> input:

     • transistor Q5 is off, the base of Q6 remains pulled up to the positive supply, and Q6 is off;
     • transistor Q8 is also off, the base of Q7 is pulled up, and Q7 turns on.
       As a result, the negative supply (ground) appears at the <Out> output.

   The <Out> output is connected to the Indicator block.

4. Indicator Block

   

   The Indicator block consists of a current-limiting resistor, LED D1 (with its anode connected to the <Out> output of the Driver block), and diode D2, connected in reverse polarity relative to the LED.

   • When a positive supply voltage is applied to the block, current flows through LED D1 toward the <Sig2> output.
   • When a negative supply voltage is applied, current flows through diode D2 toward the <Sig1> output.

   Connecting two such indicator blocks together (which is the main purpose of the detachable unit, with the cable acting as the connection between the indicator blocks) makes it possible to determine the direction of current flow.

   For example, if Sig2 of the first block is connected to Sig2 of the second block, and a positive voltage is applied to Sig1 of the first block while a negative voltage is applied to Sig1 of the second block:

   • the LED in the first block will light up, indicating that the current flows through it;
   • the LED in the second block will remain off, as the current flows through the regular diode instead of the LED.

   When the supply polarity is reversed, the LED in the second block will light up.

   The diode D2, connected in reverse polarity relative to the LED, may be replaced with an LED. This allows visual observation of the return current path to the voltage source.
   However, this configuration increases sensitivity to component parameter variations. It is recommended to use LEDs with closely matched forward voltage drops and resistors with closely matched resistance values. Even without careful matching, the device will still operate, but the visual result may differ from expectations.

   In the Indicator block:

   • Sig1 is connected to the control signal;
   • Sig2 is connected to the connector.

5. Detachable (Remote) Unit

   

   The detachable unit consists of multiple Indicator blocks connected to a connector.
   In this unit:

   • the Sig1 input of each Indicator block is connected to the connector;
   • all Sig2 outputs of the Indicator blocks are tied together.

2) Short Cable Tester

For testing short cables where both ends can be connected close to each other, the tester uses a direct LED continuity indication scheme.

• On one side of the connector, each conductor is connected to an LED with its cathode connected to the connector pin.
  The LED is connected in series with a current-limiting resistor to the positive supply voltage.

• On the opposite connector, each conductor is connected to an LED with its anode connected to the connector pin, while the cathode of each LED is connected to ground (negative supply).

When a conductor in the cable is intact, current flows from the positive supply through the LED and resistor on one connector, passes through the cable conductor, and returns to ground through the corresponding LED on the opposite connector.
As a result, the LEDs associated with intact conductors light up, indicating continuity.

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Prototype Photos & Implementation Notes

The images below show a physically assembled and tested prototype of the Smart Cable Tester.

The prototype was built using through-hole components and modular wiring, without a dedicated printed circuit board (PCB). As a result, the final device occupies significantly more physical space than would be required in a PCB-based implementation.

This approach was intentionally chosen to:

• quickly validate the electrical concept;
• test the operating principle on real cables;
• allow easy modification during development.

Implementation Limitations

• No custom PCB was designed prior to assembly
• Through-hole (non-SMD) components were used
• Internal wiring is point-to-point and not optimized
• The enclosure is purely functional

Due to these factors:

• the device is larger than necessary;
• mechanical layout is not optimized;
• cable routing and component placement are not ideal.

With a dedicated PCB and SMD components, the overall size could be reduced significantly. Additionally, the enclosure could be made much more compact and visually clean using a custom case, for example manufactured with a 3D printer.

Assembled Prototype

Possible Improvements & Future Extensions

The current design focuses on simplicity, robustness, and minimal component count. However, the architecture allows for multiple functional extensions.

1) Microcontroller Integration

A microcontroller (e.g. STM32, AVR, ESP32) can be added to extend functionality:

• Resistance measurement of individual conductors
• Detection of high-resistance or partially damaged lines
• Automatic continuity testing without manual button control
• Measurement of voltage drop across long cable runs

This would require:

• Analog multiplexers or ADC inputs
• Known reference resistors
• Controlled current injection into cable lines

2) Crossed / Swapped Conductor Detection

By sequentially driving conductors and monitoring responses on other lines, the system could automatically determine:

• Conductor-to-conductor mapping
• Crossed wires
• Swapped pairs
• Open or shorted combinations

This would allow:

• Automatic pinout detection
• Cable type recognition
• Verification of patch cables and custom wiring

3) Digital Interface and Data Output

Adding a digital interface would enable:

• USB or UART output to a PC
• Test result logging
• Automated testing scripts
• Integration with production or service tools

4) PCB Design and Mechanical Improvements

• Dedicated PCB for main and remote units
• Connector standardization
• ESD and overvoltage protection
• Enclosure design for field use

These extensions can be implemented incrementally without changing the core operating principle of the tester.

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LICENCE

MIT License Licence