LCD-LED DisplayLights and Display Board Circuits

Cable Tester Schematic Circuit Diagram

Cost-Effective Cable Tester Design

Creating an intricate cable tester with microcontrollers, often used alongside PCs, offers flexibility but can be overly complex for hobbyists. This circuit presents an affordable yet straightforward solution for testing cables with up to ten conductors. The core concept involves applying distinct voltages to each cable conductor’s end and detecting these voltages at the other end using LEDs. To generate eight reference voltages, nine LEDs (D1 to D9) connected in series serve as indicators. The first and tenth conductors connect to the power supply’s positive and negative terminals. Utilizing a constant current source eliminates the need for current-limiting series resistors. The LM317 voltage regulator, forming the constant current source, sets the current at 5 mA through resistor R1, calculated with the formula:

Iconst = 1.25 V / R1

Illuminating LED Indicators

In this setup, the transmitter end allows cables under test to connect conductors in any order. The receiver section features five LEDs, connected directly to terminal block X3, mirroring the transmitter’s LED arrangement. When a functional cable links corresponding points in both transmitter and receiver circuits, LEDs on both ends illuminate. However, if a cable fault is present, various scenarios may unfold, indicating specific issues within the cable’s conductors. This straightforward design provides clear visual cues for cable functionality, making it accessible for hobbyist use.

Cable Tester Schematic Circuit Diagram

Identifying Cable Faults: Visual Indicators

When two LEDs positioned opposite each other remain unlit, it signals crossed or shorted conductors, indicating a wiring fault within the cable.

Transmitter LED Alight: Conductor Breakage

If only the LED on the transmitter side illuminates, it signifies a broken conductor within the pair or both conductors, calling attention to potential issues in the cable’s integrity.

Even-Numbered Transmitter LED Failure: Short Between Pairs

Failure of an even-numbered transmitter LED (D2, D4, D6, or D8) indicates a short between the outer conductors of neighboring pairs, pinpointing a specific short circuit.

Multiple Unlit LEDs: Complex Cable Issues

Several neighboring LEDs failing to light indicates multiple cable problems. It could be a crossing between the first unlit LED’s corresponding conductor and the last unlit LED’s conductor or a short circuit between them, highlighting a complex wiring issue.

Button Testing for Pair Verification

Pressing buttons S1 to S5 helps identify swapped pairs. If the LED response doesn’t match on both ends, indicating incorrect LED behavior on the receiver side, it signifies the need to swap pairs.

Considerations for LED Selection and LM317 Performance

Uniform LED colors are crucial due to different forward voltage drops. Calculate R1 based on the required current, which can be adjusted if necessary. Note that these considerations exclude the power indicator LED (D15). The LM317, functioning as a constant current source, delivers the calculated current only if its input voltage exceeds the output voltage by approximately 3 V. The load voltage varies based on the number and forward voltage drop of LEDs in the transmitter, requiring a minimum of 20 V for nine red LEDs

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