Description:
The Multi Wire Cable Tester is equipped with individual LEDs for each wire. Enabling it to indicate various cable conditions, including open circuits, short circuits, reversals, earth faults, and continuity. This versatile tester achieves these functions through the use of four integrated circuits (ICs). Although initially designed for my intercom system, it can also prove valuable for testing alarm wiring, CAT 5 cables, and various other applications.
Circuit diagram
Circuit Notes:
It’s important to clarify that in this circuit, power supplies to the CMOS 4011 and CMOS 4050 ICs are not explicitly depicted for the sake of simplicity. In practice, the positive battery terminal should be connected to Pin 14 of each IC, while the negative terminal should be linked to Pin 7. For the CMOS 4017, Pin 16 and Pin 8 are used in a similar manner. Additionally, it’s worth noting that the CMOS 4050 only comprises six buffers. So to achieve the required 8 gates, two 4050 ICs are necessary. The unused inputs on these ICs should be connected to ground, specifically the battery’s negative terminal.
Circuit Description:
The circuit consists of both a transmitter and a receiver, with the cable under test connecting the two components. The transmitter essentially functions as an “LED chaser,” where the 4011 IC is configured as an astable oscillator, generating clock pulses for a 4017 decade counter divider. The arrangement of the 4017 ensures that after the 9th pulse, the count is reset. Consequently, each LED illuminates sequentially, progressing from LED 1 to LED 8 and then looping back to LED 1, creating a chasing effect. To ensure adequate current for lengthy cables and both the transmitter and receiver LEDs, each 4017 output is buffered using a 4050, which enhances the driving capabilities.
As for the receiver, it comprises eight LEDs connected in parallel, sharing a common wire. Please continue reading for more details.
Wiring the CMOS 4017
The pinout for the CMOS 4017B is shown below. Please note that in the main schematic above, alternate naming of the pins has been used. The pin equivalence is as follows:-
CP0 (clock pulse zero) is the Clock input, Pin 14 on the diagram above.
CP1 (clock pulse one) is the clock inhibit or Pin 13 on the pinout above.
MR (master reset) is the reset pin 15 in the diagram above.
Q0-9 represent the decoded decimal outputs. Hence Q0 is Pin 3 on the pinout and Q8 is Pin9.
7 Led’s 8 Wires:
This is not a typographical error. The challenge with testing each wire individually lies in the requirement for an additional common wire when using 7 individually addressable LEDs, or in the case of testing 8 wires, necessitating a ninth wire. While using a household earth connection might seem like an option, it’s not particularly practical. Furthermore, if the cable were already shorting to earth, it would render this approach ineffective.
I pondered this challenge for a while, but since this is a logic circuit, it operates based on two conditions: logic high or zero. Given that the 4017 outputs are binary, either high or low, any output can serve as a common return path for an LED.
To illustrate, LEDs 1 to 3 are connected to the 4th output of the 4017, which is in a low state (zero), and the 4th LED is configured with reverse polarity. When the 4th pulse occurs, output 4 goes high, while output 3 goes low, causing the LED to illuminate. If the common return wire is open circuit, then LEDs 1 to 4 will remain unlit. This same principle applies to outputs 5 through 8.
The common input wire can be drawn from any output terminal of the 4017, yet the same logic prevails. The convenience of rapidly testing all wires outweighs this minor drawback. When assessing a cable with only 4 or 6 wires, it must utilize the wires corresponding to LEDs numbered 1 to 4 or 1 to 6. This rationale explains the numbering scheme for the LEDs.
Testing:
In the case of a well-functioning cable with all wires properly connected, LED 1 will illuminate at both cable ends, followed by a sequential lighting of LED 2, 3, 4, and so on, up to LED 8. This sequence then repeats cyclically. However, when using a 4-wire cable, it should be connected in a manner that utilizes the common return wire, as explained in the preceding paragraph. In this scenario, the sequence would consist of LED 1, 2, 3, 4, repeating with a delay as the 4 unused outputs are cycled through.
To identify faults related to grounding, the probe labeled “to earth connection” is physically connected to a local earth. If a wire is grounding, it will cause the LED’s brightness to diminish or go out entirely at both ends of the cable. If an LED fails to illuminate at the receiver, it signifies a break or open circuit in the cable. In the event that two wires are short-circuited, for example, wires 3 and 4, the sequence at the receiver would be 1, 2, 34, 43, 5, 6, 7, 8.
Any deviation from the expected LED pattern would indicate a reversal or irregularity in the cable’s connections. For instance, consider this example: when the probe is connected to an earth source at the transmitter, and the cable exhibits extensive faults, wire 1 is functioning correctly, wire 2 is grounding, wires 3 and 5 are switched, wire 4 is functional, wire 6 is open circuit, and wires 7 and 8 are short-circuited. This scenario is illustrated below.
Test Result for Above Faulty Cable:
The transmitter pattern: …………………. The receiver pattern would be:
1 ON………………………………………1 ON
2 OFF or Faint………………………….2 OFF or faint
3 ON………………………………………3 (would show LED 5)
4 ON………………………………………4 ON
5 ON …………………………………….. 5 (would show LED 3)
6 ON ………………………………………6 OFF
7 ON …………………………………….. 7 (would show 7 & 8)
8 ON ……………………………………..8 (would show 7 & 8)
The LED sequence of course is stepped through, as you know the transmitter “pattern” it is easy to tell the state of the cable by viewing the receiver pattern. The earth condition will only show up if the contact to earth is less than 1000 ohms. A better but more time consuming method foe earth faults is to use a meter on the Megaohms range.
