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Can someone tell me on what how to detect a short circuit and LED open circuit for the below circuit?

Is it possible to detect which LED?

Could someone provide a circuit which would help to detect an LED short circuit or LED open circuit if all the below LEDs are connected in series as a single string?

I just require a circuit which will detect LED Failure (LED open or LED Short) when the LEDs are working during its normal operation.

enter image description here

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  • \$\begingroup\$ My first thought is to use the following: 74HC244; 74HCT244 Octal buffer/line driver; 3-state:assets.nexperia.com/documents/data-sheet/74HC_HCT244.pdf.TIP30x can be inputs to the buffer. \$\endgroup\$
    – tlfong01
    Apr 2, 2021 at 13:43
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    \$\begingroup\$ @tlfong01 , the link is not working. Could you tell me how to use this IC and how will it help to detect the open and short condition? \$\endgroup\$
    – user220456
    Apr 2, 2021 at 14:38
  • \$\begingroup\$ What does "help to detect" mean here? How do you expect the "circuit" is going to inform you that an LED has failed? Exactly what information do you expect the circuit to provide, and in what form? \$\endgroup\$ Apr 2, 2021 at 17:03
  • \$\begingroup\$ You didnt mention what is desired output of watch circuit (0V ok and 5V fail, some Led visualization?) Simplest open Led detection is cross the Led with D+Led, or cross with blue. \$\endgroup\$
    – user208862
    Apr 2, 2021 at 17:06
  • \$\begingroup\$ I just need some indication. Like assume I have some microcontroller pins according to the number of LEDs. So, I need a circuit that will convey the Open or short to the Microcontroller \$\endgroup\$
    – user220456
    Apr 2, 2021 at 17:18

2 Answers 2

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Connect the black (COM) lead of your voltmeter to ground. Now, connect the red (+V) lead to each of the test points, TP7301 through TP7303, in turn. If the voltage you read is equal to BG_SUPPLY then the LED is open, if the voltage you read is zero then the LED is shorted. Since each test point corresponds to a single LED, it is clear which of the LEDs has failed.

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    \$\begingroup\$ @Newbie "detect [...] if all the belpw LEDs are connected in series" - the same should work if the LEDs are connected in series, so you would read BG_SUPPLY on open and 0V on closed LEDs. You have to place TPs between all the LEDs. \$\endgroup\$
    – Sim Son
    Apr 2, 2021 at 13:50
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    \$\begingroup\$ @Elliot Alderson, No, I am asking like, is there a circuit which can detect the LED OPEN OR SHORT during the circuit working conditions. \$\endgroup\$
    – user220456
    Apr 2, 2021 at 14:18
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    \$\begingroup\$ @Newbie You need to edit your question to clarify what you want. How exactly is this "circuit" supposed to let you know if an LED has failed? \$\endgroup\$ Apr 2, 2021 at 16:51
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    \$\begingroup\$ Sure, sorry, let me edit my question \$\endgroup\$
    – user220456
    Apr 2, 2021 at 16:53
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    \$\begingroup\$ I've edited my question \$\endgroup\$
    – user220456
    Apr 2, 2021 at 16:56
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Question

How to design an alert system to do the following:

  1. Detect if any one or more of the LEDs below are open or short circuited,

  2. Report the bad LEDs, if any, and whwther they are open or shorted.


led detect 1


Answer

  1. To make things as simple as possible, the OP's specific circuit is abstracted and generalized below.

led detect 2


  1. But the OP's specific LED is not going to be generalized, but would be studied in detail, because the LED's specific I-V characteristic/parameters are critical in the circuit design.

led spec


LED Spec summary and assumption

The design parameters used would be the following:

2.1 Forward LED current I = 10mA

2.2 Forward LED voltage V = 3.1V

Power LED Lamps are usually of the power 1W or 3V, and current 350mA to over 1A. So I assume that the OP's LEDs are just typical indicator/status LEDs, though not in classical Red, but fashionable White.

I made another assumption that the Vcc is = +5V, as can be inferred from the current of limiting resistor of value 1k2.


  1. What voltage level at the test points T1, .. T8 will be if a LED is open or shorted? This question is too simple, so I won't bother to explain, or my reputation would be damaged (But also see Appendix C )). Anyway, the following is a summary.

    3.1 If a LED is open, corresponding test point would show approx 5V.

    3.2 If a LED is shorted, it will be approx 0V.

    In short, simply 5V TTL logic.


  1. So the OP's problem boils down to simply the following:

    How to read 7 TTL 5V logic High Low signals using 5V Arduino or 3V3 Rpi?

    4.1 For 5V Arduio, it is a piece of cake: Just use 7 Arduino GPIO pins to direct read the High/Low levels at the 7 test points, and day is done.

    4.2 For 3V3 Rpi (or similar 3V3 guys), a convert/shift down chip is need to setp down the 7 test points logical level to 5v to 3V3 (See Appendix A for more details). Then the 7 shifted down test point signals can direct connect to Rpi to read.

    4.3 For either Arduino or Rpi, if there are not enough GPIO pins to go around, the OP can consider using GPIO pins extenders such are MCP23008 (See Appendix B for more details.)


References

(1) LED VCEW1151CDS-3BZH3 Product Sheet - Stanley

(2) CD4050 Hex Inverting Buffer and Converter - TI


/ to continue, ...


Appendices

Appendix A - Logical level signal up/down converter/shifters

For newbies, I would recommend the CD4050 Non-inverting Buffer/Logic Level down Converter.

There are many other logic level converts around, eg TBX/TSX 0102/0104/0106/0108.

Open drain NPN BJT, eg. 2N2222, MMPQ2222, HC03 Quad NAND logic ICs can also be used 3V3/5V0 up/down converters/shifters.

/ to continue, ...


Appendix B - GPIO Pins Expanders

Common GPIO pins expanders are I2C/SPI MCP3008/30017/3s08/30S17. But their learning curves are too deep for newbies.

For newbies, I would recommend I2C PCF8574 Remote 8-Bit I/O Expander, but the learning curve is still a bit high for newbies.

/ to continue, ...


Appendix C - Two Level or Three Level Logic Actually?

Let us look at the LED active/open/short voltage levels.

  1. If the LED is open, no current can flow through the current limiting resistor (1k2), there is no voltage drop from Vcc = 5V. So the voltage at the test point is ~=5V.

  2. If the LED is short circuited, the voltage across the LED is zero, so the test point is at the same level or ground, also ~= 0V.

  3. But there is a catch here. At normal operation, according to the spec, current is around 10mA, and voltage across the LED is ~= 3.1V ~=3V

In other words, we have the Three Level Logic here: 0V, 3.3V, 5V. But GPIO pins can only detect two levels, Low level ~= 0V indicating LED is short, but High level may mean 3V ~ 5V. In other words, GPIO pin cannot differentiate between LED in normal operation, and open.

What should we do now? Comments welcome! :)

Hint: perhaps we might consider opAMP comparator, or Schmitt logic gate (eg, HC14 Schmitt inverter)


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