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I have a LED in circuit which I want a circuit to detect whether it's on or not.

I have found ground and +ve of the circuit.

And I checked the voltage across +ve of LED and Ground, when it is ON, it came 0.36V

Now trouble is transistor need 0.7V to trigger, and Op-Amp Comparator(LM358) doesn't seems to work for this, also not sure why I can't detect this voltage on ADC of Arduino.

Please tell me if there is a way to clamp this voltage so that it trigger a transistor, or if there is other way to detect that LED.

Please help me on this !

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closed as unclear what you're asking by Trevor_G, Leon Heller, RoyC, TonyM, Finbarr Mar 7 '18 at 9:38

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ Is it just me...........? Lost! \$\endgroup\$ – Trevor_G Mar 6 '18 at 18:24
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    \$\begingroup\$ Please add a schematic to show how your LED is being driven. It's unlikely to be lighting up with only 0.36V across it so you have something wrong. \$\endgroup\$ – Finbarr Mar 6 '18 at 18:27
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    \$\begingroup\$ Visible LEDs need more than 1.5V to turn on, so there is something wrong with your measurement or interpretation. Measure carefully and use the Circuitlab function to post a proper partial schematic. \$\endgroup\$ – Spehro Pefhany Mar 6 '18 at 18:27
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    \$\begingroup\$ !!!!!!!!!!! danger !!!!!!!!!!!!! ... the control board in a washing machine may have full line voltage on it 120V/220V AC \$\endgroup\$ – jsotola Mar 6 '18 at 19:43
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    \$\begingroup\$ I assume "won't" should be "want" \$\endgroup\$ – Oldfart Mar 6 '18 at 19:49
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Optically couple a photodiode to each LED you want to monitor. Interface the photodiodes to your arduino. You will then no longer be concerned with what voltages exist under the thick mask covering the circuit board, and you and your arduino will be isolated from potentially lethal voltage.

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  • \$\begingroup\$ Yes that's the first thing I tried, but as it did not trigger Optocoupler, and when I checked the voltage across the LED I was getting 0.36V, what could I be doing wrong ? \$\endgroup\$ – Vidit Katlana Mar 7 '18 at 6:04
  • \$\begingroup\$ The suggestion is to use a photodiode to detect the light emitted by the existing LED. With this, no electrical connection to the board is required. It doesn't matter what voltage is on the board. \$\endgroup\$ – AlmostDone Mar 7 '18 at 13:31
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You are doing something wrong. An LED does not have a 0.7v forward voltage like that of a signal diode.

Red, orange, amber, and yellow will have a forward voltage of about 2 volts. White, blue and green will have a forward voltage of about 3 volts.

Without a schematic, no one can help you much, post one.

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  • \$\begingroup\$ Not sure how I can take out schematic out of that PCB, that too without continuity test available :( \$\endgroup\$ – Vidit Katlana Mar 7 '18 at 6:05
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If the LED does not emit a wavelength visible to the human eye. Just like the infrared LED used in remote controls. You can use a cell phone camera to make sure it's on.

Remember that when measuring the electronic component connected to a circuit, consideration should be given to the voltage drop on the components in parallel.

It is important to check:

=> the nominal input voltage of the circuit is correct. => if the pin of the integrated component output (search in data sheet) of the component is able to power the LED. Meanwhile I recommend using a separate trigger circuit if you are developing. => look for other power losses in the circuit.

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  • \$\begingroup\$ So that means due to parallel circuitry to LED I might be getting wrong voltage ? \$\endgroup\$ – Vidit Katlana Mar 7 '18 at 6:07

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