Simplest way of sensing if an led diode is on or off.

Thread Starter

bthovda

Joined Jun 11, 2020
2
Hi.

I wonder if anybody can tell me the easiest way of sensing if a led-diode is on or off.
I need the led as a binary sensor in an home automation project.

What I want is a potential free output. that i can set up as a normally closed (NC) or open (NO) sensor.

Br,
Bjoern
 

Alec_t

Joined Sep 17, 2013
11,416
Welcome to AAC!
You could pass the LED current through the base junction of a transistor to switch the transistor on, and use the transistor to drive a relay.
 

SamR

Joined Mar 19, 2019
2,259
The LED is a visual indicator used in conjunction with or instead of a buzzer which is an auditory indicator. You state you want a binary input? Do you have a circuit in mind? To do what? And Welcome to AAC!
 

DickCappels

Joined Aug 21, 2008
6,461
Or, if desperate, just point a phototransistor at or attach a phototransistor to the LED and monitor the collector-emitter current. That is the least invasive solution for existing installations.
 

Thread Starter

bthovda

Joined Jun 11, 2020
2
Thank you for all the answers! Great! Yeah, what I want is a interface to an existing circuit. I will look into your suggestions. Thank you again!
 

dl324

Joined Mar 30, 2015
10,998
Thank you for all the answers! Great! Yeah, what I want is a interface to an existing circuit. I will look into your suggestions. Thank you again!
You still haven't done a very good job of defining the problem or solution space.

  1. Do you have access to both terminals of the LED?
  2. Is it being driven high side or low side?
  3. Is it on continuously or driven AC (e.g. PWM)?
  4. Do you want a solution that doesn't involve any physical modifications to the circuit?
  5. If you want a non-contact situation, in addition to specifying whether the LED is on continuously (or blinks so fast that it appears to be on all the time), you need to give constraints on positioning an optical sensor.
  6. Etc...
 
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