# How can I indicate that an IR LED is functioning without detecting its emissions?

I have salvaged, non-documented, IR LEDs. Without a datasheet I do not know how much current they can handle.

I have run into this before: I confirm the LED is good with by viewing it through a camera. I set up my circuit and begin troubleshooting. Only after eliminating all other possibilities, do I resort to a camera to discover that along the way the IR LED stopped emitting.

It would be great if I could be confident that I am using the right resistor but even then accidents happen. What I would like is a simple indicator that the IR LED is functioning without detecting its emissions.

I would prefer an "idiot" light such as a visible spectrum LED that is lit when the IR is functioning properly and can be relied on to go dark should the IR LED fail.

I was thinking that I could trigger this from a voltage drop using a transistor, pull-down resistor, and a variable pot that I could tune to toggle my visual indicator.

Is this feasible?

• Show LED and where it came from – Tony Stewart EE75 Jun 19 '18 at 20:03
• Yes, this can be done. Measure the voltage on the LED. It will be at the normal forward voltage of an IR LED when the LED is good, and at the supply voltage when it is burned out. Limit the current to something safe when you are fiddling with things. That means a higher value series resistor. You can lower the series resistor value later to get more power - at the risk of burning out the LED. – JRE Jun 19 '18 at 20:06
• if your voltage supply is high enough, why not adding a visible light LED in series with your IR LED? – MAB Jun 19 '18 at 20:08
• When Vf(@If)- V(t) rises due to temperature rise -4mV/‘C you can anticipate failure if you sense current, but better if you regulate current accurately. – Tony Stewart EE75 Jun 19 '18 at 20:12

Rig up a photosensitive resistor - they usually are sensitive far into the IR, and, as a bonus also into the UV. they have huge resistance without light, so you can use a battery without depleting it too fast. Another, normal resistor in series with that, to cap the maximum current to a few mA, and then an old school mA-indicator. Put the photoresistor at the end of a shiny (aluminium?) tube, and stick the LEDS you are testin into that (or, if they are SMD, put the tube over them, you'll have to isolate the tube then, though)

• I like the IR Indicator Card link from the post below but I am on a tight budget. Even so time is money and I do have the above components with exception of an "old school mA-indicator". I asked Mr. Google and he just gave me a bunch of charts. Precisely what do you mean? It would be worthwhile to have a small battery powered IR indicator I could just switch on when needed. – Jean-Paul McCoy Jun 19 '18 at 21:43
• Sorry, english is not my L1. I wanted to say moving coil instrument. Those can be salvaged from old hifi equipment, or bought for cheap. Use one in the 1 to 10 mA vicinity, and have a variable resistor in series to calibrate with a visible LED. – loonquawl Jun 21 '18 at 4:38

Generally if the forward voltage is appropriate (usually about 1.2V +/-0.1 or 0.2V) then the LED is working.

There is no point checking an IR LED with a camera if it has 0V or 2V across it, assuming a reasonable current of, say, 10mA.

There are IR indicator cards such as this one, but I think it would be easier to use a camera or a simple phototransistor sensor. Description says:

Applications: Testing IR LEDs and IR laser diodes in prototypes, remote controls, audio transmitters, light sensors, optical fibre systems, etc.

• Really like the IR Indicator Card. I'm pondering the cost benefit analysis of fabricating a sensor vs. purchasing one. – Jean-Paul McCoy Jun 19 '18 at 21:45
• You can reverse bias another LED and measure current detected used now as a PD or get a 5mm PD (Cheap and detect IR with daylight blocking lens) – Tony Stewart EE75 Jun 19 '18 at 22:00
• @TonyStewartolderthandirt Example phototransistor with filter plastic LED-like housing here not much sensitivity below 750nm and only 50 cents or so one-off. One of those, a battery and resistor will do it. – Spehro Pefhany Jun 19 '18 at 22:32
• Sure and calibration for temp variations in hFE with a 2:1 range at room temp at 2mA/mW . But for accuracy, I would always choose the same 5mm style in a PD with 0.5mA/mW +/- 5% typ from Sharp/Vishay and use a bigger R for gain. – Tony Stewart EE75 Jun 19 '18 at 23:00
• But I see after Sharp licensed to Vishay then sold to Fairchild it is now On Semi who changed the specs and part number .. but anyway detection will be too late if it burns up in <1 second. after calibrating the efficacy loss due to self-heating so fault-prevention is the best design as in my answer but may warrant further questions by the OP – Tony Stewart EE75 Jun 19 '18 at 23:24

Maybe but fault prevention is easier by understanding specs and temperature rise of similar sized LEDs and then using current sensing or temperature sensing for optimal control.

Usually all these solutions just require an understanding of Vf at If then compensate for temperature rise $$\Delta T=Rja*V(t)*I(t)*t$$ for similar size chip’s junction to ambient Rja.

Heatsink design will help greatly on the cathode to lower Rja as the anode is more thermally insulated with it’s gold whisker wire.

• Temperature/heat sinking had not occurred to me. – Jean-Paul McCoy Jun 19 '18 at 21:34
• Are you planning on driving 100mA? or 1A? and for how long is the critical question with answers in power-time curve guidelines – Tony Stewart EE75 Jun 19 '18 at 22:03
• 5mm case and leads have a thermal velocity of about 1mm/s and thermal resistance depends on length of lead 5mm suggested by OEM for fragility reasons and surface area of copper plane for 2sqin/W (Rms) SMD devices have better thermal conduction. – Tony Stewart EE75 Jun 19 '18 at 23:27