LDR puzzle

On a sunny day (Fri, 13 Jul 2012 08:56:42 +0200) it happened Jeroen Belleman wrote in :

You moved the IR remote when pressing the buttons, so more light fell on the LDR?

On a sunny day (Fri, 13 Jul 2012 07:44:15 GMT) it happened Jan Panteltje wrote in :

LDR? eeh, I ment LESS.

Not answering your question but one of quickest ways to see if an IR remote control is doing anything is to point it to a digital camera, web cam, phone camera etc. The IR is picked up and you can see (or not) the IR LED's illuminating.

"The Hemulen"

** The easiest way is to use a portable AM band radio.

The pattern of current pules in the IR LED is clearly audible when the radio is tuned off station at the low end of the band.

Every button sounds a bit different too.

... Phil

On a sunny day (Fri, 13 Jul 2012 19:19:26 +1000) it happened "Phil Allison" wrote in :

But that does not check if the LED is actually emitting IR. I have had defective (black) LEDs that had current in them. Webcam is much easier. There is a webcam in every thing you can buy thse days, from phones to laptops to bags of chips.

On a sunny day (Fri, 13 Jul 2012 08:56:42 +0200) it happened Jeroen Belleman wrote in :

I have just tested a small LDR with a remote from cable receiver. With some surrounding light it is about 43.2 kOhm, and pressing a button on the IR remote makes it go DOWN to about 40.2 kOhm. Not much does not seem very sensitive to IR, It seems changing, and some memory effect, and sensitivity changes after the IR radiation, but could also be remote flat batteries drop output after a second. And it is pulses of course, so YMMV. Need a light proof box, fixed setup, and calibrated DC current to make conclusions.

Good tornado detector too. So is Channel 13 TV white noise.

The ultra-high threshold sensitivity (compared to TUBES) of the DMM electronics acted like a receiver and detector; your remote _is_ an active transmitter. Very Shakesperian..

For testing IrDA FIR transcievers I hooked up an OPA657 to an inexpensive PIN photodiode (all inside a metal tube) and looked at the results on a 'scope. Cheap(ish) and easy.

A failed LED goes "ohmic". It still draws current, but it isn't a diode nor does it emit light. Jan has the correct technique.

--
I clamped an LDR and an IR remote about 1/2" apart, opposing each
other axially, hooked a cheap digital multimeter across the LDR, and
got the same result.

The only mechanism I can think of that would cause that to happen is
if the IR is far enough out of band and strong enough to keep some
incident light from affecting the LDR's element.

20+ years ago I had a customer complaint that his remote control didn't work. I checked it out, found no problem and sent it out. The customer called later and said the remote was not working. We had a back and forth discussion and I found he had just installed new fluorescent bulbs in the room where his VCR was located. I suggested he turn off the lights, he did and the remote worked fine. The sensor was overloaded and didn't respond to the remote. I told him to shield the sensor from the lights or relocate the VCR. He turned out to be a happy customer to have his problem figured out. Mikek

"amdx"

** Not to anything like the degree it does with the LED connected.

... Phil

Hmm, that's intreesting. I don't really know how an LDR works. But light can't block other light. It could be that the IR puts the charge carriers into some trap states? Or causes faster decay of the excited charge carriers? Weird.

George H.

Cadmium sulfide is weird. I once did a semiconductor lab wherein we used a xenon flash strobe to illuminate a photocell; I discovered a rough N^0.5 decay rate dependency. The lab manual said N^1. So much for that...

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

rare.

IFYPFY.

eb

the

radio

This is a seriously flawed view of reality. An LED left on continuously for years and years (decades, probably) will wear out as crystal defects propagate, adding resistive paths (dark current) in parallel with the junction. But it will continue to emit light even when it technically has reached end of life (generally defined as 50% of initial light output). It becomes a diode with some resistance across it, nothing more.

But this will NOT apply to an LED operated at most a few milliseconds a day, as in the remote control.

Another way to propagate defects is for the encapsulation to apply mechanical stress to the die. This will occur, for example, if the environment is cold while the die is hot from the LED current. But this will NOT apply to an LED operated a few milliseconds a day, in a comfortable room temperature environment, because the die will never self-heat and the encapsulant will never shrink.

g on

light can't block other light. =A0It could be that the IR puts the charge c= arriers into some trap states? =A0Or causes faster decay of the excited cha= rge carriers?

Find (or draw) the energy band diagram.

On a sunny day (Fri, 13 Jul 2012 18:27:07 -0500) it happened John Fields wrote in :

I just realized what happens, I mentioned the pusles (from the remote before):

----- ---- peak IR light | | | |