Possible IR retro reflective denature after just a few weeks? - Page 1
Forum section: electronics / projects

author: rfleming
author_rank: Regular Contributor
timestamp: September 27, 2021, 11:30:04 pm
content: Hi all,I have been working with a Vishay TCND5000 for several weeks now and have been trying to measure items in a moving stack at ~15-20mm. and NOT detect the carrier for them at 40mm. After first populating my board with several of these driven by a couple opamps, this seemed to work extremely well. Though after just a few weeks, the output response appears to have dropped off. Initially I was getting a nice 4V output from my opamps, now its down to 2.5Volts. While there are many things that could obviously affect this, these are the ones for sure I know didn't:- distance. In my setup, this hasn't changed at all.- voltage. I can confirm that the input voltage is still the same.- sunlight. This would increase the output, not reduce it.I am running the sensor constantly rather than doing the 20ms pulses like they state in their typical operation. Though this isn't descried well in the datasheet from what I can tell.https://www.digikey.com.au/en/products/detail/vishay-semiconductor-opto-division/TCND5000/1681163?s=N4IgTCBcDaIC4GMB2ATArABiyAugXyASchematic of a sensor/opamp pair. Where Detect1 goes directly into my microcontroller into an ADC.

author: jpanhalt
author_rank: Super Contributor
timestamp: September 27, 2021, 11:34:19 pm
content: Quote from: rfleming on September 27, 2021, 11:30:04 pmRe: BoldedNot really. An intense background IR will reduce gain. Can't you just use the change, rather than intensity? That is, modulate the IR source appropriately, and look for its presence.

author: rfleming
author_rank: Regular Contributor
timestamp: September 28, 2021, 12:29:24 am
content: I did some checks by enclosing the product in a dark box with a scope probe attached and it had no effect on the signal...well maybe 1%?Its quite hard (maybe just me) to measure presence without repeatability on the given project. The sensors output ~1V with "no presense" and 2V+ with presense. Though seeing how this changes after just a couple weeks, if I set hard thresholds for an ADC, this could change again.Currently I have no way on startup to confirm these thresholds between "nothing" and "something" unfortunately.I am wondering if I have over driven my IR Led at all, like perhaps it should've only been used for 20ms per second or something. Though I have kept it well within the absolute specs, even if im running continuously instead of time pulsed.

author: jmelson
author_rank: Super Contributor
timestamp: September 28, 2021, 01:28:30 am
content: 55 mA seems to be a lot of current for a small emitter. Are you sure it is rated for continuous operation at 55 mA?Jon

author: rfleming
author_rank: Regular Contributor
timestamp: September 28, 2021, 01:36:35 am
content: --Absolute Max--Emitter maximum: 100mAMaximum pulsed: 500mAMax power disipation: 190mWMax Junction Temp: 100C--Typical--1.2-1.5V @test condition of 50mA (tp = 20ms)So despite specs showing its definitely capable, perhaps the 20ms pulse (assuming that is what tp means) is why I have worn out the LED.

author: jpanhalt
author_rank: Super Contributor
timestamp: September 28, 2021, 10:14:09 am
content: I assume you have this application note:https://www.vishay.com/docs/81449/81449.pdfI am referencing primarily the section on ambient light. If that is the problem, a common way to avoid those effects is to switch to pulsed ("AC") mode.