Behavior of IR decoder and IR LED in a proximity sensor

I have a proximity sensor setup which I'm trying to fix. It was removed from a Hand Dryer.

It consists of an IR LED and IR Decoder which are side by side and separated by a plastic housing. When your hand comes close the IR LED light gets reflected back into the IR Decoder.

I suspected the IR Decoder of being damaged and the markings on the original didn't help in identifying the actual IR Decoder make/model ( 11 3C 0017 were the number on the back ) so I replaced it with a standard type ( HIM602H ). It didn't work.

I got my hands on a working proximity sensor and checked the IR LED output. The signal output to the LED is a 1.1kHz pulse train. When a hand is close enough the pulse train is reflected on the IR Decoder output ( inverted ). This is surprising as the IR Decoder is meant to have a band pass filter centered around 38kHz. Hooking up a scope the IR Decoder does decode a signal from a standard infra-red remote control so it must have this filter.

So how does it also pick up the 1.1kHz signal on the working detector. The replacement IR decoder I fitted ( HIM602H ) does not pick up this 1.1kHz signal.

I've seen a comment here: Distance sensor from 38kHz receiver | Let's Make Robots!

"If the LED pulses at 38kHz (and 1kHz too if just pure 38kHz doesn't work) "

It mentions pulsing at 38kHz and 1kHz. Could this be correct?


My sensor

Re: IR Decoder Conundrum

You seem to be mixing two related concepts. Typical IR Tx/Rx used a "carrier" of tens of KHz (38 KHz is common) and on-off modulate that about 1 KHz as shown here in the datasheet from the TSOP1838:



That approach effectively reduces interference from IR noise and increases range. It can be used for detection and data transmission. The link you provide is for ranging. It is a neat variation. This additional link to a product announcement from Vishay goes into it in some depth as well:
**broken link removed**

I suspect an automatic hand dryer simply uses detection, hence the mention of the two frequencies.

John

Re: IR Decoder Conundrum

The working IR Detector decodes signals from infra-red remote control ( I tried it with TV remote control ) perfectly so must have 38kHz carrier demodulation but it also sees the 1.1kHz pulse train from the IR LED. Does this make any sense that it can do both? The 1.1kHz signal is not modulated.

Re: IR Decoder Conundrum

Yes, that is what the output should show. See the attachment I posted, or go to any the datasheet for standard TSOP-like detectors, if the image is to blurry to see.

John

Re: IR Decoder Conundrum

I see the output but the input signal in the datasheet you mention is a modulated 1kHz signal ( modulated at 38kHz ). The 1.1kHz pulse from the IR LED on the proximity sensor I have is not modulated, just a pure 1.1kHz square wave?


Signal capture of working signal when hand is placed close to the sensor. No signal is seen when hand is at distance.

Channel 1 is input to IR LED
Channel 2 is output from IR Decoder ( cap on output give the slow rise time but is working )

Re: IR Decoder Conundrum

Just to update. I've received some TSOP4838 IR receivers and they too don't show the 1.1kHz square wave. What type of sensor could the original be? Anyone any thoughts on why this sensor is showing the 1.1kHz signal but any of the IR receivers I've tried do not?