IR remote detector and EMF noise

[Ken]

Hello.

 

This is a HO scale model railroad application for an under track proximity detector using a common 38khz sharp IR det. (RS 276-640) driving a pic12f675. The IR diode and detector leads are no more than 2-3 inches from the pcb (currently a breadboard). 10k pullups are on the pic input lines.

 

The circuit and software works fine, however when running the loco in pulse mode, the emf noise one would expect is intermittantly showing up on the IR detector’s output line only.  I have several IR detector replacements, same thing.

 

It does not show up anywhere else as the detector and main circuit are decoupled. The tracks are reasonably clean. My logic probe with a clip lead attached shows this noise (as expected)  the track is acting like antenna,

(again as expected).

 

A workaround is to pad down the detector’s output line with a 4.7uf tantalum

so the time constant is sufficiently long enough to ignore the noise.  

 

Question:

Is this possible, that radiated noise can cause false triggers in these receivers?

 

 

Thanks.

[AJISH ALFRED]

Hi Ken,

Noise will be generated and affect to circuit especially when there is an electric motor near by. I guess your loco’s motor is causing this trouble.

[Ken]

Hi, Ajish.

 

Thanks for responding.

 

 

Still somewhat puzzeled..

 

I suspect this is the case because of the high gain nature of these

devices. I have previously breadboarded other PIC projects around this

noisey environment and the only experience with noise is the usual cause,

insufficient supply bypassing/ no pullups etc. -which is not the case here.

 

I can open the path between the IR receiver and the pic input and

the only place I see the noise appear is on the unterminated receiver

output, the pic is unaffected as long as GPIO termination is present.

 

I am sure another contributing factor is using an uncoded IR carrier,

which I hope to fix shortly and should improve noise discrimination

through software. 

 

 

Thanks again -Ken.

 

 

 

[Author]

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