Vishay IR Detector picks up noise

[abicash]

Hello

I have developed a module employing the Vishay IR detector TSOP 38236 and a Microcontroller.It works satisfactorily,but for an issue.

If the TSOP is mounted far from the board via long wires ,the MCU is not able to pickup the signal.I investigated furthe with a DSO to find some electrical noise on the o/p pin.This noise manifests only when an electronic ballast CFL lamp is switched on the same phase.My ckt is not isolated from mains.
If the TSOP is poplulated on the board itself,then i have no issues.

How can this be taken care of?
I am thinking of putting 3 separate wires coiled from the IR det to the board.Will this help?

Please get back..

Thanks

 

[Nigel Goodwin]

Don't run long wires to the IR detector, use short wires. It's also absolutely essential that you decouple the supply to the chip, close to it's pins.

 

[abicash]

Thank you Nigel for getting back

The supply is decoupled appropriately.
The other option of using shorter lead lengths is always present but what if longer lead lengths is a feature that needs to be added?

 

[Nigel Goodwin]

Thank you Nigel for getting back

The supply is decoupled appropriately.
The other option of using shorter lead lengths is always present but what if longer lead lengths is a feature that needs to be added?

Is it decoupled right next to the IC? (not at the end of the long wires).

Add a transistor to give a lower output impedance to drive the wire.

 

[abicash]

Is it decoupled right next to the IC? (not at the end of the long wires).

Add a transistor to give a lower output impedance to drive the wire.

It is decoupled next to the chip i mean on the chip when it is far placed from the board.

Is it that the long wire is acting as an antenna to pickup noise from an electronic ballast?

 

[ronv]

Ballast

The problem is they both run at about the same frequency. IR at about 38KHZ and the ballast around 43KHZ. I guess your sure it's electical pick up and not light from the fixture. Might try twisting the signal and ground together, maybe even shielded cable, then the next step is as Nigel says, power it up at the sensor end. Might also check that the fixture is grounded.

 

[abicash]

The problem is they both run at about the same frequency. IR at about 38KHZ and the ballast around 43KHZ. I guess your sure it's electical pick up and not light from the fixture. Might try twisting the signal and ground together, maybe even shielded cable, then the next step is as Nigel says, power it up at the sensor end. Might also check that the fixture is grounded.

Hi ron thanks for your feedback

Its certainly not light from the ballast ,since i have also put a red filter in front of the detector and have tried moving the face away from the light source.
What do you imply by saying to twist ground and signal together?
Shielded cable hasn't solved it.

 

[ronv]

More info

Maybe you could post a schematic of your set up.
I noticed a comment in your post:
when the lamp is turned on the same phase. Does this mean if you plug your circuit into a different outlet the problem goes away? If so a line filter is the choice. If not it is probably radiated. If you have done a really good job on the power supply filtering it is probably pickup on the signal line. So if you can post the circuit. If not here are some ideas.
1- high freq. filter for power
2- emi shield of receiver
3- low impedence driver (probably best)
4- Shilded twisted pair wire
5- Schmitt trigger at main board

 

[ronv]

PS. These guys have a habit of putting out a pulse every now and then. Will your micro throw out an occasional pulse?

 

[abicash]

Hi ronv thanks for further inputs

See attached image

i want to add something ,if you have missed one of my posts here.

I am using a non-isolated PSU where 5v is derived w/o a transformer.So ground is common with the mains power.
The board is in Industrial design stage so its not possible to add more parts unfortunately.I can however suggest and try points #2 and #4.Point #1 is fairly taken care of since otherwise the PSU chip operates at a higher frequency and would have necessarily sabotaged the setup.
As i said the problem manifests only when the lead length of the IR det is increased w/ wires.
The other thing you mentioned about an occasional pulse which i don't send or generate.Of course there are few timers and RTC used.But when this problem occurs they have a secondary part to play.

 

[Nigel Goodwin]

Your circuit shows no decoupling components on the IR IC, it's essential that it's fed via a resistor, with a suitable electrolytic close across the supply pins.

Check the circuit I use:

 

[abicash]

Hi Nigel and thank you for that

I had connected a cap across Vcc and GND on the IR Det.Its not shown there in schematic but its present.

But i will definitely try your RC

Thnaks

 

[Nigel Goodwin]

Hi Nigel and thank you for that

I had connected a cap across Vcc and GND on the IR Det.Its not shown there in schematic but its present.

But i will definitely try your RC

It;s done that way in all equipment I've ever seen - if you leave the resistor out you usually get problems.

 

[abicash]

It;s done that way in all equipment I've ever seen - if you leave the resistor out you usually get problems.

You mean you have seen in equipments, what i have done? or the RC?

I am a bit ignorant here so please forgive me...How is the IC even powered with the RC setup?
Since what i understand is the capacitor charges at the first instant and no voltage is applied at the Vcc terminal of the det.

 

[Nigel Goodwin]

You mean you have seen in equipments, what i have done? or the RC?

No, the RC - all equipment uses it.

I am a bit ignorant here so please forgive me...How is the IC even powered with the RC setup?
Since what i understand is the capacitor charges at the first instant and no voltage is applied at the Vcc terminal of the det.

The resistor and capacitor decouple the supply - it's nothing to do with the 'first instant', essentially the IC is powered from the capacitor.

 

[abicash]

Oh bad mistake; the pinout is different in my case

TSOP has pin3 as Vcc ...
pin1=o/p ,pin2=gnd