Hi, I have a problem with IR light barrier.

I use velleman MK120 kit. The two pieces light source and light sensor work fine when powered by separate 9V batteries.

My problem is that I need to power them from same power source. Then they do not work.

Here is light source schematic: http://www.velleman.eu/downloads/fil...ransmitter.jpg
Here is sensor schematic: http://www.velleman.eu/downloads/fil...a/receiver.jpg

Manual: http://www.velleman.eu/downloads/0/manual_mk120.pdf

Please help. I use my own built 5AMP LM338K based 1.25V-25V power supply set at 9V.

 

hi,
Looking at the two circuit diagrams I dont see any decoupling on the power rails.??

What I would suggest is to power each module thru its own blocking diode, say a 1N400x and decouple each module with a 22uF to 47uF cap and a 100nF.

EDIT: you could just add decoupling on the cathode ends of the two 1N4007's

Do you follow oK.?

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I connected 0.1uf cap and 500uf electrolytic cap to transmitter and the same to receiver. The result was it still does not work. I tried to add just diodes but it did not help either.

Both of schematics I added were connected to single power supply.

Attached Thumbnails

 

 

when u say it doesent work-what is happening when the ir is recieved by the rx unit-what happens when no ir is recieved?a scope would be handy here to see exactly what is happening when the two units are powered up but add decoupling caps across the supply for both units -what type of supply are u using to power them it may be that which is causing the problem.....

 

I'm using this PSU, in dual mode meaning the other PSU was conected to transformer with no load. I did blow my fuse 250mA at the transformer for some reason. I wonder if the fuse rating is correct and if LM338 needs a dummy load, remember I have dual PSU?

I attach my PSU schematics and photo.

Attached Thumbnails

       

 

I can't see why you want either such a big power supply or dual supplies.

Have you got anything else connected to the transmitter and receiver? If so, there could be a problem with connected power supplies.

How are you powering the two AC inputs on your power supply? To run them from separate sides of a centre tapped transformer would be wrong.

The receiver is just looking for any variation in light levels, and it will also operate when there is noise on the power supply. That will make it fail to detect a break in the beam. Power supply noise could come from the power supply or from the transmitter. Either way, you should isolate the receiver from that noise with something like a 10 Ω resistor in series, close to the receiver. That, along with the 1000 µF capacitor in the receiver (C5, shown near the buzzer) should remove noise at the frequencies that the receiver is sensitive to.

Does it work when both transmitter and receiver are on one battery?

Does it work when the transmitter is running from your power supply and the receiver is running from a battery?

Does it work when the receiver is running from your power supply and the transmitter is running from the battery?

When you have done all those tests, does it still work when receiver and transmitter are running from separate batteries, or have you blown something?

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I will power a 3W LED and motor. The Transmitter and receiver will be powered by 9V (first PSU). 3W LED will be powered by first PSU too.

The second PSU will be used to power the motor via PWM driver. I made separate supplies to reduce noise.

See above.

I use simple transformer with 3 secondary outputs. The first is 15V AC goes to 9V, The second is 18V AC goes to make 12V to power motor.

I tried to take single battery 9V add resistor 10 Ω in series to the receiver and power both circuits. The result is it does not work. With digital camera I can see transmitter IR LED on both ways with single battery and on separate batteries.

No.

Yes.

Yes

No everything is still working from separate bateries.

Do I need to do this? http://www.williamson-labs.com/480_dec.htm
What RFC choke and capacitor I need?

 

Well I tried to connect transmitter on wall adapter and receiver on another wall adapter (the design includes DC jack). Guess what it does not work. Or should I say the receiver does not react when beam is cut meaning the buzzer makes short beep while switching on? When if working right the buzzer should be on.

 

I solved the problem: D

It seems the wires to IR sensor and IR LED were too close together and interference from the wires made the problems I described. It worked fine from battery, but not from any kind of power supply.

I resolved the problem by using microphone shielded pair cable to connect the both leds to their circuit boards. Now it works as expected.