Mk120 IR light barrier won't run from same power source

Kits Mini Kits Range
1

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: velleman.eu/downloads/fil…ransmitter.jpg
Here is sensor schematic: velleman.eu/downloads/fil…a/receiver.jpg

Manual: 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.

2

Try increasing supply to 10-12VDC

3

Are you sure it’s save to increase the voltage? By not working I mean that receiver does not react to transmitter but both seem to be working. I can see transmitter IR LED on with my digital camera.

They do not work from single 9V battery too, so that means it must be interference from transmitter?

4

In your first post you’ve stated that the units work fine with a 9V battery.
If this is not the case, then it will not help to increase the voltage.
If you have a multimeter that can be set to measure frequency, measure Xmtr. frequency (3-5KHz)
Check your receiver circuit for assembly errors.

5

[quote=“VEL417”]In your first post you’ve stated that the units work fine with a 9V battery.
If this is not the case, then it will not help to increase the voltage.
If you have a multimeter that can be set to measure frequency, measure Xmtr. frequency (3-5KHz)
Check your receiver circuit for assembly errors.[/quote]

I stated that the receiver and transmitter usint work fine by using separate batteries.

“work fine when powered by separate 9V batteries.”

And in my second post I wrote "They do not work from single 9V battery "

So that excludes PSU as a problem, the problem must be transmitter interference that makes receiver unable to detect when beam is cut.

Did you test that receiver and transmitter are working if powered by same power supply? I don’t have a multimeter that can be set to measure frequency :frowning:

6

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.

7

Buzzer should be on continuously if the beam is cut (or if the TX is not on).
If not, there is a problem with the receiver circuit.

8

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.

You should mention in the Kit documentation that if one removes LEDs from PCB then they need to use shielded cable only.

Thanks for help.

9

If you would have mentioned that you’ve modified the circuit in this way, the problem would have been solved sooner. We assumed that you’ve applied the kits ‘as is’.