Hi,
I was able to see the receiver.
Looks like there are a lot of pins on the IC that did not get soldered.
Also lot of incomplete solder joints
It also looks like you have LD1 and T1 swapped.
Dear Velleman,
I have two Mk120 kits bought a few months apart from Maplin. The first kit worked fine. The second does not.
The problem is with the transmitter. The “good” transmitter works with both receivers.
If I look at the IR leds with a usb camera I can see the they are lit on both the “working” and the “broken” transmitters.
When I put an oscilliscope on the working transmitter, I can see a modulated voltage on the IR LEDs. But, there is only straight voltage on IR LEDs on the “bad” transmitterl; it is not modulated. I have checked and replaced all three transistors with new ones and it still does not work. Please describe the modulation mechanism or theory of operation so I can find the fault. Or suggest where to look.
thanks
Hi,
Just a guess.
You may want to check C1 and C2 on the transmitter and make sure they are marked 152.
If not you may have mixed them up with the ones from the receiver.
C1 and C2 for the receiver are marked 473.
I hope this helps.
I eventually found the problem: open between earth and the emitter of T2.
Here is some advice that may help others:
Use a webcam or digital camera to see if the IR LEDs are lit.
If they are not, the problem is more basic than described here, go an check your components are correct numbers and right way around.
If the LEDs are lit but the receiver still does not trigger, check the following:
T1 and T2 form a flip-flop (astable multivibrator). If one of the capacitors or resistors or an earth connection is disconnected, the circuit will not oscillate. Thus the IR light will not modulate and receiver will not detect it.
At startup, small random difference between the two transistors make one (T1) turn on quicker than the other (T2), bringing low the collector of T1. The negative going collector sends a negative pulse via the capacitor to the base of transistor T2 turns off transistor T2. After a while the voltage on transistor T2 rises enough because the capacitor is charging that transistor T2 turns on, sending a negative pulse to the base of transistor T1, turning off T1…and so on.
checking the voltage on the emitter, base and collector of the two transistors will give you an idea which one is staying on all the time, and thus where to look for a bad connection. Also check earth connections to all components supposed to be earthed.
If this helped you get yours working, please let me know: ccwbats@gmail.com
I had two kits, bought weeks apart. One worked the other did not. I noticed the componets for the Bad transmitter are different brand to the good one. eg. the resistors are smaller and have thinnner color bands and the transistors have a silver side on the good one and are all black on the bad one. Obviosuly there are different production batches (and factories?). I suggest there is something in Velleman quality control that needs to be fixed. Velleman seem to like throwing the problem back on the customer when it is blatantly obvious there is a problem with these kits.
At a minimum, they should provide a trounbleshooting guide as to what voltages shoudl be found at what point in the circuit. The support is abysmal.
I am also diagnosing a broken MK 120. After fixing the obvious faults it still doesn’t work. Turning on the receiver causes the buzzer to sound no matter if it is exposed to the transmitter or not. But the bright side is that it is forcing me to understand the circuit.
Contrary to what others have posted here, you will not be able able to see the LEDs on the transmitter flash using a digital camera or camcorder. As cmp260 posted, the transmitter circuit is based upon an astable multivibrator. See en.wikipedia.org/wiki/Multivibrator . For this flip-flop circuit, the frequency equals 0.721 / (R * C). In this circuit R = 100 kohms and C = 1.5 nf. So f = 0.721 / (1.0E+5 * 1.5E-9) which is about 5 kHz. So yes the LEDs flash, but at 5000 times per second! At that rate they will appear solidly on all the time but with a brightness of 1/2 of what they would be if the flip-flop circuit was not involved.
So without an oscilloscope, you can’t tell the difference between flashing LEDs and always-on LEDs.
I will just assume that they are flashing, and try to diagnose the receiver…
Hi,
I have built MK120 and I can see the LED’s light up on 120T transmitter in my digitial camara. They don’t flash but both show white when I switch the transmitter on.
When I switch the receivor on, I get the buzzor sounding but it does not go off when I shine the transmitter unit at the receivor LED. I checked the forum and C1 and C2 on the transmitter are marked 152 so I have not mixed these up.
What can I do next to trouble shoot the receiver ? I assume if LED’s light on transmitter the transmitter is working fine. Do you have voltages on particular components I can check or something like that ?
Tks,
Alan
Sorry, this forum does not allow picture uploads
If you can provide LINKS (e.g. by using a free online picture base) to HI-RES and SHARP pics of your assembly (both solder- and component side)
then we can take a look and provide some feedback.
Hello! I purchased this same IR barrier today. I assembled it, but nothing happens…
I checked every polarity, and it all seems right.
I have taken images, here’s a link to them:
dropbox.com/sh/baeakn0s8w9b5ez/QlPohwR-57
I would appreciate any help!
Hi,
It seems that the soldering needs some help.
I see a lot of incomplete and cold solder joints
The solder should cover the pad and have a cone look to it
Please resolder all of the points
Cover the complete pad but do not let the soldering iron stay on the joint for to long as this will damage it.
Also the buzzer on the receiver has polarity.
You might want to check it to ensure it’s correct.
Ok thanks!
Well the buzzer seems right, so I’ll have to redo the solder joints i guess…
Hello! I have resoldered the components. However, the issue still isn’t solved. Now the buzzer and led just won’t stop.
I checked the two IR LED’s through my camera, but they don’t seem to be shining. So I think the problem is on the transmitter.
Here’s a link to the new pictures dropbox.com/sh/wpg0yq3dnj3tolt/CD9p0hrrNT
Sorry, but soldering looks rather poor and might need re-work.
Can you also supply pics of the component side of the board to check component placement?
Ok, the other pictures are uploading right now, so you’ll have them in a few minutes…
edit: the pictures are located in the same link (dropbox.com/sh/wpg0yq3dnj3tolt/CD9p0hrrNT)
Not trying to offend.
There are a couple of joints that it looks like the solder is just balled up on the component lead and not connected to the pad.
If you are just starting to solder you may want to consider going to http://www.youtube.com and search for
soldering tutorial
There are a few good ones out there.
Well, ok I’ll do that. I must admit that the joints don’t look really good indeed.
What I see when I look through my camera at the IR LED though, is that they flash once for a very short time, and then turn off.
They do that everytime I turn on the switch.
Could someone however locate the problem? (It’s probably a wrong contact, but where…)
Thanks!
Hey, problem solved!
One of the joints was indeed not touching the board… now it is, and it works! 
Thanks for the help!
Very glad to hear that.
Just wondering because I can not get to the one I have.
Can you see the IRs on the transmitter flashing in your camera?
Well, the pad was broken, so I made a bypass with some wire I had left. Unfortunately that stopped working after 5 minutes… The LED’s are still working, I don’t see them flashing (there’s another topic where someone calculated it, they should flash at about 5000Hz, so you couldn’t see it…)
Anyway, IR’s are working, but the receiver (buzzer) doesn’t stop buzzing anymore… I really don’t know it anymore…
Maybe you should try it with new batteries.
If that doesn’t work could you post another picture with the patch?
Well, for some mysterious reason, after I put it aside for some hours, it worked! So I could continue my project of putting a relay instead of the buzzer, which is then connected to a 2.5mm audio jack. This way I can use it to take pictures of animals (e.g. birds, which are quite difficult to photograph) passing through the beam. I hope it won’t stop work again!
(Apparently I really need to learn how to solder correctly, as it would save me some time and worries)
Here’s the photo of the progress: dropbox.com/sh/q3t6uc7m383mms0/30nEIZE-Ds
You can see the red wire, which is the bypass for the bad joint.
Thanks!