My MK111 (http://www.velleman.eu/downloads/0/manual_mk111.pdf) doesn't work properly.
It will always close the relay unless both RV's are completely turned to the left.

When I adjust the pulse the LED just stays on and the relay starts to make a buzzing noise.
When I adjust the pause, it just stays on too.

And even when both RV's are completely to the left it doesn't do what its should, then it goes on and off very fast (too fast).
Until it slows down and eventually stays on all the time.

What could be wrong?

 

Was it working before, and it recently stopped working?

Or did you just assemble it and it isn't working?

 

It never worked properly, it was like this from the first time i tested it.

 

Then you have made some error like a bad solder joint, or placed a component wrong. It could also be a bad component.

 

I've already checked if all the components are placed the right way, they seem to be.
Also, all solder joints look like they should be.

How can I find out which component is defective (if there is on) or if a solder joint is bad (other than by looking at them)?

 

I've already checked if all the components are placed the right way, they seem to be.
Also, all solder joints look like they should be.

How can I find out which component is defective (if there is on) or if a solder joint is bad (other than by looking at them)?

Inspecting solder joints if you are not trained may not always reveal faults. But will work in most cases. What kind power supply do you use? Is the voltage correct. And is DC or AC output?

 

I'm using the 12V DC from a computer power supply.
The kit says it needs a 12 to 15V DC supply, so this should be correct.

 

Check for a solder blob/solder whiskers between any of the 555's pins.

Are you certain that C3 was installed the proper way? If it were very low-value (open internally) it would cause problems.

 

How can I check if it's open internally open?

 

I just found out now that the charge in C3 changes up and down while the relay is clicking, but the charge slowly rises to 7.06V and then it just stays like that and the realy stays closed.

 

Can you take some close up photos of your project, top+bottom and post the photos

 

http://users.telenet.be/augurk/MK111/mk111a.jpg
http://users.telenet.be/augurk/MK111/mk111b.jpg
http://users.telenet.be/augurk/MK111/mk111c.jpg
http://users.telenet.be/augurk/MK111/mk111d.jpg

Here you go.

 

You have two empty holes. What are they for? Should it be a strap connecting those two. I also see some solder joints that could be bad. Pull gently in the components leg on the top side. They shall not move at all

 

Those 2 holes are beneath the RV1 and RV2.
In other velleman kits i've assembled with RV's there were holes too, and they didn't have to be used either.

 

I just found out now that the charge in C3 changes up and down while the relay is clicking, but the charge slowly rises to 7.06V and then it just stays like that and the realy stays closed.

While C3 is charged up, check the voltage on pin 7 of the 555 timer. If it's at the same voltage to ground, then there is too much leakage in C3 to allow D2 and the resistors R1 and RV2 to supply enough current to charge C3.

Measure the voltage on pin 5 of the 555 timer to ground. The capacitor has to charge to that voltage in order to toggle the timer. Pin 5 should measure about 8v, which is 2/3 Vcc.

If pin 7 is near 0v, then RV1 or R2 are open.

 

From C3(+) to pin 7 there's 4.68V
From C3(-) to pin 7 there's -5.93V

When I measure between pin 5 and the ground I get 7.10V,
while measuring C3 results in 7.09V en C2 in 10.68V.

 

From C3(+) to pin 7 there's 4.68V
From C3(-) to pin 7 there's -5.93V

OK, that doesn't make a lot of sense. C3(-) should be the same as ground.

Try increasing the setting of RV1 somewhat.

Are you certain that you did not reverse RV1 and RV2? One is about 50k Ohms, the other is 1 MEG. If they were installed in the wrong places, it won't work.

When I measure between pin 5 and the ground I get 7.10V,
while measuring C3 results in 7.09V en C2 in 10.68V.

Set both pots to mid-position, and re-measure the voltages.

 

I'm sure the RV's are in the correct place, I've already checked that and I just checked it again to be shure.

With both pots in mid-position i get:
- Still 7.09V between pin 5 and ground
- C3: 8.30V and C2: 10.67V

 

I'm sure the RV's are in the correct place, I've already checked that and I just checked it again to be shure.

With both pots in mid-position i get:
- Still 7.09V between pin 5 and ground
- C3: 8.30V and C2: 10.67V

OK, so if C3's voltage is greater than pin 5, the comparator inside the 555 should have tripped and caused pin 7 to turn on (it's an open-collector NPN transistor with the emitter connected to ground) which should cause pin 7 to measure under about 200mV; nearly ground.

If pin 7 is measuring about the same voltage as C3(+), then the 555 timer is bad or installed incorrectly.

If pin 7 is measuring nearly ground, then something is open between pin 7 and C3(+), which is RV1, R2, or the soldering connections or traces or 555 socket.

 

Pin 7 measures exactly the same voltage as C3...
Which means there's a problem with the (installation of) the NE555?