MK120 Receiver sensitivity


Is there a way to boost MK120 IR barrier’s receiver’s sensitivity ?

I already use the emitter at max current for the IR leds, and made use of tiny reflectors on both the emitter’s leds and the receiver phototransistor.

I was thinking about changing value(s) of R1, R2, R3 or R4…

What do you think ?

Thanks for your answer.

Found something:

I use 2 separate IR barriers (2 MK120 kits) in my application.

I just switched the phototransistors of the two receivers, and it appears that one of the phototransistor is less sensitive than the other.

It works (it detects the beam at short distance), but at 1 meter distance, it does not detect it anymore.

This is thus not a matter of alignment of the emitters vs. the receivers. The LM324 is also in good shape, and the circuits are correctly assembled.

Only one of the phototransistors seems to be somewhat deficient.

How can I get a new one ?
My usual internet supplier does not distribute it, but I see that Velleman distribute them. Could I buy one directly by post at Velleman ? I’m living in Belgium.

Sorry, no direct sales.
In ideal conditions (indoors, no direct light), you can reach up to 4m
Alignment is VERY important. The slightest misalignment will trigger the receiver, as IR is highly directional.
If you supply an address, we can supply a spare phototransistor, as a one time courtesy.

Thanks for your proposal; it’s very kind of you.
Fortunately, I’ve found one at one of your resselers in Brussels.

BUT: it did not solve the problem. And misalignement is not the cause, since, if I place my other receiver at the same place, it works…

So, let’s consider a moment the alignment is not the issue.

I checked the frequency of the emitter pulses, and got circa 6.2 kHz.
Both LED’s are working (digital camera shows it).

Could you provide me a series of test measurments I could perform on either the transmitter or the receiver ?
I do not have a scope, but a DVM68 Velleman multimeter including a frequency meter.

Following a short description of my application so that you can easily understand what I’m trying to do:
The goal is to automatically light a staircase (exactly 1 meter large).
At both ends, I installed a MK120 set to detect the passage. The staircase is out of reach of sunlight beams or fluorescent lamps, so there could be no interference of that kind.
The MK120 receivers outputs trigger a small PIC-based timer that lights up high power LED’s for a given period of time.
One IR barrier is working perfectly. The other not.
When I install the working receiver in place of the defective one, it works.
I also swapped the two LM324N and they are also both working.
So that’s why I’m very affirmative when telling that the alignment is OK, and so is the emitter beam.

Thanks again for your help.


Did you check the caps on both units?
Transmitter C1, C2 1n5
Receiver C1, C2 47n

I have built these and found crossing just one cap between the 2 units will shorten the distance.

I hope this helps.

Caps are well in place, as intended.

Another idea ?

Hi there,

Anyone here to help ???

I’m still struggling with this problem.

Remember: I have two IR barriers. For each, the distance between RX and TX is exactly one meter.

I installed a LED on the output, and use a TV remote control.

With the barrier that is working, at one meter distance, the output flashes with the remote control pulses.

With the other barrier, it works up to 50 cm. After that, it goes blind.

I doucle-checked each component. I replaced the phototransistor and the LM324N. With no success.

I double checked all components connections, I made measurements on each resistor and capacitor to check its value: everything seems fine.

What could I do to boost the sensitivity ???

Please help !

Many thanks.