MK120 question

I was able to assemble this kit successfully and it worked properly however, when i modified it by replacing the buzzer with a relay and wiring the LED’s and switches so i can put it in a nice enclosure i noticed that when the beam is blocked the small LED and relay flckers and does not latch constantly meaning the voltage across the relay is fluctuating. what could be the possible cause of it?

The circuit has not been designed to drive a relay, there is no debouncing.
You could try to increase the value of C3 (sorry, no values, please experiment).
Also: do not forget to provide a protection diode over the relay coil.

I have the same problem - only I haven’t attached a relay. I have assembled the units. In fact, I’ve purchased 5 of these same kits. I did not solder on the 9V battery holder, using the barrel connector and a 9V supply instead. I have not attached the buzzer, but I haven’t connected anything to replace it either.

When I put the units about 50cm apart, the red LED is off solid, and when I break the beam, the red LED goes on solid. However, when I move the units apart to about 100cm, the red LED starts to flicker. The red LED goes on solid when I break the beam, but the voltage across the buzzer connector does vary quite a bit. It’s supposed to work to 4m, but it doesn’t seem to, and again, I have multiple entire sets that are all doing the same thing.

Can you advise? I can modify C3, but why I should I have to if it’s stated to work to 4m?

Thank you! I just want it to work as stated, and I only need it to work to 1m, not 4m.

Any help is appreciated!