Help on LM358 based Infrared Beam Detector

Can someone help me remove the delay on the circuit above? It is a infrared beam cut detector, much like a burglar alarm. It works fine except for one thing, delay on the relay.

I'm trying to use this on a race beam-cut system and delay is causing problems. The Red LED reacts in realtime with the beam cut but the relay doesn't. I've tried removing caps but still the problem persist.

The circuit works this way:

1. If the beam is present, the relay is powered, meaning it is always on the Normally Closed position. If the beam is cut, power is taken off the relay. This part has no delay, relay click is instant.

2. Delay comes when the beam is back again, it takes roughly around 1 second before you hear the relay click again.

Since on a race, the starting line beam is reverse of everything. The beam is cut normally, and the beam comes back as you move out of the starting line (reaction time). This is the reason why I need to remove the delay. On the other beams like, midway, or finish line, the delay is not a problem since beam-cut is in realtime.

Thank you!

Where to the coils on the relay go to? They dissapear onto the other side of the board.

It probably runs to the one of the Q components which would be a transistor that is being controlled. The stuff connected to the gate pin of that component is what you're interested in.

Pine 2 on the LM358 seems unconnected. Does it go through a via or somethign to the other side of the board?

dknguyen said:

Where to the coils on the relay go to? They dissapear onto the other side of the board.
It probably runs to the one of the Q components which would be a transistor that is being controlled. The stuff connected to the gate pin of that component is what you're interested in.

Pine 2 on the LM358 seems unconnected. Does it go through a via or somethign to the other side of the board?

Click to expand...

Thank you for replying dknguyen, the board is not double sided and Pin 2 is the trace that runs below Q8 going to C16. I suspect that it is thru the Qs and they are not labeled.

About the relay coils, 1 pin goes to +12V source, the Ground trace goes to Q7 passing thru the Flyback diode.

Do you think If I try removing every capacitor in the circuit one by one I would be able to find it?

If you have an analog meter, try measuring the voltage on the electrolytic while interrupting the beam, if the voltage goes up & down when the relay clicks then you know its involved in that part of the circuit.

biboymusic said:

Thank you for replying dknguyen, the board is not double sided and Pin 2 is the trace that runs below Q8 going to C16. I suspect that it is thru the Qs and they are not labeled.

About the relay coils, 1 pin goes to +12V source, the Ground trace goes to Q7 passing thru the Flyback diode.

Do you think If I try removing every capacitor in the circuit one by one I would be able to find it?

Click to expand...

Do a connectivity test. Find the cap connected to the pin of the Q component that the relay coil goes to.

dr pepper said:

If you have an analog meter, try measuring the voltage on the electrolytic while interrupting the beam, if the voltage goes up & down when the relay clicks then you know its involved in that part of the circuit.

Click to expand...

I just did what you said and I got it. The smaller electrolytic cap discharges as the relay clicks. I removed it and delay was gone. Thanks.

Super, well done.
Glad to have been of assistance.