Large Scale Central

IR Switch for Crossing Signal

My basement hobby room overhead layout is a bit of a hodgepodge. A friend gave ma a 1:1 single red signal light. I repaired it, replaced the bulb with a 12V RV LED bulb powered by a flasher circuit based on a 555 timer, and mounted it beneath the overhead track.

I ordered one of these IR Sensor Switch/Relay

and wired it to turn on the timer. With the delay set for a few seconds it turned the signal on when the train passed and went off a couple seconds after it passed. After about 4-5 cycles now it stays on constantly. When power is applied the relay turns on and stays on. Contacting the seller wasn’t much help. A replacement unit acted exactly the same.

I tried isolating the TX from the RX by placing a short piece of opaque plastic tubing over the TX but no change. I have another unit but am afraid to try it until I know what’s wrong.

Has anyone tried one of these or have any suggestion for troubleshooting?

Harold Wilson

so the relay is rated pretty high on the contact capacity, so the relay is fine.

what are you using to power this circuit? is your supply solid?

Greg

Greg

On the bench I used an old router power supply rated 12V 1A. On the signal I have 2 of those large 6V dry cell lantern batteries in series. It acts the same both places.

v

so power should be no issue… so the first unit worked for 4 or 5 times… the second unit did the same or failed to operate properly at all?

Greg

Yes, both acted the same, operated normally 4-5 times then they’re always “on”, that is the relay is always energized as if it has detected movement. If I power it off then power it back on the relay energizes as soon as power is applied and stays on. If there’s no movement the relay should de-energize when the timer setting is satisfied.

It sounds like there is a capacitor across the relay and after a few pulses, it charges up enough, and holds charge, to keep the relay active. A big value resistor from the relay’s + to ground may be enough to bleed this current off and restore normal operation.

I looked at the back of the board with a magnifying glass (seem to need it more often nowadays) and looks like there is a 10,000 ohm resistor on the relay primary to ground. Then I noticed a cracked case on the transistor connected to the relay primary. I next removed the second unit to check it. No cracked case! I retested it on the bench and it worked. Placed it by the track and cycled it a couple dozen times and it still works.

What I think I did: When I placed the second unit on the track I hid it in a structure with a window grill between the TX & RX sensors. I’m guessing this interfered with the signal.

I broke 2 of the basic troubleshooting rules. 1) Make sure it fails twice before condemning the part. 2) Change only 1 thing at a time (I made a slight change in the location on the sensor when I installed the second unit).