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Topic: LEDs driving me crazy. (Read 881 times) previous topic - next topic


Folks, usually I can work these things out, but this has me stumped.

(See attached "schematic")

This "circuit" is duplicated twice in the setup.

The two leds are used to indicate that the relays are active.  These leds are in the switches/buttons.

Ok, the circuit is a stock 12v one.  So 560 ohms would be the resistor of choice, but as it was going to be used in dark places, I upped it a bit to 680 ohms.

To make it easier to find the buttons to press, I would make the leds glow slightly even when not active.  Thus the second 22k resistor.

So:  All being good, the two leds glow gently.
The diodes are there to stop any leakage through the "bleed" resistor into the coil of the relay which is also turned on.
I push one button and it's led glows BRIGHTER - indicating the relay is on - and the other led continues to glow dimly.

I release that relay and push the other button.
It's led glows brightly, but the other led is now OFF!

Looking at the photo, you can see the two parts of the circuit.  I have tried to highlight the two areas/circuits applicable and put lines showing the tracks and where/how the connect.  Sorry if it is not the best.

Measuring voltage at the junction point of the two resistors, on both circuits I get 1.7-ish volts when off.
One circuit, it goes up about .2 volts when on.  The other circuit does nothing.

When I do the "bad" one, it's voltage increases by about .2 but the other circuit's voltage drops by about .2 as well.

They are two completely separate circuits! I have checked for solder blobs, etc.  But that kind of fault would cause the problem to happen both ways, not just one.



Its hard to be sure what you are trying to do but I assume that you are trying to tell when the 12 volt relay (which drives the 240 volt relay?) and 240 volt relay (contact rating not coil) are on. Just connect an LED with series resistor across the coil of the 12 volt relay and repeat again for the 240 volt relay.  If you are driving the 12 volt relay with a transistor, remember to put a diode across the coil to stop spikes ruining the tranny.


Without freewheel diodes across each relay winding you'll blow-up every semiconductor attached to those windings - these diodes are never optional.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]


Yes, I know of the diodes.

In this part of the circuit the switching is done by the relay contacts, and not a transistor.

I shall put some diodes in anyway, to protect the LEDs.

Now:  Drum roll please.

I am stupid!

There I said it.

I stuffed up with the vero board and the tracks.

Where I was getting the +12 to power the 22k resistors was not actually +12v.  It was from the N/C contacts which have +12v on their common.

So when that relay operated, the contacts were opened.  Thus I lost my +12v.

Alas as the board is 20+ years old and the cut was done with a drill and "super neat" all the other dags and stuff around it I missed it on my initial look and just thought the track was complete.

Oh well.  It happens.

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