Having a blonde moment: Switching LED's on -Ve instead of +ve rail is OK yes?

HI good folk of Arduino land!

OK, so I seem to be having a bit of a blonde moment here and just need some reassurance!

I am creating a subtle night-time home lighting rig, using an Arduino, LED's, relays and PIR's etc. Now, I already have two sets of externally powered LED strips that have their own 32V supply and controller, so all I was looking to do was to switch the power to the LED circuit using the PIR detection to switch a relay etc. Easy yeah!

However - what I failed to spot was that the LED strips comprise two sets of LED's that when in flickering/dimming modes (as set by a sequential button press on the power supply) alternate to provide a better effect. This means obviously, that there are three leads coming out of the sealed power supply unit.

Can't find the datasheet on the net for the supply/controller (IC11M) so I am assuming the three are:

Supply to LED row 1
Supply to LED row 2
-Ve / Ground

My relays have 3 connectors, common, Switch 1, Switch 2 and I have a boatrd with 2 relays on it - so no big deal. But why waste a relay by using the two of them to switch power to each strip of LED's, when I can simply switch the -ve rail yes? That's what I am having a blonde moment with.

Can I simply use one relay to switch the LED power supply -Ve rail upon detection at the PIR?

Ta all... and apologies !!

Its "iffy".

If one of the outputs is "ON" and the other is "OPEN" then you should be OK.

BUT if in the off state the output is pulled to ground for some reason then you could damage the string of LED's connected to it.

To show the "bad" condition:

LED output 1 = on
LED output 2 = gnd
Ve = open.

Now the LED strip 1 is forward connected (normal) but led strip #2 is reversed connected (BAD).

In this scenero LED #2 will likely be damaged.


Thanks for the reply, John!

Here's what I am trying to do, apologies about the hand drawn schematic!

The Power Unit and Controller is what I have, and is a totally sealed and glued unit powered from 240V Ac and with 32V output. The three leads that come out of this unit to the rows of LED's - I assume are connected the way I have drawn in the top part of the diagram (except there's 20 LED's in each row).

What I was planning on doing was determining which is the -ve wire coming out of the unit to the LED's, breaking that and connecting it to one of the contacts on my relay module as per bottom part of diagram.

The PIR is already working!

Would this potentially cause the issues you highlight in your reply?

Add a suitably rated diode in series with each LED strip and a 1M resistor across each strip?
Silicon diode rather than schottky for low-leakage, then the Si diode's leakage will only
pull a small voltage across the 1M resistor as its in the nA to uA range.