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Premium Member
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Discussion Starter · #1 · (Edited by Moderator)
The other day I suggested basing some track lights circuits on cheap LED lights kits.

I mentioned I was going to use the PCB and LEDs from a 4017 running light kit purchased from the far east.

Well.. I now need to add a word of warning. There has proved to be, to use popular YouTube electronics jargon, a Trap for young players, here.

In my latest kit, all ten LEDs have been manufactured in reverse: i.e. the flats on the LED cases, and the larger metal pads inside relate to positive polarity, not negative, as we might suppose; whilst the polarity is indicated correctly/otherwise on the PCB.

This means of course that the LEDs have to be put into this, or any conventional circuit, back to front to make everything work.

I had heard of this happening but have never come across it before.

I have used lots of other kits from that part of the world, and the LEDs have always been constructed as we, in the UK at least, might expect.

So, how on earth does this come about?
 

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Rich Dumas
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3,568 Posts
Usually the shorter of the two leads goes to the cathode and should be connected to negative. When I examined some of the LEDs that I had left over from various projects I found that the flat on some of them were difficult to make out. One white LED had a flat marking the shorter lead and it lit up with negative connected to that. A red LED that I checked had the flat marking the longer lead, but it only lit with negative connected to that. I guess that it is best to check the polarity of any new LEDs that you get. One more hitch is that some red LEDs do not have a red case. Since red lets normally run at 2 volts and white LEDs run at 3-3.5 volts you would want to avoid getting them mixed up because the higher voltage would burn out a red LED instantly.
 

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Premium Member
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462 Posts
I rely on looking in the led itself and distinguishing the anode and cathode inside and following their conductors. Mainly because I file down the the led, or trim the conductors without noting which is which or sometimes I put it down after having bent one of the conductors and then forgetting which conductor i decided to bend to signify which is which. Then sometimes I have to check again and again with my multimeter on diode mode.
 

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Living the Life!
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Get your self one of these .......... there are very useful for all kinds of reasons ....
wink.png


LED tester.jpg

About £6 or so
 

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Premium Member
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373 Posts
Discussion Starter · #6 · (Edited by Moderator)
Yep. I usually follow the fat metal bit inside the LED down and assume that its lead is negative, but it was not so in this case.

Other than the multimeter, my LED tester gadget is not so fancy as mentioned above, but it serves me well none the less.

It is based on this gadget built by another respected YouTube entertainer, Big Clive.


Anyway, thanks, I did know how to determine the polarity, both in theory and practice, but I still dont understand how or why these ten LEDs were manufactured with a different configuration, so with respect, I am none the wiser.

(-;
 

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Living the Life!
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There is an old wives tale that says the largest of the bits visible within the dome of the LED is always -ve ........
ohmy.png


The easiest way to decipher the polarity of an LED is by the length of it's legs .... the longer one is the +ve one and the short one isn't .....
thumbsup.gif


This is particularly helpful when facing an infrared LED such as those used by Scalextric to communicate between the car and the in-track sensors. The problem here is that the infrared is not emitting visible light.

There is also a flat edge to the side of the dome but sometimes, the orientation of the bulb gets skew-whiffed by 180o .... :lmfao:
 

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Banned
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The problem here is that the infrared is not emitting visible light.
Smartphone camera's pick up IR light, test with TV remote first, can you see it glow when pressing a button on remote?

IF so you're good to go for testing IR LEDs with your phone.
 

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Premium Member
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Discussion Starter · #9 ·
Smartphone camera's pick up IR light, test with TV remote first, can you see it glow when pressing a button on remote?

IF so you're good to go for testing IR LEDs with your phone.
Apparently some phones will work for this using the on back phone camera and not the front... (or was it vice versa?).

I tried it with the ipad when I was building the corner flasher circuit the other day and an almost transparent wave pattern can be seen rippling over the workpiece when it is transmitting. This is quite interesting and worth a go.
 
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