Is there a way to test an LED?

[GuitarPlayer]

Hi, stumped newbie here 

Is there a way to test an LED? My effect pedal is fully functional, however the LED Indicator that shows when the pedal is on will not illuminate. All of the voltages check out, already looked for cold solders, check continuity, verified voltages and grounds throughout. All of the pots are functional. All voltages are correct along the 8 pins of the IC OpAmp. ONly one cap has polarity and I verified correct placement. Verified all resistors.

Voltage going from the PCB to the base of the LED is 9.4v, other lead which comes from switch is 0v, but does have continuity from switch to the base.

All I can think of at this point is that the LED is defective. Anyway to test it?

Thanks in advance for any suggestions.

Jake

It is an MGV kit from General Guitar Gadgets: http://www.generalguitargadgets.com/index.php?option=com_content&task=view&id=99&Itemid=26

Here is the schematic: http://www.generalguitargadgets.com/pdf/ggg_mgv_sc.pdf

Wiring diagram: http://www.generalguitargadgets.com/pdf/ggg_mgv_lo.pdf

Here is the BOM, but R14 is wrong here, it should be 1k: http://www.generalguitargadgets.com/pdf/ggg_mgv_bom.pdf

[Joe Hart]

Multimeter. Or connect it to a 9V battery (with the appropriate resistor!!). Hope this helps.
-Joe Hart

[GuitarPlayer]

Thanks Joe.

What measurements do you suggest taking with a multimeter? I have already checked the voltage at the lead that runs from the PCB to the LED, and also at the base of the LED, it is 9.4v. The other lead is 0v, but does have continuity.

There is a 1k resistor in line (which I have tested) with the LED voltage, so you are suggesting a simple run from the battery like this?

(+) ---1k R---===LED===----- (-)

Thanks,
Jake

[sixstringphil]

A while back someone recommended using a watch battery. Just squeeze the legs of the LED to each side of the flat battery. No resistors needed! I keep one with my LED's for that purpose.

[tiges_ tendres]

What is the voltage from the battery/power supply?

I dont think you should have 9.4 volts on the led if you have a 1k resistor in series from a standard 9 volt battery.  Check for a short/solder bridge between where 9 volts goes into to the pcb and where the wire comes from the 1k resistor to the led.

Also, check to see if you have shorted out the resistor.

One other check is to make sure that the switch is wired so that it grounds the other leg of the led.

[GuitarPlayer]

^^^^^^^ The schematic does show 9v at the LED before entering the resistor. It is at the bottom right of this doc:

http://www.generalguitargadgets.com/pdf/ggg_mgv_sc.pdf

I just checked and I have 9.3v on BOTH sides of the R14 1k resistor that precedes the LED. Should be less on the LED side, right?

[tiges_ tendres]

^^^^^^^ The schematic does show 9v at the LED before entering the resistor. It is at the bottom right of this doc:

http://www.generalguitargadgets.com/pdf/ggg_mgv_sc.pdf

I just checked and I have 9.3v on BOTH sides of the R14 1k resistor that precedes the LED. Should be less on the LED side, right?

Yes, it should be less on the side of the resistor that goes to the led.

That tells me that you have a short/solder bridge somewhere around that 1k resistor and 9 volts into the board

[ayayay!]

Motherboard battery.  3.2v. 

[GuitarPlayer]

Yes, it should be less on the side of the resistor that goes to the led. That tells me that you have a short/solder bridge somewhere around that 1k resistor and 9 volts into the board

I'm sure you are correct. I just came back up from the bench, haven't found problem yet. I opened up the GGG Rodent (Rat replica) that I built previous to this MGV and measured the voltage going into the LED; 1.9V

So back on the MGV, I pulled the lead that goes from the PCB to the LED, then I took out the R14 1k resistor. Checked for voltage where the LED lead was in the board (solder had refilled the hole) and 0V. Checked the now empty R14 hole along the trace leading into the LED lead, 0V. Check the other hole for R14 and 9.3V.

So I soldered one leg of a 1k R into that R14 hole with 9.3V, left the other lead pointing up in the air and measured voltage from that lead in the air to ground, got 9.3V.

I don't understand how I can put 9.3V into one leg of a 1k R and get 9.3V out the other leg. Only thing I can think of is that there should not be 9.3V going into that R, except it does show 9V leading into R14 on the schematic here (bottom right): http://www.generalguitargadgets.com/pdf/ggg_mgv_sc.pdf

Is it possible to burn out an LED with 9.3V? Maybe I have toasted it?

So I'm stumped and confused. BTW, looked inspected all of the solders and traces on the bottom of the PCB again and found no problems.

[ClinchFX]

If you have 9.3V on both sides of the resistor, it means that the LED is not drawing any current.  Are you sure you have the LED connected the correct way round. 

If you have the LED connected backwards, it will appear as an open circuit and will not draw any current.

Otherwise, the LED may be faulty.

To answer your original question, I use my Fluke multimeter on diode test to check LEDs.  Connect the positive probe to the anode of the LED and the negative probe to the cathode.  If the diode is OK and you have connected it the right way round, you should get a voltage drop reading on the meter.

[GuitarPlayer]

I didn't know there was a frontwards and backwards to an LED. It's just a DC circuit right? No polarity is there?

If you have 9.3V on both sides of the resistor, it means that the LED is not drawing any current.  Are you sure you have the LED connected the correct way round. 

If you have the LED connected backwards, it will appear as an open circuit and will not draw any current.

Otherwise, the LED may be faulty.

[ClinchFX]

I didn't know there was a frontwards and backwards to an LED. It's just a DC circuit right? No polarity is there?

Yes, there is polarity with LEDs, or any other diode.  Check the schematic - notice that the symbol for the LED is asymmetric.  This shows that there is polarity.

[GuitarPlayer]

Yes, there is polarity with LEDs, or any other diode.  Check the schematic - notice that the symbol for the LED is asymmetric.  This shows that there is polarity.

OK, I see it in the schematic. Thanks, I'm learning   I don't see any sort of markings on the LED. How do I tell which way is which? I guess I had better just flip it and see what happens?

[ClinchFX]

OK, I see it in the schematic. Thanks, I'm learning   I don't see any sort of markings on the LED. How do I tell which way is which? I guess I had better just flip it and see what happens?

On some LEDs, there is a small flat on the flange at the base of the LED on the Cathode side.  However, the usual way of telling before you cut the leads is that the anode lead is longer.  If you have already cut the leads, just try flipping it round.  If you have soldered wires to the LED leads, don't try to remove the wires from the LED - just switch the other ends of the wires.  Too much soldering heat on the leads of the LED can damage it, particularly if you cut the leads short.

Peter.

[GuitarPlayer]

^^^^^^ OK, thanks Peter. I think I may have toasted my LED because I did indeed cut the leads short and then soldered on some lead wires. I'll run out and buy a couple more tomorrow morning and give it another go.

Just to verify, the anode lead on the LED is longer, and the anode lead should get the voltage off the PCB, right?

Thanks again,
Jake

[ClinchFX]

^^^^^^ OK, thanks Peter. I think I may have toasted my LED because I did indeed cut the leads short and then soldered on some lead wires. I'll run out and buy a couple more tomorrow morning and give it another go.

Just to verify, the anode lead on the LED is longer, and the anode lead should get the voltage off the PCB, right?

Thanks again,
Jake

Jake, you may not have toasted the LED and it's worth swapping the connections and trying it.

Yes, the Anode is the longer lead on the LED, and it should connect to the 1K resistor R14 on the PCB.  The short lead on the LED, the cathode, should go to the switch.  When soldering the LED leads, it's a good idea to clip a small alligator clip to the leads close to the body of the LED, to act as a heatsink to prevent too much heat getting to the LED junction inside the plastic.  Don't forget to remove the clip after soldering.  If I solder directly to the LED, I cover each solder joint and the LED lead with pieces of heatshrink after soldering.  In my pedals I use sockets for the LEDs.

I'm not sure that I explained properly before, but, if there is no current flowing in a resistor, you will measure the same voltage at both ends.  This is an example of Ohm's law.  The formula is V=IR, where V is voltage difference between the ends of the resistor, I is the current through the resistor and R is the value of resistance.  If you have 1milliamp (.001 Amp) flowing in a 1K (1000 Ohm) resistor, you will see 1V measured from end to end of the resistor - .001 Amp x 1000 Ohms = 1 Volt.  If the current is 5mA (.005 A) the voltage across the resistor will be 5V

Good luck,

Peter.

[GuitarPlayer]

Jake, you may not have toasted the LED and it's worth swapping the connections and trying it.

Yes, the Anode is the longer lead on the LED, and it should connect to the 1K resistor R14 on the PCB.  The short lead on the LED, the cathode, should go to the switch.  When soldering the LED leads, it's a good idea to clip a small alligator clip to the leads close to the body of the LED, to act as a heatsink to prevent too much heat getting to the LED junction inside the plastic.  Don't forget to remove the clip after soldering.  If I solder directly to the LED, I cover each solder joint and the LED lead with pieces of heatshrink after soldering.  In my pedals I use sockets for the LEDs.

I'm not sure that I explained properly before, but, if there is no current flowing in a resistor, you will measure the same voltage at both ends.  This is an example of Ohm's law.  The formula is V=IR, where V is voltage difference between the ends of the resistor, I is the current through the resistor and R is the value of resistance.  If you have 1milliamp (.001 Amp) flowing in a 1K (1000 Ohm) resistor, you will see 1V measured from end to end of the resistor - .001 Amp x 1000 Ohms = 1 Volt.  If the current is 5mA (.005 A) the voltage across the resistor will be 5V

Good luck,

Peter.

Thanks for the explanation Peter, I really appreciate it. I did understand earlier in the thread that with no current through the R there would be no voltage drop. But thanks for the example with Ohms Law, as I will save that off to a file for future reference.

I did go back down to the bench and checked my LED. There is a flat mark on the body indicating the anode side and I did have it on the positive lead coming off the PCB. So at this point I think I've either toasted the LED or it was faulty to begin with. I'm going to run over to the electronics store tomorrow and pick up some more. I'll also look for some ready made heat sinks, and if they don't have them make some of my own from copper & alligator clips. Thanks for the advice on those.

I think I'll also look for sockets to plug the LED into, so they don't have to be exposed to heat. I have been using the shrink wrap already.

Well it's late here, I'll report back to this thread with my findings from tomorrow. Thanks again to you and all the others who provided advice.

Cheers!
Jake

[ClinchFX]

There is a flat mark on the body indicating the anode side and I did have it on the positive lead coming off the PCB.

I think I'll also look for sockets to plug the LED into, so they don't have to be exposed to heat. I have been using the shrink wrap already.

Well it's late here, I'll report back to this thread with my findings from tomorrow. Thanks again to you and all the others who provided advice.

Cheers!
Jake

The flat on the LED body is the Cathode.  The long lead is the Anode.

Although you can buy special sockets for LEDs, I usually use a similar socket to the 2 pin version of these http://www.jaycar.com.au/productView.asp?ID=HM3402&keywords=header&form=KEYWORD because they are easier to get.

Peter.

[slacker]

If you've already cut the leads or can't see a flat on the body then if you look at the LED it's got 2 metal parts inside it attached to the leads, the smaller one is the anode.

[Paul Marossy]

Last week I made a simple LED tester using a piece of scrap wood with a battery holder that holds (4) AA batteries, two small alligator clips screwed to the wood, and a momentary switch. Of course, a current limiting resistor is needed, or else the LEDs will fry upon testing. I blew one up the other day because I didn't use that current limiting resistor at first. Interesting to watch one side of it sort of fizzle and then make a loud POP as the top half of it blows off! Luckily it didn't hit me in the eye and flew across the room instead.

Anyway, it's very simple to test an LED with this arrangement and I get to use up the AA batteries that my kid's Leapsters eat up. It's a win-win situation.