Any ideas about "testing" an LED?

[BJR]

I've got 2 LED circuits I need to test on my assembly line for my customer. I've come close but need better or different ideas. One circuit is a 390ohm resistor in series with 2 LED's powered by 13.5 VDC, the other is one LED with 910ohm resistor.
Customer's concern is the LED's not being installed on the circuit board or being installed backwards. No vision to be used here or analog card either since there is already 2 machines built previously that we would like to integrate this checking feature into for them without adding much on.
The basic idea I've had from the beginning is to insert a relay coil in series with everything else so if the LED conducts the relay coil will turn on, giving me a way to also trigger a PLC input that says the circuit is OK. The problem is I have not come up with a relay coil that has the correct characteristics to work in the circuit. And any attempts to "modify" the coil resistance by putting a resistor in series or parallel with the coil would only get me further away. I had one sort of working earlier but it is not reliable. 12VDC coil with 3200ohm resistance only has about a 7V drop across the coil and the specs. say it will reliably turn on at 80% and guess what - they were right. It hardly ever turns on.
I wish I had some solid state relays here or information about them. I can't find it. I know there is nothing mechanical in there and that the switching is triggered by an LED or optical circuit but since there are different voltage coils available I can't believe it would gain me anything over a mechanical relay coil as far as the characteristics go because there is probably another resistor in there to limit current to the optical or LED circuit. Anybody have some good information on SSRs?
The customer suggested running an output from the PLC through the circuit into an input to the PLC. I know that won't work because the LED will never conduct without turning it on with some voltage but it may prompt some other ideas.
I keep getting stuck on my first idea and can't find a relay that will work in my circuit and have not thought of any other ideas as of now.
I would appreciate any ideas you may have.
thanks

 

[stasis]

try putting a relay in parallel with the LED...this will also give you an easy way to fire the input

 

[BJR]

But won't the relay turn on whether the LED circuit that will be in parallel with it is working or not?

 

[randylud]

When the LED is forward biased, the light on, the relay being parallel will essentially be shorted out. Try it!

 

[stasis]

good point...I was thinking you just needed to know when the circuit was 'on'. If you want to see if the circuit is conducting, series it through an input & a relay output.

 

[BJR]

both good ideas but-

I found that if I put a relay in parallel with the LED circuit the relay will turn on with the LEDs installed correctly and lit up or with them missing. I can't easily test what happens if they are reversed but there should be a difference between correct and missing so I have already failed detecting one situation.

Since it is a NPN system I tried removing the ground wire from the LED circuit and replacing it with the input wire. Either since the LED circuit is 13.5V and the PLC is 24VDC there is something floating ( although both power supplies are grounded to the same point) or I just am not getting enough current to turn on the PLC input. The LEDs won't turn on when hooked up this way. I'll have to look up some specs. on the PLC card and take some measurements with my meter. May be on to something though.

 

[BJR]

I think the first advice about parallel with the LEDs you intended me to put the relay in parallel with only the LEDs correct? That may work. Problem is that the pull up resistor and LEDs are on a printed circuit board inside the part and I have no way to wire to the LEDs only. Is that what you meant??

 

[randylud]

Correct, when the LED is forward biased, the forward voltage drop is going to be about 1.5V and that is not generally enough to energize any relay I know of.

 

[BJR]

oops. I wasn't thinking clearly on the PLC input. It doesn't provide a ground. You have to ground it to come on. Mondays!

Guess I'm out of ideas again.

 

[Doug-P]

correCould you put an optoisolator in series with your test circuit? That way the indicator would only come on if a component is present and correctly installed. It helps that the optoisolator only drops about .7 volts. You could drive a DC PLC input directly with the opto output or drive a relay for more power.

.02

After reading Panicmode's post I'll amend mine.

.7 volts is for germanium diodes. LED types do drop more.

 

[elevmike]

What about a photocell? Park a photo cell above the led and simply energize the circuit, and if the photocell goes on it's good. You could easly home brew a small photo-switch with a photo cell & transistor.

 

[panic mode]

what are you trying to do?
turn on relay when LED is on?
and LED is powered from 12V source through 390 Ohm resistor?
why don't you post a circuit?

you also mention SSR but which one? what is the forward voltage and current? and why do you need LED anyway? if you are messing with medical equipment and you don't know the basics, you might be endangering people.

Standard LEDs use 15-20mA and drop 1.7-2.4V depending on junction material. They will work with lower current too but brightness will be lower.
New generation of LEDs is using much less current (some 5mA is common) but the voltage drop is about 3V. Greater currents are possible and brightness remains about the same but the LEDs do get warmer (to an extent... then they die..).
12 and 24V relays use similar order currents.
Without knowing more I would change relay for 9V version and put the relay coil instead of 390 Ohm resistor. 3200 ohm is way up, you should be able to get relays with 600-800 ohm coil. that way LED (and relay coil) would be getting some 12-16mA.
you have to be carefull with the electromagnetic relays (not SSRs) because of inductive characteristic of the coil.
if the relay doesn't have built in diode (such as 1N4004 or 1N4007 for example), you might get your LEDs killed.
better way is to use optocouplers.

if the only concern is that LEDs could be oriented wrong way, change LED type. there are models of bi-color LEDs with 2pins.
inside are two LEDs connected in anti-parallel. no matter how you turn it, it will be on when energized. If turned wrong way it will illuminate in other color (red instead of green for example).

 

[MarkTTU]

I vote using a photo cell. It should work easily enough and not require any wiring to the actual circuit.

 

[elevmike]

if the only concern is that LEDs could be oriented wrong way, change LED type. there are models of bi-color LEDs with 2pins.inside are two LEDs connected in anti-parallel. no matter how you turn it, it will be on when energized. If turned wrong way it will illuminate in other color (red instead of green for example).

How about adding a full wave bridge to the circuit? Same effect.

 

[Terry Woods]

This, or similar, should apply to both test cases.

It doesn't matter if the LED under test fires or not... it only matters that it is there and installed correctly.