Flashing 12v LEDs. Help please with weird effect.

[dasatcopthorne]

Hi Guys.

 

Cleverly I thought that I would put a flashing LED on one of our Club's control panels, to remind an operator that something is switched on or over to another control panel.

 

Anyway, I ordered from Ebay and have spent hours trying to get them working.

 

Luckily I powered them from an old H&M Clipper and at 12vdc they didn't work but seemed to work fine at 6v. Increasing the voltage turned the LED off.

 

I have spent many hours trying to discover the correct resister to fit so they flash on 12v.

 

I have now, just by chance, found that they are effected by ambient light. So cutting the light level brings them on at 12v with a 10k resisted in line.

 

I've now found that if direct sunlight falls on the lens, the LED shows a steady light!!

 

Anyone know and can explain what is happening here please?

 

Cheers

 

Dave (with minor knowledge of electronics)

 

 

[KalKat]

This tends to suggest the (tiny) circuitry inside the envelope isn't very well shielded. The net effect is that the device is photosensitive. In the good old bad old days, it was possible to make photo detectors by scraping the paint from transistors -  this appears to be a similar effect

Were these cheap LEDs by any chance?

 

Emma

[Tricky Dicky]

Normally LEDs specified at 12V usually have the correct resistor built in an no further resistance is required. If it is a standard LED then a resistor with a value of 1K should be enough, increase the resistance if it is too bright. A link to the product might help.

 

Richard

[dhjgreen]

quote:

"In the good old bad old days, it was possible to make photo detectors by scraping the paint from transistors"

 

Emma

Ooooh, that's a while ago; OC71 Germanium, etc.

[dasatcopthorne]

This tends to suggest the (tiny) circuitry inside the envelope isn't very well shielded. The net effect is that the device is photosensitive. In the good old bad old days, it was possible to make photo detectors by scraping the paint from transistors -  this appears to be a similar effect

Were these cheap LEDs by any chance?

 

Emma

 

Thanks for that, Emma.

 

Yes, cheap ones.

 

Dave

[dasatcopthorne]

This tends to suggest the (tiny) circuitry inside the envelope isn't very well shielded. The net effect is that the device is photosensitive. In the good old bad old days, it was possible to make photo detectors by scraping the paint from transistors -  this appears to be a similar effect

Were these cheap LEDs by any chance?

 

Emma

 

Thanks for that, Emma.

 

Yes, cheap ones.

 

Dave

 

Normally LEDs specified at 12V usually have the correct resistor built in an no further resistance is required. If it is a standard LED then a resistor with a value of 1K should be enough, increase the resistance if it is too bright. A link to the product might help.

 

Richard

 

Thanks, Richard.

 

They have a resistor fitted but wrapped in heat shrink. I haven't bothered to find the value.

 

I'm more interested in the susceptibility to ambient light. When we exhibit, there's not much chance of knowing what the light levels will be.

 

Dave

[dasatcopthorne]

Guys.

 

Hear's the gen from the ebay ad.

 

Cheers

 

Dave

 

Lens Type : Water Clear
Lead Length : 20cm
Optimum Supply Voltage : 9v - 12v for highest brightness
Minimum Supply Voltage : 5v with much reduced brightness
Maximum Supply Voltage : 14v 
Current : 20mA @ 12v
Viewing Angle : 20~25 Deg

Flash Rate : Approx 3 per/2 Sec 

[dhjgreen]

 

Guys.

 

Hear's the gen from the ebay ad.

 

Cheers

 

Dave

 

Lens Type : Water Clear

Lead Length : 20cm

Optimum Supply Voltage : 9v - 12v for highest brightness

Minimum Supply Voltage : 5v with much reduced brightness

Maximum Supply Voltage : 14v 

Current : 20mA @ 12v

Viewing Angle : 20~25 Deg

Flash Rate : Approx 3 per/2 Sec 

 

Is it time for a "down to experience " moment.  Did you buy/need lots of these.  If not, there are many choices here is Rapid's offering.

 

http://www.rapidonline.com/electronic-components/kingbright-l-36bsrd-b-3mm-12v-super-bright-red-led-flashing-56-0765 

 

 

Regards,

[cpman46]

Here is one I've used from Rapid Online on two control panels with no problems under bright flourescent lighting.

http://www.rapidonline.com/Electronic-Components/Kingbright-L-56bHD-5mm-14V-Red-Flashing-LED-55-0140?sourceRefKey=J6W-OxFrJ&filterSearchScope=1

 

As AndyID has reminded me in his post you are trying to run them from unsmoothed dc. The control panel LEDs above were run from a well smoothed & regulated 12V dc supply. you could try smoothing the supply by placing a 100uF 35V electrolytic capacitor across the output from the Clipper but would then need to use something like a 7812 or 78L12 12V regulator to bring the voltage down to a safe level for the LEDs.

 

If you need a simple diagram please PM me.

 

Regards

[AndyID]

The output from your Clipper has no smoothing and that is confusing the flasher circuit. Try running them from a battery.

[DCB]

You are provably way over 14 volts .  You need an Hand M safety minor or something capable of delivering 12 volts irrespective of load, a computer PSU perhaps.

The Safety Minor will deliver from about 1 volt to 15 volts ish to the track off load depending on the control knob setting.  The Clipper will be nearer 19 volts off load on either the uncontrolled DC or the Controlled DC irrespective of control knob setting .

 

It is quite normal for modellers to select a resistor for an LED based on using a chart for 12 volts when the supply voltage is actually 19 volts.

[Crosland]

You don't need a resistor with a "12V" LED, nor with a flashing LED. The other advice about requiring regulated DC is correct.

[dasatcopthorne]

Thanks for all the replies, Guys.

 

I only needed three but had to buy five but still only £3 odd.

 

Lessons learnt today:

 

Thought my Clipper put out about 14.6 volts for some reason. Actually 21.3v Mmmmmmm

Buy better quality in future and check the supply voltage and then re-check.

I found the resistor value needed by experimentation and not a chart.

 

As you will see from my first post. I have the LEDs working (on 21v) but could not understand the ambient light effect.

 

The clipper is only for home testing and I will have to start again with the proper layout supply which I think is 13.5v.

 

Now.

 

As the layout already has a number of LED signals set up for 12v, I would very much like to stick with using the same supply.

 

To smooth this supply,  would I still use the capacitor value mentioned above by Cpman46 please?

 

Thanks for putting me straight.

 

Dave

[cpman46]

Dave

 

What is the source of your 13.5V supply? If it is unsmoothed i.e. raw rectified dc then I suspect you will need a 12V voltage regulator as mentioned in my previous post. If you need a simple diagram please PM me.

 

Mike

[dasatcopthorne]

Dave

 

What is the source of your 13.5V supply? If it is unsmoothed i.e. raw rectified dc then I suspect you will need a 12V voltage regulator as mentioned in my previous post. If you need a simple diagram please PM me.

 

Mike

 

Mike.

 

The supply is from one winding on a '2x12V 50VA Chassis Mounting Transformer , rectified with a Bridge and fitted with a auto reset cutout.

 

Cheers

 

Dave

[KalKat]

That power supply is not a smoothed output - the idea of using a regulator circuit is a good one. These circuits aren't generally expensive and as a bonus, it won't upset anything operating already.

 

 

Emma  (feeling rather old after 'membering OC 71 and other 'geranium' devices)

[Huw Griffiths]

This tends to suggest the (tiny) circuitry inside the envelope isn't very well shielded. The net effect is that the device is photosensitive. In the good old bad old days, it was possible to make photo detectors by scraping the paint from transistors -  this appears to be a similar effect

Were these cheap LEDs by any chance?

 

I don't think it was just transistors - or diodes - which could be used as light detectors.

 

I don't know if you remember when Tandy / Radio Shack had shops in the UK - and they sold a number of "home electronics experiments notebook" type booklets. If I recall correctly, a lot of these were attributed to Forrest Mims III - quite a few included circuits which used LEDs as light detectors.

 

Also, I can't remember where I saw it - or when (it was a number of years back) - but I seem to recall reading about LEDs (or, possibly IREDs - infra red emitting diodes) being used as both emitters and detectors in some signalling / communications circuits.

 

 

Ooooh, that's a while ago; OC71 Germanium, etc.

 

Probably a while since the things were routinely manufactured and sold new, at the sort of prices most people would wish to pay - what's sometimes referred to as the "design" and "manufacture" stages of the components' "life cycles".

 

However, it's probable that a small number of shops might still sell a few of them, albeit at rather higher prices - what's sometimes known as the "maintenance" or "discontinued by manufacturer" stages. (In some cases, even the manufacturers might be "discontinued" - Mullard, for instance.)

 

 

I've never actually used germanium devices - but I can remember them.

 

I read about them - in a Ladybird book about building radios - in a long lost set of books called "Basic Electronics" - and in some of my uncle's 1970's Mullard data books (at the time, he fixed televisions for Radio Rentals - he's also the person who got me interested in electronics). For all I know, I might even have one or two of these components myself - mixed in with other components I've acquired over the years. However, I should add that I'm not in any rush to check.

 

 

Huw.

[cpman46]

Dave

Thanks for the info. You will probably have about 16V raw rectified dc after the bridge rectifier. I have attached a circuit for a 12V 1A regulator here which I use on all of my panels. If you need the bits to build I can supply no obligation. The LED is a standard type which is mounted on the regulator board as a quick indication that the circuit has power and also acts as a small load if the rest of the circuits using the 12V are disconnected for any reason.

 

12V Regulator Circuit.pdf

 

Regards

Mike

[dhjgreen]

quote:

"I don't think it was just transistors - or diodes - which could be used as light detectors.

 

I've never actually used germanium devices - but I can remember them."

 

 

 

Huw.

Sorry, way OT now but you are not correct Huw. Your second statement confirms you are too young to remember that the OC71 (and OC44 low noise 'RF' transistor, remember those Emma) had a glass dome and were painted black. To make them into a photo-sensor you scraped off the paint, as Emma said earlier.

 

We are talking the mid '60s here.

 

Oh and yes Huw, they were made by Mullard, in Stockport IIRC.

 

David

[cliff park]

There were also some that had a metal can over them, squeezed gently at the base with a pair of pliers to break the seal this could be removed. It left the working parts of the transistor in a gel, which could be picked off with a pin to make a phot sensor. The point was that the real things cost a fortune, I don't know why, but OC71s were quite cheap.

[cpman46]

Somewhat off topic from the original question but couldn't resist putting up the following picture after Cliff Park's remark re metal cans:

 

 

from L to R OA90, AC176, OA47, OCP71 Photo Transistor, AC127, AC141 & AC187 all from a bygone age but still in my bits box!!

[DCB]

You don't need a resistor with a "12V" LED, nor with a flashing LED. The other advice about requiring regulated DC is correct.

 

Correct but this chaps 12 volt supply was  was nearer 21 volts.   More than enough to fry many "12 volt" electronic components. 

Computer 12 Volts is 12 volts   Car 12 volts is or was 13.8 to 14.2 volts in my day and Model Railway 12 volts is typically 19 volts.

 

You really need to size an LED Resistor to keep the LED within maximum forward Voltage limits when the power supply is off load, not just use the nominal rating as LEDs are fairly unforgiving if too much voltage is applied.

[Crosland]

 

You don't need a resistor with a "12V" LED, nor with a flashing LED. The other advice about requiring regulated DC is correct.

Correct but this chaps 12 volt supply was  was nearer 21 volts.   More than enough to fry many "12 volt" electronic components. 

Computer 12 Volts is 12 volts   Car 12 volts is or was 13.8 to 14.2 volts in my day and Model Railway 12 volts is typically 19 volts.

 

You really need to size an LED Resistor to keep the LED within maximum forward Voltage limits when the power supply is off load, not just use the nominal rating as LEDs are fairly unforgiving if too much voltage is applied.

 

 

You do not need AND SHOULD NOT ATTEMPT TO USE a resistor with a flashing LED.

 

There's no such thing a "computer 12V". 12V is 12V is 12V,... Anything else is 12V +/- something or "nominally 12V", just like our mains is "230V" In fact it is 230V +10%/-6% and is, in practice, 240V.

 

We are now well in the 21st century, just use a properly regulated power supply.

[dhjgreen]

Somewhat off topic from the original question but couldn't resist putting up the following picture after Cliff Park's remark re metal cans:

 

DSCN0857.JPG

 

from L to R OA90, AC176, OA47, OCP71 Photo Transistor, AC127, AC141 & AC187 all from a bygone age but still in my bits box!!

Ooooh look, a naked OC71.

[dhjgreen]

"Thanks, Richard.

 

They have a resistor fitted but wrapped in heat shrink. I haven't bothered to find the value.

 

Dave"

 

Does anyone else find this slightly worrying, shouldn't the current limit be internal to the led package?