Chinese LED tester

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Andrew Cowan

Senior Member
Hi all.

I recently bought an LED tester (from ebay). It is mainly for working out what colour superbrights are, but also for working out what current LEDS need.

I was dubious about how accurate the current provided would be, so I ran opened it up to see how it works. Answer - resistors. Not suprising given its cost.

Anyway, it seems very odd - here are the values of the resistors, and the calculated current (given a voltage drop of 2V from a 9V battery), at each test current:

2mA slot: 1540 ohms, so 4.5mA
5mA slot: 364 ohms, so 19mA
10mA slot: 364 ohms, so 19mA
20mA slot: 364 ohms, so 19mA
30mA slot: 240 ohms, so 29mA

I have no idea why these values are so wrong, but I plan to replace the resistors. There were also several solder bridges in the unit - I have cleaned these up now.

So anyway, I will replace the 2ma, 5mA and 10mA slot resistors - with 3.5K, 1.4K and 700 ohm resistors. Is 2V an average value to assume for the voltage drop?

Has anyone any idea why the manufacturer has done this?

Andrew

EDIT:

Upon looking at the PCB further, the bottom strip (for common anode LEDs), has very odd wiring. Going from left to right (looking from above the unit):
Pin 1: 364 ohm resistor to 0V
Pin 2: 9V
Pin 3: There is space for a 360 ohm resistor to 0V, but instead, there is a solder bridge to pin 4 (0V)
Pins 4-13: 0V
Weird!

The row above (for common anode LEDs) also has odd wiring:
20mA slot:
Pin 1: 364 ohm resistor to 9V
Pin 2: 0V
Pin 3: 364 ohm reistor to 9V

50mA slot:
Pin 4: N/C
Pin 5: 120 ohm resistor to 9V
Pin 6: N/C, but i assume it should go to 0V
Pin 7: Linked to pin 5

70mA slot:
Pin 8: Linked to pin 10
Pin 9: N/C, but i assume it should go to 0V
Pin 10: 120 ohm resistor to 9V

150mA slot:
Pin 11: 51 ogm resistor to 9V
Pin 12: N/C, but i assume it should go to 0V
Pin 13: linked to pin 11
 

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marcos.placona

Senior Member
I can bet that not even the manufacturer have a clue about why they are using those resistors.

Is it at least using a PCB or is it all soldered on a veroboard :D

Would you mind attaching a pic of the inner contents of this box?
 
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ylp88

Senior Member
Keep in mind that LEDs are best driven as constant current devices. In fact, their forward voltage can change quite a bit depending on their operating conditions, made especially evident due to the current generation of LEDs which can get very hot if not operating under the correct conditions.

However, red LEDs usually have the lowest typical drop at around 2V and green LEDs tend to vary depending on the semiconductor used (usually around 2-4V, from memory). Blues, whites and UV LEDs are usually around 3.5-4.5V. But as I said, all of these values are typical since they vary with current.

ylp88
 
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Andrew Cowan

Senior Member
More photos.

The quality of the soldering on this is really bad, although it's hard to see under the glue gun :).

I can imagine a factory, where they save on component drawers by having one big tub of mixed resistors which are randomly inserted in any order.

Andrew
 

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papaof2

Senior Member
But just think how many people have employment because of hand soldering - instead of those nice wave soldered boards...

John
 
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Andrew Cowan

Senior Member
Dippy - I am not at all disappointed with the product. I think it is fun, as it gives me something to work on fixing. Within 5mA is accurate enough for me, so it just needs tweaking slightly. I thought it was odd, though, that the same resistors were used for different currents - thus I posted on here. I think I will use 3V as the average voltage drop, although as I said before, I am not particually bothered about accuracy.

Andrew
 
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Andrew Cowan

Senior Member
The plot thickens...

As I started to replace the resistors, I found that the correct values were silkscreened onto the PCB (see the photo).

That shows that whoever soldered it up just used the wrong components. I think that those workers need to be paid more (or given more hugs to encourage them).

Andrew

Edit - that has to be the clearest photo I have ever taken! Macro mode, tripod, no vibration and zoomed in. The object in the foreground is the tripod leg.
 

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Dippy

Moderator
Paid more?

Read up on Chinese 'shadow' factories... though I don't not suppose for one single, solitary nanosecond it will inhibit anyone here or anywhere buying cheap stuff. After all, it save the purchaser enough money to buy some Rose Tinted Specs and blinkers.

(I should add that I am NOT saying or implying that this is the case with this product. Only the factory knows). There, a hippy-grade escape clause :)

It's a pity the board and components aren't of the same exclent quality as your photo.
 
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Andrew Cowan

Senior Member
How else can we encourage the workers to be more careful? Lets give them more hugs.

I've now fixed the problem - replacing the incorrect resistors with (blue) replacements of the correct value.

I'm glad I have figured out how to take good photos of close up objects - it's hard to get people on this forum to help, when showing them a photo that is so blury it could be anything.

Andrew
 

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marcos.placona

Senior Member
Wow, that's a darn good picture Andrew (we can even see the resistor colours), but note that sometimes the same can be obtained with an ordinary scanner (didn't believe till I tried myself)

In regards to the errors on your board, I think this usually happens when they employ kids that were supposed to be at school, and giv'em some boards to solder in exchange for food.

I'm not at all saying that buying stuff from the Chinese is bad (I'd be being hypocrite as I buy lots of stuff from them), but that's an example to show that stuff bought from them should be very well analyzed. Today you've had problem with an LED tester, and in the worst scenario you'd just lose a couple of LEDs. Of course it could be worst, and you could be putting an expensive motor/ic in jeopardize for example.

This is a very delicate situation, as we end up buying cheap because of the low prices. I used to buy all my stuff here in Europe (normally at Maplins when I was living in the UK). Now I've found the "wonderful" world of dodgy Chinese stuff and mix up my purchases between here and there, but of course, when you talk about reliability, you know where to look.

Don't know exactly why I'm saying that, I guess I just feel a bit guilty about being one of those who indirectly help the slavery in China
 
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Andrew Cowan

Senior Member
I'm in the same situation as you - I used to buy all of my stuff at maplins, but then I discovered dodgy chinese stuff. The electronics seem to all be low grade, and the specifications seem to be a bit optimistic (eg on rc speed controllers and motors). However, for anything that doesn't have to be amazingly accurate or reliable, then chinese stuff seems fine.

For example, I wouldn't use a non-branded chinese/ebay rc transmitter for a radio controlled plane, while I would use a chinese/ebay servo tester for testing servos on the ground, as it doesn't matter if it goes wrong.

Anyway, while I have time on my hands (summer holiday), broken (but fixable) electronics sounds like fun!

Andrew
 
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marcos.placona

Senior Member
Andrew Cowan said:
For example, I wouldn't use a non-branded chinese/ebay rc transmitter for a radio controlled plane
Click to expand...
Except if you knew the right guys to buy from. I found some very reliable transmitters/receivers (Dr_Acula's tip) from the Chinese guys :)

And also, I have to say that when talking about Maplins, we're probably talking about the same dodgy Chinese stuff that comes straight from China to a reseller in the UK and then to Maplins. I ain't really sure about Maplin's rc transmitter's reliability either :D
 
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Andrew Cowan

Senior Member
Half the dodgy chinese stuff on you see on ebay can also be found in the maplins catalogue. For 10 times the price. I bought 400 breadboard jumper leads from ebay for £5 - in maplins, you get 10 for £3. (By the way, I didn't want 400 - but they only come in packs for 400 :))

The transmitter I use is a Futaba 6EXA - a well know brand.

Andrew
 
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marcos.placona

Senior Member
Yeah, I've seen those jumpers at Maplins about 3 weeks ago, and found them to be extremely for what they are.

The Futaba's are indeed very good and reliable, and I sure wouldn't risk crashing my plane with the dodgy ones :D

There's also that chance of being "conned" on eBay or even real web stores as sometimes they claim to be in the US, but in reality are based in China (or somewhere around), but just say they are in US to get some credit, and use the excuse of being in the US, because when you buy (being in the UK) from them, you don't complain about the extremely delays on the shipping.

I recently bought some stuff from Futurlec, just to realize right after the purchase that everything comes from Thailand (there's no way they can hide the post office's stamp).

I should have guessed that, as the prices were really cheap when compared to any store here in the EU or the US.
 
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manuka

Senior Member
It helps to put the immensity of China's 1200 million people into perspective against the population of the USA (a mere 300 million). For every Brit. there are approx 20 Chinese. Imagine -20 Dippys ! ( And before he mentions it, this means 1200 million/4 million = 300 Stans !). From the point of view of many Chinese, they are the centre of the world & the rest of us just bit players. Put it into perspective by considering ~10 million people in Greater London against ~1/2 million in my NZ city of Wellington. As a resident of London how would YOU feel !

Even with skilled wages just ~US1$ an hour,many Chinese made goods are increasingly of superior quality, & made using higher tech., than Western made equivalents. Although their cost structure is ~10% of the West,huge regional differences mean you'll naturally get ENORMOUS quality variation!

I've found many " Made in China" Western critics tend to assume country wide "ma & pa" backstreet sweatshop operations. Although this certainly the case in some regions away from the prosperous East Coast,on recent visits to China I've been gob-smacked with the mega industrial sophistication of the likes of Shenzhen (over the border from HK). It tends to make Silicon Valley look like "Silicon back alley" in comparison.
 
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Michael 2727

Senior Member
It is well known that in the late 60's when "Transistor Radios" became affordable
and as popular as todays i-Pods, they were assembled in the back blocks of
Asia, China, Japan by 10yr to 12yr old Kids. No Factory, no parts drawer, no
ESD workbench. Probably didn't even have electricity in their Village/Town,
that would mean a gas or coal heated soldering iron, no Weller Solder Stations
there I would think.
It was called WORK Experience -
They all grew up to become designers, assemblers for PC Motherboard. ;)
 
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Dippy

Moderator
Assuming they didn't suffer from heavy-metal-poisoning as is happening in some recycling places in China.

Yes, to be serious, I think we all appreciate there is an enormous quality variation. It varies from 'perfectly good' & 'great value' right down to Andrew's LED tester and a mains plug I bought.

Example: Just about every GLCD you buy will be made in China.
However, ones bought here from reputable Western importers/suppliers will have a high probability of good quality products being made in 'acceptable' factory conditions. Indeed I know a supplier/importer representative who went to China to view the factory personally. He's been to China numerous times, just like you Stan. And he said he saw some dogdey factories as well as smart ones.

But do you feel comfortable about a 2 quid GLCD from an anonymous bloke off of Ebay or similar? Of course you do, you've saved money and blow where/how it was made. Do you complain when it doesn't work properly or conks out after 3 weeks? Most people couldn't give a monkey's. Happy to waste money saving money.

The lower the price, the higher the risk, generally. Unless it's knock-off of course.

Anyway, I still maintain that even if a super-cheap useful product was known to be made in a dodgy factory over half the people would still buy it.

I should add that I buy products made all over the place, but I try my best to avoid the real cheap nasties. I demand quality even if it costs a bit more. AndI will that little bit extra to know I have a reliable warranty and support.

Here's a starter..
http://www.laborrights.org/creating-a-sweatfree-world/sweatshops/1524

[End of Sunday Rant, phew!]
 
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Andrew Cowan

Senior Member
I made one using picaxe (08M, potentiometer, servo output), but then I wanted to add an LCD display showing the signal generated, and the signal recieved when it is connected to a reciever. Axe033 = £12. Premade servo tester = £5. Sorry!

Andrew
 
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Andrew Cowan

Senior Member
I've got an LCD counter module somewhere which would have been perfect.

08M:
Out 0: Counter reset
Out 1: Counter count
Out 2: Servo out
In 3: Servo in (from reciever)
In 4: Potentiometer

I see what you mean, that looks like a great project. I could still do it...

Andrew
 
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