UART RX TX LED Driver using Op-Amp

SunnySkyguy said:

Making a precise logic comparator and regulated current limit is harder than it looks with a wide tolerance of hFE in an common emitter. So was never done that way in logic chips. None of these compare using a proper logic gate to drive the LED with a fixed current limiting R, which would be the best solution. (LVC type only)

Sorry to be harsh, but that's the reality.

Click to expand...

I agree and understand, If i ever wanted to mass produce this I obviously would not go with this simple solution. Like you mentioned all I need is a fixed current limiting resistor to make this more reliable...so the question is do transistor packages exist that include a resistor in series with the emitter or collector line?

EDIT: Actually or an LED that has a current limiting resistor in a surface mount package?

Like every FET has an RdsON, every transistor has an effective Rce when saturated and otherwise is Ib*hFE controlled resistor into a fixed voltage. It's just that BJT's have a wider tolerance on hFE and Rce and the latter is only specified when it is really good like Diodes Inc sub 100 milliOhm parts.

THe largest integrated discrete transistor supplier with the best binned specs is ROHM. THey will probably have what you expect, but only good for samples. They prefer big customers who buy only reels of parts. Check D-K for cut strips.

Better yet define your logic level chip output P/N and I'll tell the best LED driver , which I used in my original examples of 3.

jstefanop said:

so the question is do transistor packages exist that include a resistor in series with the emitter or collector line?

EDIT: Actually or an LED that has a current limiting resistor in a surface mount package?

Click to expand...

No, because the value of the resistor depends on how much current you need and on the supply voltage. Also, a current-limiting resistor frequently gets hot. How can a tiny surface mounted resistor cool itself?

Audioguru said:

No, because the value of the resistor depends on how much current you need and on the supply voltage. Also, a current-limiting resistor frequently gets hot. How can a tiny surface mounted resistor cool itself?

Click to expand...

Well I did find some SMD LED/Resistor packages, but they are designed for 5v and can't be used in the 3.3v rail I'm using. Pretty cool though.

I use current limiting surface mounted resistors all the time. Surface mounted resistors actually cool themselves way easier than any other type of resistor. Heat is passed directly from resistor into PCB efficiently because the whole resistor is in contact with PCB.

Why is Vcc only 3.3V? Then many LED colors cannot be used like blue, bright green and white because their maximum forward voltage is about 3.6V. Your LED does not have a forward voltage of 2.0V unless you bought a few hundred of them, tested them all and found one with 2.0V because LEDs of one part number have a range of forward voltage maybe yours is from 1.7V to 2.4V.

The minimum current of some LMV358 opamps with a 3.3V supply is fairly low and its output current is also a range and some have higher current and some have lower current.

Although the quality of LED's I buy is very high, ( of course I am biased) this is an example from my order of 25,000 pcs required to be 4000~4500'K and 16 Cd 30deg 5mm white clear.

Take note that ;
- only 8% are in the range are 3.0~3.2V
- None are >3.2V or <2.8V

P/N: 2S-4000W30N16-5NFZ , 25K pcs
1 [ ( 0.36 , 0.34) , IV:15000~17500mcd ,VF:3.0~3.2VDC ]===> 1 Kpcs
2 [ ( 0.36 , 0.35 ) , IV:15000~17500mcd ,VF:3.0~3.2VDC ]===> 3 Kpcs
3 [ ( 0.36 , 0.35 ) , IV:15000~17500mcd ,VF:2.8~3.0VDC ]===> 3 Kpcs
4 [ ( 0.37 , 0.36 ) , IV:15000~17500mcd ,VF:2.8~3.0VDC ]===> 6.5 Kpcs
5 [ ( 0.37 , 0.37 ) , IV:15000~17500mcd ,VF:2.8~3.0VDC ]===> 4 Kpcs
6 [ ( 0.38 , 0.38 ) , IV:15000~17500mcd ,VF:2.8~3.0VDC ]===> 7.5 Kpcs


Since White are based on Blue LED Substrates, the results are the same;

Here were my Yellow and Red 20mA 5mm LEDs

TYPE: 2S-615R30N9-5NFZ , 15K pcs RED
1 ( 620~625nm , IV:10500~12500mcd , VF:1.8~2.0 VDC )===> 15 Kpcs
2 ( 620~625nm , IV:10500~12500mcd , VF:2.0~2.2 VDC )===> 0 Kpcs


● TYPE: 2S-592Y30N11-5NFZ , 15K pcs YELLOW
1 ( 589~592nm , IV:10000~11500mcd , VF:1.9~2.1 VDC )===> 0 Kpcs
2 ( 592~595nm , IV:10000~11500mcd , VF:1.9~2.1 VDC )===> 15 Kpcs


For many suppliers they specify very wide tolerances to handle the occasional process escapes and get 100% yield but I buy only the best quality.

For these LEDs , you can run them from 3.3V supply no problem with suitable R for desired current. or even 3V Lithium as indicators.

Keep in mind that 16 Cd or Candella is blinding at arm's length , so you won't need 20mA and this reduces Vf according to Ohm's Law and the ESR of the LED.

Many people buy no-name-brand junk LEDs from ebay that do not even have a datasheet. They are probably rejects from a manufacturer. Their forward voltage could be anything.
I buy ordinary quality LEDs from a well known parts distributor who I trust who buys from a manufacturer who I trust. If they print spec's in a datasheet then I follow them because I do not want to test and select parts.

I have tighter specs than any other distributor and the results all meet spec, but I need no spec for Vf. as they are always ok when Iv meets spec in my case.

But I agree Ebay/Amazon sales never have specs are most likely rejects. A recent test on a good distributor's colour quality of green indicated they were all over the map.

Also Warm and cool are easy to make.. 4000~4500'K is the perfect balance of blue substrate LED and Phosphor conversion to red/yellow to give the apparent perfect daylight white using only 75μm of phosphor thus achieve equal CIE corrected eye energy for RGB.
(recall Blue + Yellow = Green) These specs and results listed are exceptional rare in 5mm LEDs

Also newbies note that LED Luminous Intensity , Iv ( like hFE) can vary as much as 4:1 even in good specs from Max.:Min and binned parts cost more with 2:1 range. See the tolerance on the parts I get? for mcd Intensity? These have been certified by an independent lab in UK for Philips EU. Just FYI, that it is possible to get high quality and consistency, if you know how and where and have adequate demand.

My very cheap Chinese flashlight has 24 white LEDs that all look the same color and brightness. It is powered from three AAA cells in series without any current-limiting resistors.
Somebody in the LED or flashlight factory must be testing and binning the LEDs.

LEDs are always 100% tested and sorted, which is why it so easy to expect rejects for colour , intensity or voltage on Ebay. Some OEM's know better to use from same bag per assembly. Others have to mix it up. But flashlight mfg's would not have the automated test equipment to do this kind of sorting.

Yea im not buying junk eBay LEDS. Im using 0603 SMD Yellow and Red leds from good supplier at digikey. They have a 1.9v-2.2v range in forward voltage which is completely fine for a 3.3v source.

I am glad that you are using good LEDs.

You may have had a reason for choosing 603 LEDs but D-K has a lot of LEDs in 603 case size but they aren't very bright.

Now I compared the best 5mm parts in 603 and you would need to add a driver to use 20mA whereas with a good 5mm part for the same price you could drive it directly from the UART with a bigger R <1mA and still be brighter.

The very best D-K has to offer is 150060RS75000 Wurth Electronics Inc is only 250mcd 160deg @20mA
I only use 5mm with zener 2S-615R30N9-5NFZ 9000 mcd 30 deg @20mA

Whereas the cheapest parts are only 40 mcd !!! ( which is about as good as EBay )