How can I determine the resistor value for an unknown type of LED? - Page 1
Forum section: electronics / beginners

author: xenium
author_rank: Newbie
timestamp: January 29, 2020, 03:54:48 am
content: I'm working on a little project to build a modem emulator for my 8-bit retro computers. Basically, a Raspberry Pi in an old Hayes modem case, which is running a modified version of Jim Brain's tcpser modem emulator software. This will allow me to make Telnet (and other TCP) connections from the retro computer's serial port, using an old-school terminal emulator. Fun for "dialing in" to Internet-connected BBSes and such. The modifications I'm making to the tcpser software will allow the Raspberry Pi to blink the modem status LEDs, which will be attached to the Pi's GPIO lines.I want it to look just like a real Hayes modem, so I want to re-use the LEDs from the original donor modem. The problem is, I haven't been able to find *any* specs for these LEDs, so I don't know what size current-limiting resistors to use. I have both a 1200 and 9600 baud Hayes modem which I can take the LEDs from. I checked both of them, and found that one of them is using 200 ohm resistors, while the other is using 400 ohm resistors. I have no idea what voltage they are running the LEDs at though, so that info does me no good (and I can't measure it, because I don't have the power supplies for the modems, and I don't think they are in operable condition anyway.) They use 13.5 volt power supplies, so it could be any voltage up to that I suppose.The LEDs are square, with the anode and cathode coming out of the side of them, so they emit light at 90 degrees to the circuit board they are mounted on. They emit red light, but are in a clear package. There is a dome on the side of the LED that emits the light. I haven't even been able to find a photo of these LEDs online, and the modems are from the 1980's, so it's entirely possible they aren't manufactured anymore.I've found a number of formulas online for calculating current limiting resistor values, which say to measure the voltage drop across the diode and then use the formula to select a value based on the desired current, but I don't even know what current these should be run at. I've experimented a little, and have gone down as far as 50 ohms @ 5V. The LEDs seem to operate well at that value, and I've left a test one running like that for a few days and it did not burn out or get warm, but that seems like a really low resistor value to me, so I'm afraid I'm overdriving the LED and will cause it to die prematurely. If I use a more typical 200 ohm, or even 100 ohm resistor the LED seems a bit dim though.Any ideas on what I should use? Does anyone know where I might find specs for these LEDs?

author: Brumby
author_rank: Unknown
timestamp: January 29, 2020, 04:17:21 am
content: Don't worry too much. You have done enough practical work to get all the information you need.I suggest you take this experiment:Quote from: xenium on January 29, 2020, 03:54:48 am... and measure the voltages across the LED and across the resistor.Using Ohms Law for the resistor will give you the current running through the circuit. Take note of this value. Also, take note of the voltage across the LED.Using these two figures, you will then be able to calculate the resistance required for any given supply voltage.In relation to your concern:Quote... you could try other resistor values between 50 and 100 ohms - choosing the highest value that gives you acceptable brightness and re-do the calculations.

author: tooki
author_rank: Super Contributor
timestamp: January 29, 2020, 08:58:51 am
content: Quote from: xenium on January 29, 2020, 03:54:48 am50 ohms sounds risky to me. Indicator LEDs are typically rated for 10-20mA drive current, while 50 ohms on 5V is about 70mA for a standard 1.8V red LED.Remember thatLEDs from that era weren’t nearly as bright as modern ones, so if you drive them to modern brightness levels, you’re overdriving them significantly and will reduce their lifespan.*Now, some researchfound a sourcethat says that the very first red indicator LEDs (1970s) were specced for drive currents of 50mA, while late 70s to mid-80s-onward high efficiency (“low current”, “super high brightness”, and “high efficiency” types) red LEDs were typically rated for 20mA, often with 30mA absolute max.I’d either measure the actual current across them in their original circuits (just measure the voltage across the dropper resistor and use ohm’s law to calculate the current), or just stick with the usual 220 ohms for 5V, which will drive them at a conservative 15mA, or 180 ohms which is around 20mA. If you need more brightness, find a modern substitute. There’s a good chance something is still available in the same package. Heck, I’ll even help you look, since I’ve spent unhealthy amounts of time looking at LEDs on eBay, AliExpress, and other suppliers and so have a good grip on the right keywords and vendors. :p*When I resumed my electronics hobby after a decade+ hiatus, I got various modern LEDs, and also dug out my bin of late-80s radio shack and mail-order grab bag LEDs. When testing, I was surprised at just how dim those LEDs are. Like, I knew they’d be dimmer, but I’d forgotten just how big the difference was! (And that’s not even considering the new-type ultra-efficiency green and blue LEDs, whose intensity is just insane.)P.S. #1 It’s almost guaranteed that the modems’ internal circuitry used 5V for the logic that drove the LEDs, so using the same resistance values they used is a safe bet. In the unlikely chance they used a higher voltage, then you’d merely be running them at lower current.P.S. #2 Another potential solution is to use modern light pipes and modern LEDs, or even to saw apart the old LEDs to turn them into light pipes for use with modern LEDs.

author: atmfjstc
author_rank: Regular Contributor
timestamp: January 29, 2020, 09:14:07 am
content: I couldn't find any schematics for Hayes modems, but if you could take a hires pic of the top and bottom of the PCB, we might be able to figure out what voltages the original LEDs would have been driven at. Possibly 9V for the 400Ohm resistor ones.

author: Jwillis
author_rank: Super Contributor
timestamp: January 29, 2020, 11:40:28 am
content: This is how I get an approximation of the forward current of a LED.I take a resistor and measure the exact resistance. Record that resistanceSet up a the led and resistor in series on a breadboard. Apply a voltage until you Just achieve the brightest LED.I do this in the dark because it helps me to see small changes to the brightness. You want the transition where the difference in brightness is very noticeable .Adding more voltage will only make very small changes. Measure the actual Voltage of the source. I do this because the readout on my supply is a little out. Record that voltage. Measure across the LED. and record that voltage.Plug the numbers into R=(Vs-Vled)/IledOhms law I= E/RFor example I have a resistor that measure 470.5The voltage applied to circuit is 10 volts for the brightest LED. I kind of play with the voltage up and down.and the voltage across the LED is 2.070470.5= (10 - 2.070)/I is is to say I= 7.93/470.5 = 16.8mAClose enough to 20mA rating .I find this works even for some high power LEDs I have . Some with Vfas high as 60V and Ifas high as 1AThe brightness of LEDs and the Ifis non linear so you kind of have to play with the source voltage and make the best judgment you can.

author: MagicSmoker
author_rank: Super Contributor
timestamp: January 29, 2020, 11:47:10 am
content: Quote from: tooki on January 29, 2020, 08:58:51 amDespite being bolded, that's still an understatement! There could be a 1000x difference in brightness between an 80's-era T1-3/4 red LED and a modern one in the same package. A quick perusal of Digikey shows brightness ratings at 10-20mA ranging from ~100mcd to >5000mcd (!!!), compared to maybe 3mcd for a vintage LED. If you replace a 3mcd LED with one putting out 5000mcd at the same drive current it will be like looking at an aircraft landing light the first time you flick that beast on.However, the PTH right-angle LEDs still available seem to have very 80's-era brightness levels of 2-4mcd at 10mA:https://www.digikey.com/products/en/optoelectronics/led-indication-discrete/105?k=&pkeyword=&sv=0&sf=1&FV=-8%7C105%2C69%7C411921&quantity=&ColumnSort=0&page=1&stock=1&pageSize=25

author: Fungus
author_rank: Super Contributor
timestamp: January 29, 2020, 12:34:53 pm
content: LEDs work by current, not voltage.Most LEDs have a maximum working current of20mabut you're better off using less than that if you want them to last for a long time.(20mA is usually far too bright anyway with modern LEDs)So:a) Set up your multimeter to measure currentb) Briefly measure the current going through your LED with a 220 Ohm resistor.c) If it's more 20mA, or if the LED is too bright for your application, increase the resistor and go to step b.d) If the LED is very dim and the current is less than 15mA, decrease the resistor (unlikely that you'll ever go below 100 Ohms).Trying to getexactly20mA with standard size resistors is pointless. The only way to do it is with a 20mA current limiting circuit. The difference in brightness between (eg) 17mA and 20mA will be tiny anyway.Don't ever go over 20mA on an unknown LED.If in doubt, buy a load of LED controller chips and use them instead of resistors. I'm fond of these:https://www.ebay.com/itm/152148026472They're SIP packages so they don't take up much space. You simply wire pin 8 high to get five 17mA current sinks. Connect the anode of your LED to your microcontroller, the cathode of your LED to the chip, job done. No calculations or resistors needed.

author: exe
author_rank: Unknown
timestamp: January 29, 2020, 01:46:09 pm
content: Interesting, for the sake of lifespan I don't use currents above 10mA, more like a few mA. Since not all LEDs are born equal, I just buy the brightest ones.I also have leds that are dim no matter how much current I give'em. I bought them recently (I bought ten different kinds of leds bc I wasn't sure which one I need), and I found no downsides so far of using bright leds. So, I prefer bright leds simply because to me they work the same way as "normal", but save a lot of power.

author: rdl
author_rank: Super Contributor
timestamp: January 29, 2020, 06:39:16 pm
content: I bought a 525nm green LED last year planning to use it as a power indicator for a PC. At 20 mA it will cause temporary blindness. At 5 mA it's almost too bright to look at directly. At 300 uA (yes, microamps) it's still much brighter than the average 1980s era green LED.edit: photo addedStandard old green LED on left at about 15 mA, new green LED to the right at about 320 uA. Far left is an old green 7 segment display. The photo is not all that great because it's hard to get the exposure of ambient light vs LED to look right, but you get the idea.https://www.mouser.com/ProductDetail/Kingbright/WP7083ZGD-G

author: james_s
author_rank: Super Contributor
timestamp: January 29, 2020, 06:46:32 pm
content: Those old LEDs were never very bright, they just kind of glowed, nothing like the modern ones you can get. Don't exceed 20mA, I probably would aim for around 15mA and call it good. As others have explained, measure the forward drop, subtract that from the supply voltage and use Ohms law to calculate the resistor, easy.

author: Mr.B
author_rank: Unknown
timestamp: January 29, 2020, 07:12:36 pm
content: I once had a similar application where I wanted to use the original LEDs in the original case of an old wyse terminal.Rather than powering the original LEDs, I glued a surface mount LED onto the back of each of the original packages.The light emitted from a modern surface mount LED was more than sufficient to make it appear that the original LED was working.

author: tooki
author_rank: Super Contributor
timestamp: January 29, 2020, 08:58:54 pm
content: Quote from: rdl on January 29, 2020, 06:39:16 pmAbsolutely! I have modern green LEDs like that and I also run them at half a milliamp or less, when used as an indicator. (Another thing i do is to sand down the front of the LED, since the frosted surface diffuses it a fair bit.)

author: Fungus
author_rank: Super Contributor
timestamp: January 29, 2020, 09:34:10 pm
content: Quote from: tooki on January 29, 2020, 08:58:54 pmI thought I was the only one who did that.(making super bright, diffuse LEDs by hand)

author: Brumby
author_rank: Unknown
timestamp: January 29, 2020, 11:31:55 pm
content: In reading through these posts, I admit I overlooked one point... not finding out the measurements of your test circuit.I think we would all like to see the actual figures you got from taking the measurements I suggested. That will tell us a lot - and we will be able to make better recommendations!LED rule #1: The lower the current, the longer the life you can expect.

author: iMo
author_rank: Super Contributor
timestamp: January 29, 2020, 11:46:15 pm
content: 20mA advice is a BS.All modern LEDs work fine at <1mA, any color (as said above).The far east vendors recognized that a couple years back and started to deploy 1k resistors on newer boards.I did a measurement with about 20 different LEDs I got at local store and most of them lit fine at 3k6 (3.3V) and an orange or white one even at 10k..

author: rdl
author_rank: Super Contributor
timestamp: January 29, 2020, 11:56:24 pm
content: Keep in mind he is re-using LEDS from some 1980s vintage equipment. 20 mA is not out of line for LEDs of that era.

author: iMo
author_rank: Super Contributor
timestamp: January 30, 2020, 12:01:24 am
content: Ok, LEDs from 70ties and 80ties are 10-20mA, sure.

author: james_s
author_rank: Super Contributor
timestamp: January 30, 2020, 01:30:01 am
content: LEDs from the 70s-80s is exactly what this thread is about.There are reasons to run modern LEDs at 10-20mA, but not if you're using them as indicators. I can't count the number of times I've removed LEDs or changed resistors on various boards because they came with blinding bright SMT LEDs on them.

author: tooki
author_rank: Super Contributor
timestamp: January 30, 2020, 03:41:54 pm
content: Quote from: imo on January 29, 2020, 11:46:15 pmI think it would behoove you to actually read the original post before calling BS... If you had, you'd know that this entire thread exists because the OP needs to know how to determine the Vf of theoriginal, vintageLEDs.

author: tooki
author_rank: Super Contributor
timestamp: January 30, 2020, 03:48:58 pm
content: Quote from: Fungus on January 29, 2020, 09:34:10 pmIt baffles me that most THT indicator LEDs now come in water-clear packages, instead of a diffused package.I've been experimenting with all sorts of things to make good indicator LEDs. Currently, the best bet has been to start with a 2x3x4mm LED, paint it all over with either white or metallic silver paint (to reduce leakage out the sides, as well as give a more uniform appearance from the top) and then sand the top matte. White heat shrink works, too, but I need to find a thinner tube that fits more snugly before shrinking, and it's more finicky to get flush. I've been considering trying this clear spray paint I saw in a shop that is specifically for making glass/acrylic look frosted.(If this sounds reminiscent of the 80s-style rectangular LEDs that had a white plastic frame and diffused epoxy inside, that's basically the effect I'm going for. Those LEDs aren't available at all any more, never mind in modern colors...)

author: xenium
author_rank: Newbie
timestamp: February 01, 2020, 06:21:34 am
content: Wow! Thank you everyone for all the feedback. This was my first post here and I didn't expect the topic to be this popular.Thank you Tooki for the excellent link to the reference on old school LEDs. It was very illuminating (pun intended.)I think I may indeed have those very old 50ma indicator LEDs. I think that for several reasons:The first Hayes Smartmodem was released in 1981, so assuming the design process began a year or two earlier, that puts it in correct the timeframe for the 50ma indicator LEDs described in the reference you linked to. As far as I know they used the same LEDs for all their future Smartmodems, so my mid-80s modems would probably still have the same ones.In the reference you listed, it says that the later, higher-efficiency LEDs would change the shade of red if they were in a clear package, depending on the current. The LEDs I have are in a clear package and seem to produce the exact same shade regardless of how much current I put through them, they just get brighter or dimmer. I've tested them all the way up to 78ma, with no discernible change in color shade. They always look red, not orange.I've tested them all the way up to 78ma! I would expect a typical 20ma LED to burn out quickly at that high of a current? I left one running at 45ma for days, and it seemed fine and did not get warm to the touch. Even at 78ma I was able to run it for a few hours. At that point it finally did start to get warm and the light from it began to look orange, so it would seem that 78ma is past its breaking point.To those that asked for measurements in the original circuit: again, I can't measure them in the original circuit, because I don't have a power supply for it. Also, being an electronics newb, I have a pretty limited bench supply... it's just a converted ATX power supply (I know, I know...) so I can't easily produce the 13V the modem required. And again these were "parts" modems that I bought cheap, so I don't believe them to be operable anyway.For those that asked for other measurements: my multimeter set to diode modes measures their Vf as 1.76. Running them at 5V on my breadboard with various resistors I get the following:22ohms = 78ma(1)47ohms = 45ma(2)100ohms = 25ma (3)200ohms = 14ma (3)1Kohms = 3ma (4)10Kohms = 300ua (5)(1): I decided to sacrifice one of them in the name of science: it ran for several hours at 78ma before finally showing signs of burning out.(2): I ran one at 45ma for several days with no signs of harm. This was the only value that looked to me like a reasonable output level for an old-school indicator LED. Yes, I know that LEDs of that vintage were very dim. (I'm old enough to have used these types of modems back in the day.) Still, at any less current than this they are VERY dim, as in barely visible kind of dim. That was what inspired me to start this post in the first place, rather than just running them at 20ma to be safe.(3): They were visible at this value, but really seemed quite dim, even for 1980's indicator LEDs.(4): *barely* visible, just a tiny pinpoint of light that I needed to turn the room lights down to even see.(5): No visible light that I could see.To those that suggested using modern LEDs or cutting up the old ones to use as light pipes... yeah, that would probably be the sensible thing to do, but these old Hayes modems did have a very unique look to them. It's hard to describe... sort of a glowing pinpoint of light that I think gets magnified by the clear housing. which has a domed lens in it. I just don't think it would look the same with modern LEDs.Also, I agree with you 100% Tooki on modern indicator LEDs. I guess I'm just old, but I don't get it. I have a blue power LED on my TV that I had to cover with tape because it is so bright that I couldn't even watch the TV without constantly being distracted by the glare. What were they thinking? Now that I'm finally getting a bit more into electronics I may start swapping some of them out.)I've finished prototyping the project on a breadboard (with modern LEDs) and programming the Pi, and it's pretty awesome if I do say so myself. I even hooked up the modem's speaker to the Pi's audio output, and I have it play mp3s of appropriate call progress and carrier sounds at the appropriate times. It really does feel like I'm dialing into a BBS via a phone line in the pre-Internet days.To those that asked for photos, here you go!These are the actual LEDs we've been talking about:And here are the high-res photos of the modem circuit boards that were requested. Because they're so big I'll just provide links instead of inline images. The LEDs were soldered on over on the right-hand side:1200 bps modem - component side:http://www.nullmodem.org/eevblog-photos/1200-front.jpg1200 bps modem - back side:http://www.nullmodem.org/eevblog-photos/1200-back.jpg1200 bps modem - daughter board component side:http://www.nullmodem.org/eevblog-photos/1200-daughter-front.jpg1200 bps modem - daughter board back side:http://www.nullmodem.org/eevblog-photos/1200-daughter-back.jpg9600 bps modem - component side:http://www.nullmodem.org/eevblog-photos/9600-front.jpg9600 bps modem - back side:http://www.nullmodem.org/eevblog-photos/9600-back.jpg

author: Fungus
author_rank: Super Contributor
timestamp: February 01, 2020, 07:43:58 am
content: Quote from: xenium on February 01, 2020, 06:21:34 amIf these LEDs are from a donor modem then they may already be a bit burnt out.Running them at 45mA is a bad idea. "It seems to work!" is never a good design guideline.

author: westfw
author_rank: Super Contributor
timestamp: February 01, 2020, 09:19:27 am
content: Those look like 220 ohm resistor SIPs on the 1200bps pcb...

author: mariush
author_rank: Super Contributor
timestamp: February 01, 2020, 09:40:47 am
content: Yeah, I agree... 221 is code for 220 ohm 22 x 10^1You can see in the back that each led goes through two pins on that resistor network.As for the 9600 bps modem, I see 390 ohm resistors on the back of the circuit board (it says 391 on the resistors).You could use a multimeter to measure the voltage before the resistor on both modems and from there you can measure voltage and currentYou can also use Vin - Vforward = Current x R and you know the resistor and you can measure the current across the led with the multimeter if you want to.

author: Brumby
author_rank: Unknown
timestamp: February 01, 2020, 10:18:57 am
content: Quote from: xenium on February 01, 2020, 06:21:34 amVfwill change, according to current. Diode mode voltage readings don't necessarily reflect the Vfyou need to use in current limiting resistor calculations. It would be interesting to measure the voltage across the LED in each of the following...Quote... but there is enough information here that we can simply calculate those figures:For a 5V supply...SeriesresistanceCurrentResistorvoltageLED Vf(Ohm)(mA)(V)(V)2278.01.7163.2844745.02.1152.88510025.02.5002.50020014.02.8002.20010003.03.0002.000100000.33.0002.000... which looks pretty typical.