Sharing an LED between two sources - Page 1
Forum section: electronics / beginners

author: analogix
author_rank: Frequent Contributor
timestamp: April 16, 2021, 11:31:29 am
content: I'd like to use a single 3mm LED for two hard drives ("In use/activity" indicators). Is this possible?(the reason being to the computer case physically just providing for one 3mm LED).I was thinking a bi-coloured LED with 3 pins, where I'd use its cathode as a common GND, and the two other pins going to each drive's LED + pin. Unfortunately it appears that the - pin of each drive's LED isn't shared (no continuity), so is there another way to do it?(image source:LEDnique)

author: tunk
author_rank: Super Contributor
timestamp: April 16, 2021, 11:41:22 am
content: I think they exist - do a web search for e.g. this:led dual color common cathode

author: analogix
author_rank: Frequent Contributor
timestamp: April 16, 2021, 03:55:47 pm
content: Yes, they exist. Also "RGB" LEDs with 4 pins (common cathode and 3 separate colours).I could also use a 5mm LED if I modify the holder slightly.But how can I connect it to the two drives without causing any problems (since the negative polarity pin of each drive's LED connection isn't shared between the drives)?Could I somehow connect both drives' negative terminal through a diode each, so as to only get power from their own drive?

author: Jan Audio
author_rank: Frequent Contributor
timestamp: April 16, 2021, 04:00:10 pm
content: It are just 3 LEDs in 1 package.If you have common + then you connect the + and have invert logic on the LED.If you have common - then you connect the - and have no invert logic on the LED.

author: KrudyZ
author_rank: Frequent Contributor
timestamp: April 16, 2021, 04:06:16 pm
content: It's likely that the drive's output pulls down to activate the LED, so you will likely need a common anode LED.Then you can just use one of the positive leads from the drives to the anode and connect each drive to their own cathode.This assumes that the drives share a common ground and that they are both powered at the same time.Depending on where the series resistor is your LEDs might get half the current when both are on or not.

author: analogix
author_rank: Frequent Contributor
timestamp: April 16, 2021, 07:46:58 pm
content: Thanks for the suggestion. I'll look closer into that, but here's a quick schematic of my proposal:i'm not sure if the diode is the correct way or not, but the idea is that if I use a dual coloured LED with a common cathode I connect them as I normally would to each drive, but a diode prevents -V from drive 2 to enter -V of drive 1. Will this work?

author: analogix
author_rank: Frequent Contributor
timestamp: April 16, 2021, 08:26:18 pm
content: Quote from: KrudyZ on April 16, 2021, 04:06:16 pmHmm... That's the problem -their GND connection isn't the same.Are there LEDs that are actually 2 separate LEDs in one physical package -in other words with 4 leads but sharing the lens etc.? That would solve the connection problem if my diode schematic won't work.

author: analogix
author_rank: Frequent Contributor
timestamp: April 17, 2021, 09:21:58 am
content: It's complicated....It's a vintage Atari ST with an internal SCSI hard drive (its LED is connected to the drive itself).Then there's an after-market add-on internal PCB which adds IDE support. An IDE to Compact Flash memory card adapter is attached to this board for a super fast/silent hard drive alternative. The add-on board has various jumpers and pins where a couple of them are for attaching drive activity LEDs.

author: Ian.M
author_rank: Super Contributor
timestamp: April 17, 2021, 02:23:14 pm
content: Use two >100% CTRR optocouplers to acquire the LED signals, and you can do whatever you like with their phototransistor output side, including connecting them in parallel to a single LED + series resistor.

author: tunk
author_rank: Super Contributor
timestamp: April 17, 2021, 04:27:38 pm
content: There's this (5mm and expensive though):https://no.rs-online.com/web/p/leds/2471511Edit: You could also use a 3mm light pipe and two rectangular LEDs behind it.

author: james_s
author_rank: Super Contributor
timestamp: April 17, 2021, 05:51:05 pm
content: Did you check how the LEDs are actually wired in the drives? I would expect that the + pin is wired directly to +5V, though it may have the current limiting resistor in series with it, then the - pin will pull to ground to turn the LED on, low side switching is more common than high side. If this case you only want to connect one of the + pins.I do like the idea of using optocouplers as someone else suggested, that's a much safer and more flexible approach.

author: analogix
author_rank: Frequent Contributor
timestamp: April 19, 2021, 06:21:06 pm
content: Quote from: james_s on April 17, 2021, 05:51:05 pmYou're right!I double checked and found that indeed, the LED + pin goes through a 220 Ohm resistor and then on to the +5V pin of the harddrive Molex connector.The LED - pin apparently goes through some circuitry to generate the signal.I'm having trouble locating the LED pins of the other drive and suspect I've been tricked again by a current limiting resistor which would bypass a continuity test. I'll keep checking and see if one of the the voltage pins are common between the drives. In that case I'll be able to use a dual-coloured LED with a common cathode, or common anode (depending on the common voltage pin), right?QuoteYes, I don't fully understand it except it physically separates circuits. More complex than connecting a dual/multi-coloured LED, but something I'll look into more detail if that's not possible.

author: analogix
author_rank: Frequent Contributor
timestamp: April 19, 2021, 06:23:11 pm
content: Quote from: tunk on April 17, 2021, 04:27:38 pmGreat idea about the light pipe.Regarding the above LED: the datasheet wasn't quite clear about it (or I didn't understand it), but is this indeed multiple LEDs with totally separate cathode pins? In other words, two individual LEDs, but in one physical package?

author: james_s
author_rank: Super Contributor
timestamp: April 19, 2021, 08:49:10 pm
content: Ok well you've confirmed then that the LEDs are low side switched. So find a common anode LED and connect the anode straight to a convenient +5V source. Then put a resistor in series with each cathode and connect those to the LED - terminal on each drive.

author: analogix
author_rank: Frequent Contributor
timestamp: April 22, 2021, 02:35:19 pm
content: Sounds like a good planThanks!