Modify Blinking LED to breathe or pulse - Page 1
Forum section: electronics / beginners

author: pball
author_rank: Newbie
timestamp: September 09, 2019, 12:35:42 am
content: I'd like to try making the power led on my home theater PC pulse instead of flash when it's asleep. The led is on and off for 1 second intervals. I have a basic understanding of electronics but not creating more than really simple circuits. I stuck a capacitor in parallel with an led on a breadboard and it didn't affect how the led turned on but after power was turned off the led did gradually dim until it went dark, which is half of the effect I'm looking for.I'm guessing a more complex circuit would be required to make the led gradually ramp up and down. I'd like if it could be be done with a passive circuit. Appreciate any help, thanks.

author: ledtester
author_rank: Super Contributor
timestamp: September 09, 2019, 01:14:17 am
content: Have a look at this schematic:https://www.axiris.eu/en/index.php/project-corner/breathing-ledThe stuff to the left of D1 is just a multivibrator - a square wave generator. So I would try replacing that with your LED control signal. You'll have to play around with the resistor and cap values.Note this circuit requires a +5 supply.

author: pball
author_rank: Newbie
timestamp: September 09, 2019, 02:07:57 am
content: Thanks for the schematic. I'll have to get some capacitors and other pieces to try this out. Seems simple enough and after some googling I think I get the principle for it's function.

author: ledtester
author_rank: Super Contributor
timestamp: September 09, 2019, 02:10:09 am
content: The web page also has a good discussion about it.

author: edavid
author_rank: Super Contributor
timestamp: September 09, 2019, 02:48:47 am
content: The multivibrator circuit isn't necessary, since the PC already has a blinking circuit. You were on the right track with the capacitor, but you also need a resistor in series with the LED (the capacitor goes in parallel with the LED + resistor). Your breadboard test was misleading since you didn't allow for the series resistor that's already present in the PC's LED wiring.

author: pball
author_rank: Newbie
timestamp: March 22, 2020, 08:12:22 pm
content: Dredging up this old post since I'm finally getting around to trying this out.Does using different transistors values have a big affect on circuits in a similar way that resistors or capacitors do? I can't find a kit with the BC847 transistor in and I'm curious if a BC3xx or BC5xx series would function the same for a circuit like this. Thanks

author: Zero999
author_rank: Super Contributor
timestamp: March 22, 2020, 11:04:45 pm
content: Quote from: pball on March 22, 2020, 08:12:22 pmThe transistors aren't critical. I think the idea was not to build the entire astable multivibrator, but the RC+emitter follower part of the circuit i.e. everything to the right of D1, inclusive and connect it to the square wave signal flashing the LED. It would need a permanently live 5V supply to work properly.

author: rstofer
author_rank: Super Contributor
timestamp: March 23, 2020, 03:03:59 pm
content: I would probably use an 8 pin PIC, a resistor and an LED. Or maybe an ATtiny <something> if I wanted a smaller package.

author: edavid
author_rank: Super Contributor
timestamp: March 23, 2020, 05:32:47 pm
content: As I mentioned before, you don't need any transistors, and certainly not an MCU

author: pball
author_rank: Newbie
timestamp: March 23, 2020, 09:27:41 pm
content: I see a transistor to the right of D1, is that not required? I'm also planning to make the whole circuit as shown so I can play with it to get values correct and make sure it works before hooking it up to my PC.

author: edavid
author_rank: Super Contributor
timestamp: March 23, 2020, 09:58:13 pm
content: You need the transistors if you want to build that circuit, but not for your original project that will be connected to the PC.

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: March 24, 2020, 06:26:56 pm
content: I knocked up this circuit in a couple minutes. Seems to do what you're asking for. It has a pleasant fade on and off.[ Specified attachment is not available ]

author: pball
author_rank: Newbie
timestamp: March 24, 2020, 09:26:50 pm
content: Thanks for the schematic Ground_Loop. I'll try that out along with the previous circuit. As long as my parts arrive, I have loads of free time currently to play around.

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: March 24, 2020, 10:29:20 pm
content: It appears that the circuit I did is fundamentally the same as Ledtester's above; at least the LED portion is.