555 alternate led fader - Page 1
Forum section: electronics / projects

author: digik
author_rank: Regular Contributor
timestamp: November 07, 2020, 02:09:56 pm
content: I have build this circuit:I have only 5Vpower supply to power this circuit.I want to power two strings of this type of LED that drows about 150mA (so it's fine for the 555 timer):Now the problem: I have only about 2.40V at the output of the capacitor at pin3 and I need at least 3.5V. I presume I have to add a transistor but how can I preserve the alternating (and fading) effect? I have noticed that at the capacitor at pin3 the output goes from 2.40V to -2.40V

author: Zero999
author_rank: Super Contributor
timestamp: November 07, 2020, 07:19:25 pm
content: Yes, that's exactly what you should expect to happen. I would even expect it to be a lower voltage than that, when the LEDs are connected.You could add another 555 connected as an inverter, or use a 556, to form an H-bridge, to double the output voltage.

author: digik
author_rank: Regular Contributor
timestamp: November 07, 2020, 09:12:23 pm
content: Thank you, but how can I connect another 555 as an inverter? Where?

author: S. Petrukhin
author_rank: Super Contributor
timestamp: November 07, 2020, 10:14:06 pm
content: Quote from: digik on November 07, 2020, 09:12:23 pmInstall 2 solid-state DC relays instead of LEDs on the circuit and control anything, even a spotlight.https://lcsc.com/product-detail/Solid-State-Relays_Cosmo-Electronics-KAQY212A_C194713.html

author: Benta
author_rank: Super Contributor
timestamp: November 07, 2020, 11:11:09 pm
content: Add a couple of OC71 transistors to buffer the output, perhaps followed by an AC127. They're all equally ancient.Sorry, but I just hate the 555.

author: Zero999
author_rank: Super Contributor
timestamp: November 07, 2020, 11:11:52 pm
content: Quote from: digik on November 07, 2020, 09:12:23 pmhttps://electronicsclub.info/555buffer.htm

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 08, 2020, 02:38:50 am
content: Try this circuit.But, you MUST use the Cmos version of 555, (like TLC555), with bipolar 555, it will not work.NB: only for LED that drows about 150mA as specifiedFor other current, R1 must be modifiedNB2: Schematic as been modified: Output logic of NE555 inverted for higher brightness.OK for bipolar NE555

author: digik
author_rank: Regular Contributor
timestamp: November 08, 2020, 11:21:11 am
content: that's a huge disappointment: I wanted only two alternating led strings that fades GRADUALLY in and out and all the result is a mess: this IC is not suitable for anything but some elementary experiments. Despite all the hype around it ("Benta" was right) it's a useless old nonsense. I think I will give up.@ akimpowerscr: thank you but in your schematic I see just one led but I have two strings@ S. Petrukhin: if I will use relays, what about the fading effect?

author: Zero999
author_rank: Super Contributor
timestamp: November 08, 2020, 11:22:54 am
content: Quote from: akimpowerscr on November 08, 2020, 02:38:50 amThat won't fade the LED. It'll just flash.Here's the circuit I was talking about, but on reflection, it probably won't be very good.

author: digik
author_rank: Regular Contributor
timestamp: November 08, 2020, 11:35:52 am
content: why, it probably won't be very good.?

author: Zero999
author_rank: Super Contributor
timestamp: November 08, 2020, 11:47:03 am
content: Quote from: digik on November 08, 2020, 11:35:52 amIt won't drive the LEDs at the full 150mA and each LED will turn on, before gradually fading off, rather than fading on and off. I also made a mistake, the 470µF capacitor needs to be non-polarised, so should be replaced with two 1000µF capacitors connected back-to-back.

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 08, 2020, 11:56:39 am
content: Quote from: Zero999 on November 08, 2020, 11:22:54 amIt seems to me that you did not understand the principle of operation of my diagram.Do you know the working principle of cathode ray tube television?Every 1/50 of a second, a half frame is drawn on the screen, so every 1/25 of a second, an entire image is drawn by the cathode ray .... The image should therefore flash, but it does not because of retinal persistence.In my project, the same principle is used.The oscillation frequency of the NE555 is 1KHz with the potentiometer at 0 and 126 Hz when the potentiometer is at maximum 100K.The eye obviously cannot follow a flashing at such frequencies and sees a fading and not a flashing.

author: digik
author_rank: Regular Contributor
timestamp: November 08, 2020, 12:00:29 pm
content: Quote from: Zero999 on November 08, 2020, 11:47:03 amSo it's not possible to achieve this sort of effect with a 555 IC?

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 08, 2020, 12:08:28 pm
content: QuoteHere is the modified schematic for two strings. (75mA each)NB: for more fading effect, change R3 from 10K to 1K

author: drussell
author_rank: Super Contributor
timestamp: November 08, 2020, 12:23:37 pm
content: Quote from: digik on November 08, 2020, 12:00:29 pmIf the 555 is producing the effect you want with the single LEDs, then all you need to do is buffer the output to provide more current to drive a whole string of LEDs. Most buffer-type op-amps won't do 150ma, but some special high power ones will, or perhaps one intended as a small audio amplifier like the LM386, maybe? That's what I would have tried first, "back in the day" if I were trying to so something like that. Otherwise you can just make a discrete circuit to buffer it.I absolutelyhatePWM'd fading LEDs! Despite what some people claim above, I absolutely CAN see the flashing, even at relatively high frequency and it drives me absolutely bonkers! I suppose, though, that if one were to actually add some sort of smoothing capacitor, it might be able to be made tolerable, but I like your linear fader idea much better.

author: drussell
author_rank: Super Contributor
timestamp: November 08, 2020, 12:24:34 pm
content: Quote from: akimpowerscr on November 08, 2020, 12:08:28 pmThe OP wants to fade back and forth between the two strings (or two colors within a single physical string), not just drive two strings in parallel.Edit:Hmmm, now that I think about it, if it were within one physical string, the original circuit won't work because how are you going to connect the second color to drive it in reverse if one end is already commoned together in the same polarity...

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 08, 2020, 12:26:30 pm
content: QuoteMost of the flashlights use it.QuoteThis is not what I understood from reading the first post of digik.

author: drussell
author_rank: Super Contributor
timestamp: November 08, 2020, 12:32:06 pm
content: Quote from: akimpowerscr on November 08, 2020, 12:26:30 pmOh, I know... They're absolutelyeverywhere!That's why it drives me nuts!! Flash Flash Flash Flash Flash

author: drussell
author_rank: Super Contributor
timestamp: November 08, 2020, 12:44:54 pm
content: Quote from: akimpowerscr on November 08, 2020, 12:26:30 pmI'm pretty sure that's what the OP is looking for, since the topic is "alternate LED fader" and the first post includes a schematic that appears intended to operate that way, with the caption:Quote from: digik on November 07, 2020, 02:09:56 pmEdit:The dual-purpose with fading is why the OP is taking the output from the timing capacitor ramp instead of the usual 555 flippy-floppy output.

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 08, 2020, 12:56:16 pm
content: I'm going to have to develop a whole different circuit for that.The NE555 astable oscillator circuit is not very satisfactory because it only allows the current to be varied from 99% to 60%, not fading to 0I am going to work on a circuit that allows the current to be varied from almost 0 to 100% with two alternating channels, as digik requests, but I don't have time to do it now.It will use two NE555 or one NE556.

author: Zero999
author_rank: Super Contributor
timestamp: November 08, 2020, 01:12:50 pm
content: Quote from: akimpowerscr on November 08, 2020, 11:56:39 amQuote from: akimpowerscr on November 08, 2020, 12:08:28 pmOh I see, PWM, but neither of those circuit will accomplish what the original poster is trying to achieve. They're just dimmers. The original poster wants the LEDs to gradually fade on and off, automatically, rather than a dimmable LED circuit.Here's another idea, using buffers and constant current sources. It still isn't very good. A mircrocontroller would do the job, but I have a feeling the OP doesn't want to use a progammable device, in which case, I'd go for a different approach, such as the LM324, or LM339.

author: drussell
author_rank: Super Contributor
timestamp: November 08, 2020, 01:20:53 pm
content: If we're trying to stay simple here, what about some sort of circuit that would alternately ramp the adjust pins on two adjustable voltage regulators?Just a thought.

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 08, 2020, 02:46:38 pm
content: QuoteManually,it is still possible with NE555, but automatically, you better look for another solution.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: November 08, 2020, 03:19:15 pm
content: Quote from: digik on November 08, 2020, 11:21:11 amThis is a solid state relay - opto-transistor. It can transmit a PWM signal and even has a linear mode. See the diagram.

author: BrianHG
author_rank: Super Contributor
timestamp: November 08, 2020, 04:37:29 pm
content: Make a fader PWMing the leds at 10KHz and there will be no visible flicker.With today's ~10 cent MCUs, plus a 10cent mosfet per LED fader channel, you cannot get cheaper than that with the option for software control features or patterns.

author: digik
author_rank: Regular Contributor
timestamp: November 08, 2020, 05:31:44 pm
content: we all know about MCU (simple, programmable and of course ugly because of that PWM effect). I just wanted to know if it were possible to make a simple alternate LED FDING EFFECT (or breathing, or whathever you want to call it) using one or two 555. Just that. I thought that since the 555 is so well known that would be possible, but it's clear that is not

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 08, 2020, 05:45:47 pm
content: You just need to be a little less demanding: manual fading instead of automatic.With two fading units, you would only need to use a double (stereo) linear potentiometer but wired in reverse so that when you turn the potentiometer in one direction, one of colors increases and the other decreases, and in the opposite direction. , it will be the opposite.

author: drussell
author_rank: Super Contributor
timestamp: November 08, 2020, 05:52:17 pm
content: Quote from: akimpowerscr on November 08, 2020, 05:45:47 pmThat's not what the OP is trying to do, though.They're trying to do the breathing/fading effect with a speed adjustment POT in a simple, non-MCU-controlled circuit instead of PWM, not trying to manually dimmer-control one LED to the other via a POT or something.

author: digik
author_rank: Regular Contributor
timestamp: November 08, 2020, 05:55:28 pm
content: @ drussel: yes, and so far without any success@ akimpowerscr: sorry but staying on the back of a Christmas tree all the day turning a pot is not that fun

author: Zero999
author_rank: Super Contributor
timestamp: November 08, 2020, 09:41:37 pm
content: Quote from: S. Petrukhin on November 08, 2020, 03:19:15 pmWhy would you use a solid state relay/opto-isolator, when a plain old BJT/MOSFET is much cheaper? Solid state relays are also often too slow to do flicker free PWM with.Quote from: digik on November 08, 2020, 05:31:44 pmJust use a high frequency and it won't flicker. This circuit uses the old LM339, with the PWM at 1.6kHz. If that's still noticeable, increase the frequency to around 8kHz, by reducing C2 to 220pF. Of course a microcontroller is much cheaper and easier.