Looking for a simply circuit using a 555 timer that will fade an LED in and out. - Page 1
Forum section: electronics / beginners

author: level42jeff
author_rank: Contributor
timestamp: December 05, 2019, 03:35:37 pm
content: I'm looking for a super simple circuit that I can use to fade an LED in and out at a constant rate. The problem is that there are too many tutorials out there, and they all seem to vary slightly.I'm quickly becoming overwhelmed and frustrated trying to figure out what caps and resistors I'd need. I already tried following one tutorial, which ended up in the LED just coming on constantly, and not fading at all.when I did this on breadboard, it sort of worked, but when I tried transferring this to a PCB, that's when I had the issue with the solid LED only.Anyone have any suggestions here. Hoping to have this done by Christmas.

author: Wimberleytech
author_rank: Super Contributor
timestamp: December 05, 2019, 03:57:34 pm
content: 1) Did you translate these instructions into a schematic first?2) Can you show us your breadboard?3) Can you show us your PCB?Then we can help.I think the video instructions have reversed emitter and collector. At least from what he said. Maybe he connected it right but said it wrong. <<No, it appears that is what he did. Using the transistor is a diode essentially (base-collector diode)>>

author: level42jeff
author_rank: Contributor
timestamp: December 05, 2019, 05:13:23 pm
content: Thanks for the feedback,Here is my breadboard, and I'll have to get shots of my PCB when I get home.https://imgur.com/a/C3uowpAI did my best to translate this video into schematics before I did anything else. I took the shopping list of components to my hobby shop and got as close as I could parts wise to the video.

author: nali
author_rank: Frequent Contributor
timestamp: December 05, 2019, 06:28:16 pm
content: There's a link to a schematic in the video description - yes it is a fixed freq astable but the LED (or LEDs plural in the schematic) are driven by emitter followers from the timing capacitor.Can't really decipher the birds nest on the breadboard!

author: Wimberleytech
author_rank: Super Contributor
timestamp: December 05, 2019, 06:38:54 pm
content: Quote from: level42jeff on December 05, 2019, 05:13:23 pmLooks like it is connected correctly. Bear in mind that the base of the transistor is a sawtooth varying from 1/3 to 2/3 of the supply. This voltage is dropped by the base-collector junction (should be base-emitter imho) and that is the voltage available to turn on the led. Depending on the led choice and the power supply chosen, you get widely varying results.I suggest that you swap the collector and the emitter connections.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 05, 2019, 06:39:52 pm
content: Expanding on that further --Note that the LEDs won't turn all the way off at high supply voltage. The reason for this is that, the transistor drops ~0.6V, and the LED drops a minimum, whatever it does -- depends on color, 1.5-3.6V for red to blue types. Meanwhile, the capacitor voltage is controlled between 1/3 to 2/3 of the supply, by the 555. If the supply is 9V, the range is 3-6V. Subtracting 0.6V for the transistor leaves 2.4V as the minimum across the LED. A blue LED will be quite dim at that voltage, if not completely off; whereas a red LED will probably still be dimly lit. A 6 or 7V supply would be more appropriate in that case, or a resistor can be connected in parallel with the LED to make it see a lower voltage.Note that the fade is approximately linear, which doesn't look right, visually. Ideally you'd have a gamma correction circuit, which causes the current to increase (towards full brightness) more rapidly, and decrease (towards cutoff) more gradually. Unfortunately, there isn't a simple way to implement this on the breadboard.Tim

author: Wimberleytech
author_rank: Super Contributor
timestamp: December 05, 2019, 07:32:36 pm
content: How about using two 555s and a comparator and drive the LED with a PWM signal? I just threw this together in LTSPice. It doubles your complexity but is going to work a lot better.I chose a random comparator from the LTSpice library...use whatever you have in your parts bin that meets the supply limitations (watch for open collector outputs).Update:I built this in the lab. It works as expected. I used an LM311 (the first comparator I could lay my hands on).

author: wilfred
author_rank: Super Contributor
timestamp: December 06, 2019, 01:55:17 am
content: The Talkingelectronics website is always worth a look. I found this example you might find of interest.http://www.talkingelectronics.com/projects/30%20LED%20Projects/30%20LED%20Projects.html#51

author: Jwillis
author_rank: Super Contributor
timestamp: December 06, 2019, 05:53:18 am
content: In your design the negative side of the capacitor should be connected to ground. But you have it connected to pin 6. Pin 6 should be connected to the positive side of the capacitor . Also as mentioned before recheck the orientation of your transistor. For this circuit the collector should be connected to the positive battery supply and the emitter to the led resistor (ballast or current limiter resistor ) then to the anode or positive side of LED ( the long pin). Short pin negative or Cathode of LED connects to ground.Sorry I didn't have my glasses on.

author: Wimberleytech
author_rank: Super Contributor
timestamp: December 06, 2019, 01:52:42 pm
content: Quote from: Jwillis on December 06, 2019, 05:53:18 amLOL Great post!