Dimming LED fairy lights - Page 1
Forum section: electronics / projects

author: nevillet
author_rank: Contributor
timestamp: November 11, 2022, 06:05:12 pm
content: Hi All,I am looking for a circuit for controlling the brighness of LED fairy lights.The object of the circuit will be to dim the LED’s by adjusting the positive and negative pulse width.The LED’s are wired in inverse parallel and are always supplied with a ‘effects controller’ which give 8 variations of flashing with one effect being ‘steady on’.I have examined the ‘steady on’ output on a oscilloscope and have found that:The frequency is 160Hz, with the positive & negative pulse being 2.7V (but this would vary on different sets).Some sets are single colour LED’s, others are wired with groups of colours e.g. yellow & red wired in one polarity and blue & green in the other.It is for this reason that it would be advantageous to be able to control the positive and negative pulses separately.<a target='_blank' href='https://usefulwebtool.com/'>tieng viet onlineSo the specification of what I’m after is:1. The ability to adjust the positive & negative voltage (± 2.5 to ±25V)2. A variable control to adjust positive waveform duty cycle from 0-100%3. A variable control to adjust negative waveform duty cycle from 0-100%4. 500mA current capability per polarityThere may well be a circuit design already in exsistance that will do what I’m after.In which case what terminology should I be searching for?Many thanks all.

author: Zero999
author_rank: Super Contributor
timestamp: November 11, 2022, 06:28:26 pm
content: Your circuit shows the LEDs in reverse series, not parallel.The only sensible way to do this is to use an H-bridge and microcontroller and program it with your own code. It might be possible to reuse some of the circuitry, with your own MCU.

author: nevillet
author_rank: Contributor
timestamp: November 12, 2022, 04:06:10 pm
content: Please forgive my ignorance. I think the diagram below is correct:<a target='_blank' href='https://usefulwebtool.com/'>tiengviet online

author: Zero999
author_rank: Super Contributor
timestamp: November 12, 2022, 10:32:50 pm
content: Use a motor driver IC such as the TB67H451 and an microcontroller for the PWM.https://docs.rs-online.com/c239/A700000007487120.pdfMore informationhttps://esphome.io/components/light/hbridge.htmlhttps://auschristmaslighting.com/wiki/Controlling-2-wire-Multi-Function-LED-StringsA link to a video which explains how an H-bridge controls this type of Christmas light. The link is set to the part where the schematic is revealed and explained, but it's worth watching from the beginning.https://youtu.be/uQJv9EcaKh4?t=457

author: nevillet
author_rank: Contributor
timestamp: November 13, 2022, 05:13:08 pm
content: Many thanks for your help.I've been a fan of Big Clive for many years!

author: Zero999
author_rank: Super Contributor
timestamp: November 13, 2022, 06:08:26 pm
content: I've missed part of your original post.Quote from: nevillet on November 11, 2022, 06:05:12 pmIf that's true, your second schematic is wrong. It will be a string of single LEDs in reverse parallel, like those in Big Clive's video, except all different colours. In fact they'll all be blue of violet LEDs with a phosphor on top to produce a longer wavelength.QuoteNo, to change the brightness of LEDs, you want to vary either the duty cycle or forward current, not voltage.

author: IanB
author_rank: Super Contributor
timestamp: November 13, 2022, 06:36:10 pm
content: Quote from: Zero999 on November 13, 2022, 06:08:26 pmI think the OP means that different LED light sets may be configured differently, and may expect a different (fixed) voltage to drive them?

author: nevillet
author_rank: Contributor
timestamp: November 13, 2022, 07:31:39 pm
content: Hi Ian, Your ascertian is corrent.

author: Zero999
author_rank: Super Contributor
timestamp: November 13, 2022, 08:45:25 pm
content: Quote from: nevillet on November 13, 2022, 07:31:39 pmGenerally, the power supply is designed specifically to drive the LEDs. You ideally need a current source, which will be adjusted according to the number of LEDs in parallel. It's possible to use a constant voltage and a resistor, but isn't optimal.