EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: nevillet on November 11, 2022, 06:05:12 pm
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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.
(https://i.ibb.co/VMxBmdS/Screenshot-2022-11-11-175646.jpg) (https://ibb.co/j3z4RdM)
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So 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 polarity
There 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.
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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.
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Please forgive my ignorance. I think the diagram below is correct:
(https://i.ibb.co/n0zKJkw/LED-Fairy-Light-Pulse-Diagram-V02.jpg) (https://ibb.co/kyQPCcG)
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Use a motor driver IC such as the TB67H451 and an microcontroller for the PWM.
https://docs.rs-online.com/c239/A700000007487120.pdf
More information
https://esphome.io/components/light/hbridge.html
https://auschristmaslighting.com/wiki/Controlling-2-wire-Multi-Function-LED-Strings
A 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
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Many thanks for your help.
I've been a fan of Big Clive for many years!
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I've missed part of your original post.
the positive & negative pulse being 2.7V (but this would vary on different sets).
If 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.
(https://www.eevblog.com/forum/projects/dimming-led-fairy-lights/?action=dlattach;attach=1638413;image)
1. The ability to adjust the positive & negative voltage (± 2.5 to ±25V)
No, to change the brightness of LEDs, you want to vary either the duty cycle or forward current, not voltage.
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The ability to adjust the positive & negative voltage (± 2.5 to ±25V)
No, to change the brightness of LEDs, you want to vary either the duty cycle or forward current, not voltage.
I think the OP means that different LED light sets may be configured differently, and may expect a different (fixed) voltage to drive them?
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Hi Ian, Your ascertian is corrent.
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Hi Ian, Your ascertian is corrent.
Generally, 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.