Interactive Tunable Designs

LED Dimmer

Have you ever wondered how to dim an LED? Now is your chance to try it! LEDs are very efficient light sources that can be turned on and off rapidly without affecting their operating life. So dimming can be achieved just by changing the percent on-time, or "duty-cycle." In this example dimmer design, you can select that duty-cycle and other circuit parameters, to get a better understanding of how it works.

This design is "Live:" change any parameter (in blue) and run a simulation to see the results.

This control method is called Pulse-Width Modulation (PWM). It allows the LED to operate at its nominal current during the “ON” portion of the cycle, so there is no color shift. There are many ways to implement PWM dimming in an LED driver circuit. The “Live” (interactive) example above shows one method. It uses a 555 Timer to control the PWM switching frequency and duty cycle.

In that schematic, you can change parameter values and run new simulations to see the effect. The key dimming parameters are the potentiometer setting, which controls the relative RC charge and discharge time constants of the 555 timer and therefore the duty-cycle of the PWM. Also, the capacitor c2 value controls the PWM switching frequency.

Internal Block Diagram of the 555 timer
Image courtesy of Wikipedia;

Because the frequency response of human vision is limited, using a sufficiently high PWM frequency avoids the perception of flicker for the observer. To see the effect of that capacitor controlled frequency on flicker, the LED model has an internal monitor for the "perceived" light output (blue waveform). This is just a low-pass filtered version of the instantaneous light output, with filter frequency set to 15 Hz to represent the nominal bandwidth of the human eye. You can try increasing or decreasing c2 and observe the effect on the dimmer switching frequency and corresponding flicker amplitude.

You can also change the emitter resistor r2, to adjust the on-state LED current, and the battery voltage which has little effect until it is too low to overcome the forward voltage drop of the LEDs.

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