Simple circuit for controlling LED brightness

Recently I was asked to produce a circuit to create a variable ‘dim’ control for an existing LED based exhibition.

I decided to go with a simple 555-timer based design which provides control of the pulse width of an output, whilst keeping the oscillation frequency fixed.

The circuit is shown below.  It’s an unusual design, because the output is taken from pin 7.  This ‘output’ is not capable of driving current (at least not without affecting the circuit behaviour) so you have to be careful with the connections you make to it.

Image of Circuit
555 PWM Circuit

The circuit works because of some very simple rules:

  1. When pin 7 is high, the output pin (3) is low.
  2. When pin 7 is low, the output pin (3) is high.
  3. Pin 7 will be high when the trigger/threshold pin is > 2/3 of VCC.
  4. Pin 7 will be low when the trigger/threshold pin is < 1/3 of VCC.

When the circuit is first powered on, pin 7 is low because C2 is discharged.  This causes pin 3 to go high, and C2 begins to charge via D1.  When C2 reaches about 2/3 of VCC pin 7 goes high, causing pin 3 to go low.  Now C2 discharges through D2 until its voltage falls below 1/3 of VCC and then the cycle repeats as an oscillation.

The resistance in the charge and discharge cycles is controlled by RV1.  When the resistance is increased for the D1 side of RV1 the resistance falls by the same amount on the D2 side of RV1.  This changes the relative ‘speed’ of the charge/discharge cycle, which has the practical effect of pulse width adjustment.
Since the total resistance of RV1 (for the total charge/discharge cycle) is constant, the frequency of the output is stable and is determined by the values for RV1 and C2.

The frequency of the output is governed by a simple formula:

FREQUENCY = 1.44 * RV1 * C2

The power switch is a PMOS FET.  It controls the switching of a 12V feed onto the existing LED array at the exhibition.  RV1 is adjusted to provide the desired brightness for the display, and LED D3 provides an indication of the level of ‘dimming’ that the circuit is currently providing.
The circuit oscillates at 100Hz, so there is no perceived flicker.

Happy circuit building!

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