The visible brightness of a light emitting diode or 7-segment LED type displays can be varied by applying a pulse signal and varying its duty cycle. Common seven-segment decoders usually have a “blanking input” terminal to apply this type of control. By keeping the frequency of the pulsed signal above 50Hz, the flicker will not be noticeable.
Referring to the circuit schematic, the charge and discharge times of the timing capacitor C1 vary with the setting of the R1 potentiometer. Although charge and discharge times vary, the total period remains the same. Thus the output signal frequency remains unaffected while its duty cycle is varied. Due to the fact that the average dissipated power is proportional to the duty cycle, the output signal controls the brightness of LED’s. For providing sufficient driver voltage and power to the ripple input of the decoder, a booster transistor may be connected as shown on the circuit schematic.
