Capacitor in circuit for potentiometer?

Question

In a German tutorial video about EAGLE, starting at 1:15 the author adds a trim potentiometer for regulating the contrast of an A162 display. Between the slider and ground he adds a 100 nF capacitor.

What is the capacitor for?

Screen shot showing the circuit with capacitor C4:

Note that the author stresses that the video is about the basic features of EAGLE, not about circuit design and not even about good layout of a circuit. He says that he didn't build the circuit, and it may not work. The tutorial is very good —  I learned using EAGLE from scratch.

Answer 1

The capacitor filters noise, making the voltage at V0 more stable. A capacitor resists changes in voltage. The rate of change of voltage, current, and capacitance are related by:

$$ I = C \frac{\mathrm{d}v}{\mathrm{d}t} $$

The larger the capacitance, the more current required to make a change in voltage. Since there is only so much noise current, the more capacitance, the less the voltage noise.

Possible sources of noise:

• electromagnetic interference
• thermal noise
• noise from the wiper sliding in the potentiometer
• noise from the power supply
• leakage currents from other digital IO

Another way to view it: the capacitor and resistor together make a low-pass RC filter. This attenuates higher frequencies, while not attenuating the lower frequency. Since you are interested in the nearly constant voltage selected by the potentiometer, and the lowest frequency is a voltage that doesn't change at all, this is good.

Answer 2

The VO pin is the power to the LED segments. The amount of current drawn on this pin may vary somewhat as the display is scanned, in a matter which will vary with display content. This can cause annoying horizontal streaks on the display. Adding a capacitor will ensure that any variations in voltage that may result from changing current demands are slow enough that they affect the display uniformly.