Variation of running frequency using an external RC network.

The idea

In an astable, the running frequency is set by an external resistor–capacitor network rather than by the supply voltage. The capacitor charges through the resistor and discharges through another path, so the time spent high and low depends on the resistance and capacitance values. Increasing either R or C increases the time constant, lengthening each part of the cycle and lowering the frequency. A larger capacitor stores more charge for a given voltage, while a larger resistor reduces charging current; both slow the voltage change across the capacitor. For a simple astable, the period is approximately proportional to RC, so doubling R or C roughly halves the frequency.

What earns the marks

• States that the running frequency of an astable is determined by the external resistor and capacitor values in the timing network.
• Explains that increasing resistance reduces charging or discharging current, so the capacitor takes longer to reach its threshold voltages.
• Explains that increasing capacitance increases the charge stored for a given voltage change, so the capacitor takes longer to charge or discharge.
• Links the period to the RC time constant, noting that period is approximately proportional to the product RC.
• Predicts the effect on frequency of changing R or C, for example doubling either value approximately halves the frequency.
• Selects or compares component values to achieve a required running frequency in a described astable circuit.

Common mistakes and how to put them right

• Believing the supply voltage sets the running frequency.

  Put it right: The frequency is set mainly by the external RC network, so correct by focusing on R and C values.

• Assuming frequency is directly proportional to RC.

  Put it right by using the reciprocal relationship.

• Ignoring the effect of changing only one component.

  Put it right: Changing either R or C alters the timing, so correct by considering the product RC rather than one value alone.

Exam tips

• Use proportional reasoning: if RC increases by a factor, state the corresponding factor change in period and hence frequency.
• When comparing circuits, identify which component has changed and explain the effect through charging current or stored charge.
• Quote the relationship between period and RC as approximate unless the exact circuit formula is given, and avoid inventing a precise constant.