4.1.2. The RC circuit experiment

Components:

• breadboard
• 5V power supply
• 3mm red LED
• 2 x SPST pushbutton switches
• BC547 NPN transistor
• 220Ω, 2 x 1kΩ resistor
• electrolytic capacitor 1000μF

Figure 4‑4 RC circuit experiment

Explanation: The left pushbutton serves to charge the 1000μF capacitor through a 1kΩ resistor. At the same time capacitor is charging, the voltage on the base of the transistor rises. Let’s see how much voltage we need on the base of the transistor in order to light up the LED on the emitter. We know that the base voltage must be ~0.7 higher than the emitter voltage for the transistor to start conduction. Also, we know that the voltage on the emitter to light up the LED is ~1.8V. Therefore, the base voltage has to be ~2.5V, around half of the source voltage of 5V. As already mentioned, it will take 0.7T or 0.7s to reach that amount of voltage. The right pushbutton serves to discharge the capacitor. To conduct the experiment, we should first use the right pushbutton to fully discharge the capacitor. Then we should use the left pushbutton to demonstrate that the LED lights up with the aforementioned delay.

The discharging of the capacitor is very similar but opposite to its charging, which is shown in the following figure.

Figure 4‑5 RC circuit discharging
(Source: referenced)

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References:

https://www.electronics-tutorials.ws/rc/rc_1.html