Building a Simple RC Timing Circuit

Wire a resistor and capacitor together and watch an LED fade in instead of snapping on — a visible demonstration of RC charging.

You'll need: a resistor (try 100kΩ to start), an electrolytic capacitor (try 100µF, mind polarity), an LED with its own current-limiting resistor, a switch, and a supply.

Lesson 26 covered RC filters mathematically; this build makes the same charging behavior visible. Wire the resistor in series with the capacitor across your supply — the same low-pass arrangement from that lesson — then tap your LED-and-resistor branch from the junction between them, same as the voltage-divider tap in Lesson cb-06.

When you close the switch, the capacitor starts uncharged, so almost no voltage reaches the LED branch at first — dim or off. As current flows through the resistor and charges the capacitor, the junction voltage climbs, and the LED gradually brightens over roughly a second or two (with the values suggested above), rather than snapping instantly to full brightness.

Try swapping in a larger resistor or capacitor and watch the fade take noticeably longer — a direct, visible demonstration of what a larger RC time constant means, expanding on the theory from Lesson 26.