Concept of Operation

The circuit demonstrates the effect of the charging and discharging of a capacitor on the illumination of an LED. When the switch is closed, the capacitor begins to charge and the LED slowly gets brighter until the capacitor is fully charged and the LED is at its full brightness. When the switch is open, the capacitor begins to discharge and the LED slowly becomes more dim until its light is no longer visible.

This circuit invites hands-on experimentation. The rate of charge and discharge is dependent on the values of the resistors and the capacitor. The larger R1 is, the more slowly the capacitor will charge and the more slowly the LED will become brighter. The larger R2 is, the more slowly the capacitor will discharge and the more slowly the LED will dim. A larger or smaller capacitor will affect both charge and discharge rates.

Schematic

Breadboard Diagram

Parts

• 10k Resistor (x1)
• 100k Resistor (x1)
• 1000uf Electrolytic Capacitor (x1)
• Standard LED (x1)
• Tactile Pushbutton Switch (x1)

(All parts for the project are available in our store.)

Notes on Implementation

1. LEDs are polarized and must be mounted in the proper orientation. On standard, round LEDs, the cathode (negative lead) is identified by a flat edge. Also, the cathode lead is typically shorter than the anode.
2. Electrolytic capacitors are polarized and must be mounted in the proper orientation. On standard electrolytic capacitors the cathode (negative lead) is identified by a stripe on the package. Also, the cathode lead is typically shorter than the anode.
3. The 1000uf capacitor shown in the breadboard diagram is larger than the 0.1" span of the breadboard holes. The leads should be inserted in rows 15 and 16 in order to connect with the yellow and black hookup wires.
4. The tactile switch must also be mounted in the breadboard in the proper orientation. Although the typical switch used here comes in a square package, the spacing of the leads is wider in one direction (5mm) than the other (3mm). When oriented correctly, the leads will span the gap down the center of the breadboard and fit easily into the holes. When oriented incorrectly the leads will be too narrowly spaced and will not span the gap.
5. The circuit was built and tested at 4.8 volts DC (4 AA NiMH rechargeable batteries). Any power source that supplies about 5v DC should work fine.