This is a Relay Timer Circuit, modeled after a project by Charles Platt in the book Make: Electronics. It demonstrates how an RC (resistor-capacitor) network can introduce a predictable time delay to turn off an electronic component automatically.
The circuit has been enhanced from the original to incorporate a more versatile RC timing element, constituting an NPN Transistor, Diodes and additional push switch to discharge the capacitor.
Step-by-Step Operation
1. Charging the Capacitor: The user interacts with the push-buttons on the left side of the screen. When the upper push-button is closed, it connects the 9V supply directly to the 1 mF capacitor, rapidly charging it to full voltage.
2. Discharging and Base Bias: When the push-button is released, the charged capacitor gradually discharges its stored energy through the 1 kΩ resistor into the base of the NPN transistor.
3. Driving the Load: As long as the capacitor holds enough charge to keep the base voltage above the transistor's activation threshold (~0.7V), the transistor stays switched ON. This allows current to pass through its collector-emitter path, powering the green indicator LED.
4. Timing Out: As shown in the top graph tracker, the voltage (blue line) and current (green line) decay exponentially over a duration marked at 6.52 seconds. Once the capacitor drains below the threshold, the transistor turns OFF, breaking the path to ground and extinguishing the green LED.
Circuit Specifications
Power Supply 9 V: Main voltage source for the system.
Capacitor 1 mF (1000 µF): Stores electrical charge to create the timing window.
Resistor 1 kΩ: Limits the base current to the transistor and controls capacitor discharge.
Transistor NPN type: Acts as an electronic switch to drive the inductive coil.
Indicator Green LED: Lights up to provide visual feedback while the timer is active.
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