Projects/CorridorLight: Difference between revisions

When initially applying 5VDC the capacitor C1 starts charging through R4 and almost no current will flow through R2. The initial voltage on the positive terminal of the capacitor would be zero and it starts increasing at a rate that depends on the values of R4 and C1. As the voltage in the common node increases also does the voltage on the gate of the transistor Q1 (very small current through R2 means very small voltage drop across). Some of the current also flows through R2 and R3 operating as a voltage divider. As the capacitor charges the current through R4 goes down and so does the voltage drop, increasing the voltage on R2 which in turn will increase the voltage on the gate for Q1. When the voltage is sufficient Q1 will allow current to flow from the collector to the emitter and the LED will illuminate through the current limiting resistor R1. As the capacitor charges, the voltage on the base will be higher, the current from base to emitter will increase and the LED will shine brighter.

For switching off, after removing power the capacitor will discharge through R2 and R3 but it will still provide current to discharge through R4, R1 and D1 until the current in the gate of Q1 turns the LED off.

 

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