I made this circuit, the function is to turn on the LED light, and then turn off the light after a while.
This is a voltage source for a cr2032 coin battery, and the LED is a 20mA perforated red LED light, Vf is about 2V.
Ignoring the resistance in series with the voltage source, how to describe the capacitance and resistance that determine the brightness of the red light and the time it goes out. For example, if I want it to go out faster or slower, how do I change the resistance or capacitance?
And do the parameters of the transistor have an effect?
Kasia Posté sur September 2, 2020
This is a filter circuit with RC time constant, R*C=τ, if you want to increase the time constant, increase R or C or both.
However, the current offset of the transistor must be considered in this circuit, so only the capacitance is changed.
Lainie Posté sur September 2, 2020
It should be after the switch is closed that the light will turn on after a while, because the voltage across the capacitor cannot change suddenly, so it will turn on after a while. Reduce the resistance of the 1K resistor and the capacitance of the 1uf capacitor to reduce this delay.
In addition. After the switch is turned off for a while, because the voltage across the capacitor cannot change suddenly after the switch is turned off, it must be discharged slowly until the base voltage is less than about 0.7V, the light will go out.
Lashay Posté sur September 2, 2020
The capacity of the capacitor determines the bright time.
