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The time required for the current flowing in the LR series circuit to reach its maximum steady state value is equivalent to about 5 time constants or 5Ï. This time constant Ï, is measured by Ï = L/R, in seconds, were R is the value of the resistor in ohms and L is the value of the inductor in Henries.
You control the time constant of this circuit, the time it takes for the voltage across C2 to reach 63.2% of Vcc, by varying the setting of R4. Notice that the transistor inside of the 555 timer IC is connected via pin 7 of the device to the junction between R1 and R3.