Simple delay timer transistor circuit generator#
Simple 555 Timer Projects: Fixed and Variable Pulse Waveform Generator You may like to read some more 555 Timer Projects for Beginners: Therefore, when transistor Q1 is turned ON, switch S1 must be reopened to reset the circuit.This causes the reset to lock the discharge ON and the output OFF.When transistor Q1 is turned ON , the voltage applied to the reset pin-4 is changed from 12V to 0V. When the output turns transistor Q2 OFF, transistor Q1 is supplied with base current through resistor R3 and turns on relay coil K1.When capacitor C1 has been charged to 2/3 of V CC, the discharge turns ON and the output of the timer turns OFF.Transistor Q1 remains off as long as transistor Q2 is ON.When the timer is triggered, the output activates transistor Q2 which steals the base current from transistor Q1.When switch S1 is closed, pin-2 which is connected to 0V, triggers the timer.To understand the operation of the circuit, assume that switch S1 is open and all capacitors are discharged.The time setting can be changed by changing the value of resistor R1.Resistor R2 has a relatively low value to enable capacitor C1 to discharge quickly.The Resistor R2 is used to limit the current when capacitor C1 discharges. Resistor R2 should be kept at a value of about 100Ω.Capacitor C1 and resistors R1 and R2 are used to set the amount of time delay.Since the timer can no longer operate, the output remains turned off, which permits transistor Q1 to remain turned ON.Pin-4 is also connected to ground, which prevents the timer from further operation.When transistor Q1 turns ON, the collector of the transistor drops to ground or 0V. When pin-4 is connected to a voltage that is less than 1/3 of V CC, it turns ON the discharge and keeps the timer from operating.For the timer to operate, pin-4 must be connected to a voltage that is greater than 2/3 of V CC.Since pin-4 is connected to the collector of transistor Q1, 12V is applied to pin-4.Since transistor Q1 is OFF, most of the applied voltage is dropped across the transistor, causing about 12V to appear at the collector of the transistor. When the power is applied at V CC, transistor Q1 turned OFF.Pin -4, the reset pin, is used as a latch in this circuit. Resistor R3 limits the base current to transistor Q1 and resistor R4 limits the base current to transistor Q2.Diode D1 is used as freewheeling diode to eliminate the spike voltage induced into the coil of relay K1 when switch S1 is opened.Once transistor Q2 has been turned ON, it will hold transistor Q1 OFF by stealing its base current.This charging time is only a fraction of a second, but it ensures that transistor Q1 will not turn ON before the output of the timer can turn transistor Q2 ON.Before transistor Q1 can be turned ON, the capacitor C3 must be charged through resistor R3.
Capacitor C3 acts as a short time delay circuit. Capacitor C3 is connected from the base of BJT Q1 to ground.
As long as Q2 is turned ON by the output of the 555 timer, transistor Q1 is turned OFF.