555 Timer Projects for Beginners - 555 Timer Delay Circuit Design
555 Timer Projects for Beginners - Delay circuit using 555 Timer IC
In this project, the 555 timer is used to construct an ON delay relay.
The 555 produces accurate time delays which can range from seconds to hours depending on the values of resistance and capacitance used in the circuit.
Understanding the Delay timer Circuit:
In the below figure , transistor Q1 is used to switch relay coil K1 ON or OFF. As the 555 timer can not provide the necessity current to operate the relay a transistor is used to control the relay.
- Transistor Q2 is used as a stealer transistor to steal the base current from the transistor Q1. As long as Q2 is turned ON by the output of the 555 timer, transistor Q1 is turned OFF.
- Capacitor C3 is connected from the base of BJT Q1 to ground. Capacitor C3 acts as a short time delay circuit.
- When VCC is turned on by switch-1, the capacitor C3 is discharged. Before transistor Q1 can be turned ON, the capacitor C3 must be charged through resistor R3.
- 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.
- Once transistor Q2 has been turned ON, it will hold transistor Q1 OFF by stealing its base current.
- Diode D1 is used as freewheeling diode to eliminate the spike voltage induced into the coil of relay K1 when switch S1 is opened.
- Resistor R3 limits the base current to transistor Q1 and resistor R4 limits the base current to transistor Q2. Pin -4, the reset pin, is used as a latch in this circuit.
- When the power is applied at VCC, transistor Q1 turned OFF. 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.
- Since pin-4 is connected to the collector of transistor Q1, 12V is applied to pin-4.
- For the timer to operate, pin-4 must be connected to a voltage that is greater than 2/3 of VCC.
- When pin-4 is connected to a voltage that is less than 1/3 of VCC, it turns ON the discharge and keeps the timer from operating. When transistor Q1 turns ON, the collector of the transistor drops to ground or 0V.
- Pin-4 is also connected to ground, which prevents the timer from further operation.
- Since the timer can no longer operate, the output remains turned off, which permits transistor Q1 to remain turned ON.
- Capacitor C1 and resistors R1 and R2 are used to set the amount of time delay.
- Resistor R2 should be kept at a value of about 100Ω. The Resistor R2 is used to limit the current when capacitor C1 discharges.
- Resistor R2 has a relatively low value to enable capacitor C1 to discharge quickly.
- The time setting can be changed by changing the value of resistor R1.
- To understand the operation of the circuit, assume that switch S1 is open and all capacitors are discharged.
- When switch S1 is closed, pin-2 which is connected to 0V, triggers the timer.
- When the timer is triggered, the output activates transistor Q2 which steals the base current from transistor Q1.
- Transistor Q1 remains off as long as transistor Q2 is ON.
- When capacitor C1 has been charged to 2/3 of VCC, the discharge turns ON and the output of the timer turns OFF.
- When the output turns transistor Q2 OFF, transistor Q1 is supplied with base current through resistor R3 and turns on relay coil K1. When transistor Q1 is turned ON , the voltage applied to the reset pin-4 is changed from 12V to 0V.
- This causes the reset to lock the discharge ON and the output OFF.
- Therefore, when transistor Q1 is turned ON, switch S1 must be reopened to reset the circuit.
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