Schematic Diagram Trigger Input Control Of 555 Timers

The following method allows the timer to be triggered by a normally closed switch. This would be useful in applications such as intrusion alarms where the protection circuit is broken if a window or door is opened. Trigger Input Control of 555 Timers.

The described method utilizes a 555 timer in a monostable configuration, which is ideal for applications requiring a single pulse output upon activation. In this scenario, a normally closed switch is integrated into the circuit. Under normal conditions, the switch remains closed, allowing the circuit to function without interruption. When the switch is opened, as in the case of a door or window being breached, the circuit detects the change in state.

The 555 timer's trigger input (pin 2) is connected to the normally closed switch. When the switch is closed, pin 2 is held high, preventing the timer from triggering. Upon the opening of the switch, pin 2 is momentarily pulled low, which triggers the timer and initiates its timing cycle. The output from pin 3 can then be used to activate an alarm or other alerting mechanism.

To ensure reliable operation, it is important to include a pull-up resistor connected to the supply voltage at pin 2. This resistor ensures that pin 2 remains high when the switch is closed. Additionally, a capacitor can be connected between pin 6 and ground to set the duration of the output pulse, while a resistor between pin 7 and the supply voltage determines the timing interval.

This configuration is particularly advantageous for security applications, as it allows for immediate detection of unauthorized access, providing a prompt response to potential intrusions. The simplicity and reliability of the 555 timer make it an excellent choice for such control circuits.The following method allows the timer to be triggered by a normally closed switch. This would be useful in applications such as intrusion alarms where the protection circuit is broken if a window or door is opened Trigger Input Control Of 555 Timers 🔗 External reference