trigger

In electronic circuits, a trigger is a signal, usually a pulse or level change, applied to a component to initiate a specific function. The trigger tells that component: now is the time to do your job. In oscilloscopes, a trigger signal synchronizes the sweep circuit so the display stops jumping and shows a stable waveform. In thyristors and photoflash circuits, a trigger pulse on the gate terminal opens the main current path. Without the trigger, many circuits would either sit idle or behave unpredictably.

Triggers come in several electrical forms. An edge trigger responds to a voltage transition: rising edge (0 to high) or falling edge (high to 0). A level trigger fires when the signal simply exceeds a threshold voltage and stays there. Pulse triggers require a brief, defined pulse width. In digital logic, clock edges trigger flip-flops and shift registers. In analog circuits, a comparator output can trigger an analog-to-digital converter. The trigger voltage itself is often small, millivolts to a few volts, even when the component controls large currents or voltages downstream.

Common trigger sources include manual pushbuttons, photodetectors, thermostats, limit switches, and timing circuits. A microcontroller output, a sensor threshold, or a synchronized reference signal all serve as triggers. Many industrial controllers need multiple trigger inputs with different thresholds or timing requirements. Noise and false triggers are persistent problems: a stray voltage spike can fire a power circuit unintentionally. Engineers use debouncing circuits for mechanical triggers and Schmitt triggers for analog signals to reject noise and ensure clean, single-fire operation.

Trigger hold-off and jitter

Once triggered, many circuits become temporarily insensitive to further triggers during a hold-off or dead time period. This prevents retriggering before the primary action finishes. In a welding controller, this ensures one weld pulse completes before the next can start. In sampling systems, hold-off time determines the minimum interval between samples. Jitter, an unwanted variation in the exact moment the trigger fires, degrades precision in timing-critical applications such as laser pulse generation or digital signal synchronization.

The term "trigger" itself comes from firearms: pulling the trigger releases the firing mechanism. The electrical analogy is direct and persistent across all trades. Designers choose trigger type and sensitivity based on the application's speed, noise immunity, and repeatability needs. Understanding trigger behavior is fundamental to troubleshooting circuits that fail to fire on command or fire erratically.