Trigger (electronics)

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A trigger is an electronic circuit that generates a pulse (trigger pulse) or a switching process (switching edge) in the event of a triggering event. The input signal is analog, - the output signal is binary. The triggering event can be, for example, when an input voltage exceeds a threshold (trigger threshold, trigger level) or the expiry of a period of time.

In the simplest case, circuits with a trigger threshold are implemented with an operational amplifier , at the two inputs of which are the voltage to be assessed and a reference value . Depending on which voltage is greater, the output switches to the positive or negative limit of its controllability - as a result of the almost infinite voltage gain . The two output values ​​are often referred to as HIGH and LOW.

Variants specially designed for fast switching are called comparators . In order to work with logic gates, the LOW can also be the ground potential here. Such circuits can also be constructed with discrete transistors .

For example, this is how the trigger of an oscilloscope works , triggering the creation of a new image as soon as a trigger level is exceeded or a delay time has expired or something similar has occurred. The edge of a trigger pulse triggered by the event is decisive , not its duration.

If the voltage to be assessed is close to the reference value and if it is overlaid with an interference voltage, multiple switching back and forth can occur. This can be avoided with a Schmitt trigger , which switches to HIGH when the input voltage rises when the trigger threshold is exceeded, but only to LOW when the input voltage falls below a lower threshold. The hysteresis required for this can easily be produced with a comparator or operational amplifier through positive feedback .

One example of this is the switching regulator , which, thanks to its hysteresis, avoids too frequent switching. The trigger threshold and the actuator of the controller are variable here.

In a network analyzer , the trigger is used to trigger signals in a defined frequency or voltage spectrum . This makes it possible to obtain frequency or voltage responses from a system over a range of, for example, 1 MHz, with a resolution that is similar to the pixels on the screen. At 1 MHz above, with a resolution of 1600 points (1 MHz: 1.600 pts = 625 Hz / pt steps), this would be pulses with a frequency difference of 625 Hz. The first signal would be sent with 1000 625 Hz, the second with 1 001 250 Hz and so on until 2 MHz is reached.

The process of triggering is closely related to the trigger: many devices and assemblies have a trigger input, for example to switch them to processing mode after an event. Examples are:

Various devices used in research can also send an email, for example, when a (rare) event occurs.

Cameras have a so-called FX contact to trigger the electronic flash synchronously with the work of the mechanical shutter (see also open time ).


  • Erwin Böhmer: Elements of Applied Electronics . At least 16 editions since 1979, Vieweg;
    now by Erwin Böhmer, Dietmar Ehrhardt, Wolfgang Oberschelp.
  • Wolfgang Böge, Wilfried Plaßmann (Hrsg.): Vieweg handbook electrical engineering: Basics and applications for electrical engineers. At least 7 editions since 1999; now edited by Wilfried Plaßmann, Detlef Schulz