Trigger the 555 timer IC with a positive pulse. Scheme, description

Trigger the 555 timer IC with a positive pulse. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Application of microcircuits

The timing circuit is required in many applications, and the 555 timer IC is the most commonly used. Although this timer is a versatile device, its use is limited in that it can only be triggered by a negative input pulse. However, a closer look at the timer's functional block diagram shows that pin 5, connected to the non-inverting input of comparator 2 through a resistor, can be mistaken for a positive trigger input. Thus, pin 5 can serve both as a control voltage input, for which it was originally intended by the developers of the 555 timer, and as a positive trigger input.

(click to enlarge)

Since the start pulse ends by the time the timing capacitor is charged to the control voltage level, the input start pulse, when applied to pin 5, does not affect the control voltage. The sensitivity of the circuit when a trigger pulse is applied to pin 5 is determined by the voltage difference between pins 5 and 2. The sensitivity is adjusted by connecting pin 2 to the tap of the voltage divider.

As shown in the diagram, the idle multivibrator containing the 555 timer IC is triggered by the rising edge of a positive input pulse. Pin 2 is connected to the middle of a voltage divider connected between the power rail and ground. In addition, a shunt capacitor is connected to pin 2 to ensure that the circuit is insensitive to spurious pulses from nearby circuits.

Author: Rudy Stefenel, San Jose, EA California; Publication: N. Bolshakov, rf.atnn.ru

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drex
Modeled in Proteus. It starts, but the duration of the output pulse is exactly equal to the duration of the input and does not depend on the parameters of the timing elements.

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