Thursday, September 25, 2014
Browse »
home»
555
»
ic
»
Schmitt
»
Trigger
»
using
»
Schmitt Trigger Using IC 555
Several circuits exist for using NE 555 as a Schmitt trigger.The circuits are s0 arranged that when the voltage exceeds aset value, say V1, the output of NE 555 becomes high. When the voltage falls below, say V2, the output becomes low. Almost all such circuits use the internal hysteresis pro- vided by ·the three 5k resistors. Though such circuits are simple to design, the Schmitt trigger action of such circuits puts a limitation on the designer. In such circuits, for exam- ple, the difference between Vl ( the upper trip point) and V2 {the lower trip point) is fixed at l/ 3 Vcc by the internal potential divider, and this difference cannot be varied. The circuit shown in Fig. 1 avoids this limitation.
lt enables the use of an NE555 as a general-purpose Schmitt trigger with externally adjustable characteristics. Rl and R2 form a potential divider of Vcc and apply a voltage V1 = R1/R1+R2 * Vcc to terminal 6. Similarly, a voltage V2 = R3/R3+R4 * Vcc is applied to terminal 2. The input voltage Vi is applied to terminal 5 (negative . terminal of threshold comparator).
Also, the input voltage V1 is divided into half (by the internal 5k resistors) and is applied to the positive terminal of the trigger comparator. When the input voltage exceeds twice the value of V2, the trigger comparator output becomes high and the output of NE555 becomes high. After exceeding twice the value of V2, if the input voltage reduces, no change in the timer output occurs until the input voltage goes below VI. When the input goes below V1, the threshold comparator output goes high and the timer output goes low. Thus the NE555 acts as a Schmitt trigger with adjustable hysteresis points of 2 XSV2 and Vl. ’ .
Care should be taken to keep 2xV2 larger than Vl, as is expected from any Schmitt trigger. If this is not satisfied, the timer will not behave as a Schmitt trigger but will operate as a simple comparator (with 2xV2 as the comparison voltage). Another important point is that the internal impedance of the input voltage should be very low, so that the voltage at negative input of the threshold comparator is now significantly modified by the internal potential dividing resistors
(connected to Vcc and earth).
A typical circuit is given in Fig. 2 along with input and
output waveforms.
Schmitt Trigger Using IC 555
lt enables the use of an NE555 as a general-purpose Schmitt trigger with externally adjustable characteristics. Rl and R2 form a potential divider of Vcc and apply a voltage V1 = R1/R1+R2 * Vcc to terminal 6. Similarly, a voltage V2 = R3/R3+R4 * Vcc is applied to terminal 2. The input voltage Vi is applied to terminal 5 (negative . terminal of threshold comparator).
Also, the input voltage V1 is divided into half (by the internal 5k resistors) and is applied to the positive terminal of the trigger comparator. When the input voltage exceeds twice the value of V2, the trigger comparator output becomes high and the output of NE555 becomes high. After exceeding twice the value of V2, if the input voltage reduces, no change in the timer output occurs until the input voltage goes below VI. When the input goes below V1, the threshold comparator output goes high and the timer output goes low. Thus the NE555 acts as a Schmitt trigger with adjustable hysteresis points of 2 XSV2 and Vl. ’ .
Care should be taken to keep 2xV2 larger than Vl, as is expected from any Schmitt trigger. If this is not satisfied, the timer will not behave as a Schmitt trigger but will operate as a simple comparator (with 2xV2 as the comparison voltage). Another important point is that the internal impedance of the input voltage should be very low, so that the voltage at negative input of the threshold comparator is now significantly modified by the internal potential dividing resistors
A typical circuit is given in Fig. 2 along with input and
output waveforms.
Subscribe to:
Post Comments (Atom)
No comments:
Post a Comment