SCHMITT TRIGGER (regenerative comparator) Schmitt trigger is an inverting comparator with positive feedback. It converts an irregular-shaped waveform to a square wave or pulse, also called as squaring circuit. The input voltage Vin triggers the output V0 every time it exceeds certain voltage levels called the upper threshold voltage Vut and lower threshold voltage Vlt, These threshold voltages are obtained by using the voltage divider where the voltage across R1 is fed back to the input. The voltage across R1 is a variable reference threshold voltage that depends on the value and polarity of the output voltage V0 When Vo = +VSat, the voltage across R1 is called the upper threshold voltage, Vut The input vp voltage Vin must be slightly more positive than Vut Vut in order to cause the output switch from +VSat to -VSat. Vut As long as Vin <Vut, Vo is at +Vsat. -vp Vut =R1/R1+R2 (+VSat) On the other hand, when V0 = -Vsat, the voltage across R1 is referred to as lower threshold +vsat voltage, vin be slightly more negative than vlt in order to switch V0 from +Vsat to -Vsat. In other words, for vin values greater than vlt, vo is at -Vsat. Vlt is given -vsat by the following equation Fig : I/O waveform (c) Vo vs Vin plot of . hysteresis voltage Vlt =R1/R1+R2 (-VSat) Thus, if the threshold voltages are made larger than the input noise voltages, the positive feedback will eliminate the false output transitions. Resistance ROM used to minimize the offset problems. In the triangular wave and sawtooth wave generators a noninverting comparator is used as a Schmitt trigger. When the input is a triangular wave, the output of the Schmitt trigger is a square wave, whereas if the input is a sawtooth wave, the output is a pulse waveform. The comparator with positive feedback is said to exhibit hysteresis, a dead-band condition That is, when the input of the comparator exceeds Vut, its output switches from +Vsat to -Vsat and reverts back to its original state, +Vsat, when the input goes below Vlt. 푅1 == (푉푆퐴푇) 푅1+푅2 COMPARATOR CHARACTERISTICS The important characteristics of a comparator are these: 1. Speed of operation 2. Accuracy 3. Compatibility of output The output of the comparator must switch rapidly between saturation levels and also respond instantly to any change of conditions at its inputs. This implies that the bandwidth of the op-amp comparator must be rather wide; in fact, the wider the bandwidth, the higher is the speed of operation. The speed of operation of the comparator is improved with positive feedback (hysteresis) The accuracy of the comparator depends on its voltage gain, common-mode rejection input offsets, and thermal drifts. High voltage gain requires a smaller difference voltage (hysteresis voltage to cause the comparator's output voltage to switch between saturation levels, On the other hand, a high CMRR helps to reject the common- mode input voltages, such as noise, at the input terminals. Finally, to minimize the offset problems, the input offset current and input offset voltage must be negligible; also, the changes in these offsets due to temperature variations should be very slight. Since the comparator is a form of analog-to-digital converter, its output must swing between two logic levels suitable for a certain logic family such as transistor-transistor logic (TTL). LIMITATIONS OF OP-AMPS AS COMPARATORS A general-purpose op-amp such as the 741 can be used in relatively less critical comparator applications in which speed and accuracy are not major factors. With positive feedback (hysteresis), the switching speed of the op-amp comparator can be improved and false transition due to noise can be eliminated. In addition, an offset voltage- compensating network and offset minimizing resistor can be used to minimize offset problems. However, the output voltage swing of an op-amp is relatively large because it is designed primarily as an amplifier. In other words, the output of an op-amp comparator is generally not compatible with a particular logic family such as the TTL, which requires input voltages of either approximately +5 V or 0 V. Therefore, to keep the output voltage swing within specific limits, op-amps are used with externally wired components such as zeners or diodes. The resulting circuits, in which the outputs are limited to predetermined values, are called limiters. 555 TIMER One of the most versatile IC is 555 timer. It can be used in a number novel and useful application. Applications Monostable and Astable multivibrators, dc to dc converters, digital logic probes, waveform generators, analog frequency meters and tachometers, temperature measurement and control, infrared transmitters, burglar and toxic gas alarms, voltage regulators, electric eyes, and many other. 555 timer can produce accurate and highly stable time delays or oscillation. The timer basically operates in one of two as a monostable (one-shot) multivibrator or as an astable (free running multivibrator). The device is available as an 8-pin metal can, an 8-pin mini DIP, or a 14-pin DIP. Operating temperature range of SE555 is -55 to +125 and NEE is 0 to +70C. It operates on +5 to +18 V supply voltag; It has an adjustable duty cycle; timing is from microseconds through hours; it has a high current output it can source or sink 200 mA PIN CONFIGURATION Pin 1: Ground. All voltages are measured with respect to this terminal. Pin 2: Trigger. The output of the timer depends on the amplitude of the external trigger pulse applied to this pin. The output is low if the voltage at this pin is greater than 2/3 Vcc. However, when a negative going pulse of amplitude larger than 1/3 Vcc is applied to this pin, the comparator 2 output goes low, which in turn switches the output of the timer high. The output remains high as long as the trigger terminal is held at a low voltage. Pin 3: Output, There are two ways a load can be connected to the output terminal: either between pin 3 and ground (pin 1) or between pin 3 and supply voltage Vcc (pin 8). when the output is low the load current flows through the load connected between pin 3 and Vcc into the output terminal and is called the sink current. However, the current through the grounded load is zero when the output is low. For this reason, the load connected between pin 3 and Vcc is called the normally on load and that connected between pin 3 and ground is called the normally off load. On the other hand, when the output is high, the current through the load connected between pin 3 and Vcc (normally on load) is zero. However the output terminal supplies current to the normally off load. This current is called the source current. The maximum value of sink or source current is 200 mA Pin 4: Reset. The 555 timer can be reset (disabled) by applying a negative pulse to this pin. When the reset function is not in use, the reset terminal should be connected to Vcc to avoid any possibility of false triggering. Pin 5: Control voltage. An external voltage applied to this terminal changes the threshold as well as the trigger voltage. In other words, imposing a voltage on this by connecting a pot between this pin and ground, the pulse width of the output waveform can be varied. When not used, the control pin should be bypassed to ground with a 0.01-µF capacitor to prevent any noise problems. Pin 6: Threshold. This is the noninverting input terminal of comparator 1 which monitors the voltage across the external capacitor. When the voltage at this pin is 2 threshold voltage 2/3 Vcc, the output of comparator 1 goes high, which in turn switches the output of the low. Pin 7: Discharge. This pin is connected internally to the collector of transistor Q1. When the output is high, Q1 is off and acts as an open circuit to the external capacitor C connected across it. On the other hand, when the output is low, Q1 is saturated and acts as a short circuit, shorting out the external capacitor C to ground. Pin 8: +Voc. The supply voltage of +5 v to +18 is applied to this pin with respect to ground (pin 1) 555 TIMER AS MONOSTABLE MULTIVIBRATOR A monostable multivibrator, often called a one-shot multivibrator, is a pulse-Generating circuit in which the duration of the pulse is determined by the RC Network connected externally to the 555 timer. In a stable or standby state the output of circuit is zero os at logic low level. When an Trigger pulse is applied, the output is forced to go high (≈Vcc). The time the Output remains high is determined by the external RC network connected to the timer. At the end of the timing interval, the output automatically reverts back to its logic-low state. The output stays low until the trigger pulse is applied. Then the cycle repeats. Monostable operation. Initially when the output is low, that is, the circuit is in a stable state, transistor Q1 is on and capacitor C is shorted out to ground. However, upon application of a negative trigger pulse to pin 2, transistor Q1 is turned off, which releases the short circuit across the external capacitor C and drives the output high. The capacitor C now starts charging up toward Vcc through RA. However, when the voltage across the capacitor equals 2/3 Vcc, comp1’s o/p switches from low to high, which in turn drives the output to its low state via the output of the flip-flop.