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7.5 Stimulator with Schmitt TE
One way to create pulses quadratic form is to use a special circuit called the agonist-Schmitt (Schmitt trigger). This circuit converts a sinusoidal input signal at the same frequency square waveform with a sinusoidal signal.
The Sch.7.7 depicts an exciter circuit with Schmitt operational amplifier (TE). In this simple circuit output waveform plays an important role anasyzefxi the exit to the non-inverting input of the TE (positive anasyzefxi). According to what we saw in chapter 4, the reason or factor anasyzefxis be given by:
(751)
Example 7-3
We compute a TE Schmitt stimulator with output voltage ± 12 V and
Therefore anasyzefxis b = 0.1.
Solution
The Eq. (7.5.1) gives
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Figure 7.7. Schmitt stimulator with IS and the waveforms of
The above circuit works as follows:
We assume that initially, for vi = 0 the output voltage v0 is the upper saturation level + VCC. When the sinusoidal voltage reaches the rising price +, VCC, the output of the TE will fall in the income-VCC, because the voltage vnb will then become negative. Because now the Vag = -, VCC, the v0 but will not state ¬ sions, when n, ellattoumeni fall back on +, VCC, but will wait until they reach the vi -, VCC, and so will return again n0 to-VCC. After the returns to n0 + VCC, when the input voltage reaches the value +, VCC, etc. The output will then be the square waveform shown in Sch.7.7.
So
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We value R2 = 2.2 KO. Therefore, we find
R1 = 19.8 KW 20 KW Note that we use the TE 301.
7.6 O Timer 555
O is 555 timer chip and built by many companies. O timer is depending on the stimulation syndesmogisi and can function as a ground aftodiegeiromenos, Sch.7.8a as a ground of a shot, and as stimulator Sch.7.8v Schmitt, Sch.7.8g.
Aftodieg. A ground
555
(A)
A ground single-shot
555
Stimulator Schmitt
(C)
Figure 7.8. Various modes of the timer 555.
Aftodiegermenos a ground (a), a ground single shot (b) Stimulator Schmitt (c)
555
The Sch.7.9 shows the block diagram of the timer 555 and the terminals on a DIP (there is a base TO-99). The
most significant of these connectors are pin stimulation (2), compared to the excitation of the input voltage, the threshold pin (6) for comparing the input voltage threshold
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a fraction of supply voltage, the reset pin (4), which locates in the 0 mode and the discharge pin (7),
to discharge an external capacitor.
Threshold
Discharge
Comparators
R 5 KO
R 5 KO
Comparators
Vref
Flip-Flop (FF)
Output stage
Ground
Control Voltage
Trigger
Output
Reset
Ground 1
Trigger 2
Output 3
Reset 4
8 Vcc
7 Discharge 6 Threshold
5 Control Voltage
Figure 7.9. Block diagram of the timer 555 (a), terminals (b)
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Let's look in more detail the various terminals 555.
O comparator stimulus (trigger comparator) compares a voltage VCC / 3 with
input voltage excitation. When the excitation voltage be less than or equal to VCC / 3, the comparator generates output that drives the flip-flop (FF) in state L (Low), so the FF cuts out the discharge transistor T (disharge) and leads to the exit in state H (High) by output stage (output stage, buffer-inverter).
O comparator threshold (threshold comparator) compares a continuous trend 2VCC / 3 of the input voltage threshold. When the threshold voltage become equal to or greater than the voltage 2VCC / 3, this gives the comparator voltage leads the output of the FF state H. Thus, the FF leads to the base of transistor T decay (disharge) and leads to Koros, when the collector circuit is closed (no outer resistance). O input terminal is now in state L. If applied at the entrance to a low voltage (reset), the output is driven in state L.
O pin reset (reset) allows the 555 to the reset function of the excitation is due to the excitation input (trigger). When the input is not used must be connected to voltage VCC. When the reset input is grounded, or when the voltage is below 0.4 V, the output pin and dishcarge is approximately ground potential, ie the output is in state L.
O discharge pin (disharge) serves a discharged capacitor, which we have connected externally, during which the output is in state L. When the output is in state H, disharge pin acts as an open circuit and allows the capacitor to charge a rate determined by an external resistor and capacitor.
Besides this, there is a tension adjustment pin (control voltage). Between this terminal and the earth is commonly associated, as a filter, a capacitor 10 nF. Since the capacitor is leaking to ground noise or ripple of the voltage fed ¬ munity to minimize their effect on the voltage threshold. O pin that can also be used to change both the level of the voltage threshold, and the voltage level trigger. For example, if 5 KO resistor connection between the probe and control voltage
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the VCC (pin 8) the change in threshold voltage and the voltage 0.8VCC trigger a 0.2VCC. If we apply external voltage to the terminal control voltage, so it will change the threshold voltage and the voltage trigger. In this way we can use the 555 to output waveform shape.
In the following we will develop in more detail the 3 main modes of timer 555. O this timer, ¬ what used Power Supply 4 έως voltage 18 V, can provide output current up to 200 mA.
7.6.1 Aftodiegeiromenos a ground with IC 555
The clock shows the Sch.7.10 555 syndesmologimeno aftodiegeiromeno as a ground and the waveforms. The capacitor voltage vC C (pin 6) is connected directly to the input trigger (pin 2). The voltage vC change between VCC / 3 and 2VCC / 3.
T1 = (R1 + R2) C
Figure 7.10. Aftodiegermenos a ground and waveforms of
When you connect the supply voltage VCC, the capacitor C charges to the Tai ¬ voltage VCC through resistors R1 and R2. But when the voltage of the capacitor, growing, reach 2VCC / 3, the comparator threshold
Finally, the duty cycle d, will be given by:
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stimulates the flip-flip (FF), making the transistor conductive T (see Sch.7.9), and
the capacitor C begins to be discharged through R2. When, due to
discharge the capacitor voltage drops to VCC / 3, stimulated the comparative
the trigger and the output Q of FF led to state L (Low) and
so driven to cut the transistor T, so the capacitor
C begins to charge back to the voltage VCC, and therefore all the
cycle is repeated.
It turns out that the charge and discharge times t1 and t2 are
by the equations:
t1 = 0.69 (R1 + R2) C (7.6.1)
and
t2 = 0.69 R2 C (7.6.2)
The corresponding period T is:
T = t1 + t2 = 0.69 (R1 + 2R2) C (7.6.3)
In which case the frequency of oscillation of a ground would be:
(7.6.4)
(7.6.5)
If R1 >> R2, the turnover for this will be maximum (100%) while if
R1 << R2, turnover is minimal and equal to 50%.
Example 7-4
We compute a ground aftodiegeiromeno to 555, with t = 0.7 ms
and t2 = 0.3 ms. It will also identify the work cycle and the
frequency of oscillation.
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Solution
We C = 100 nF, Eq. (7.6.2) and (7.6.1)
So, R1 = 5.8 kO
In addition, Eq. (7.6.5) give
And the Eq. (7.6.4) give
7.6.2 crystal oscillator with 555
O a ground of Sch.7.10 used in most applications as oscillator / clock (clock), with a maximum frequency of oscillation, according to the manufacturer, equal to 1 MHz. If we want the oscillation frequency has high accuracy and stability using crystal, in which the crystal Sch.7.11 connected between the RC and chronokyklomatos comparator threshold. The charge-discharge of the capacitor C is as already described, but the control signal and the two comparators (pins 2 and 6) passes through the crystal, making him the natural oscillating frequency or aliquot of resonant frequencies.
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Figure 7.11. Crystal oscillator with IC 555
The values of R and C chosen so that the crystal shorted to have preservation of oscillations near the resonance frequency of the crystal. The resonant frequency of the oscillator is given by:
(7.6.6)
RC
The values of R and C can be changed by 25% or even more without affecting the oscillation frequency of the crystal. The timing of charge-discharge voltages of the capacitor is changed to be consistent with the values of R and C which we elected to maintain a constant oscillation frequency given by Eq. (7.6.6).
If we double the time constant, having the same crystal, the oscillation frequency will be half the frequency of the crystal. Various other changes to give ypoarmonikes time constant 1/3, 1/4, 1/5, etc. of the resonance frequency of the crystal.
Control
• Threshold Voltage,
Trigger Grounc
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O variable capacitor at the edges of the crystal allows adjusting the oscillation frequency accurately. The resistance of 1 MO dc outlet allows the input of the comparator, to begin oscillation when applied voltage VCC.
7.6.3 a ground shot of the IC 555
The Sch.7.12 shows the timer 555 as syndesmologimeno polydoni THE ¬ a shot. The circuit is as follows: In steady state the FF holds the conductive transistor T (on) thus grounding the capacitor C. During this time, the output is the potential of the land, ie in state L (Low). If the comparator threshold voltage is polarized in the VCC / 3 remains at rest as long as the trigger input is maintained above the voltage VCC / 3. When the trigger input excited by the negative input of the pulse front, the comparator threshold leads to the FF reset state, so the transistor T is taken to the state cut (off), from which the C-shorted so that the output is driven by the ¬ The distance (High: VCC). Through R, the voltage across the capacitor increases exponentially with voltage VCC, with time constant RC. When the capacitor voltage reaches 2VCC / 3, the comparator will trigger the FF in the state 1 (set). This, in turn, will lead the transistor T in gross registered tonnage, so it discharges the capacitor C. This situation reflects the initial condition, ie a steady state.
Figure 7.12. The timer 555 as a ground shot and a waveform of
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It turns out that the output pulse duration (tp) is given by:
(7.6.7)
Sometimes the circuit Sch.7.12 unwanted excited by the positive input of the pulse front, even if the control voltage terminal capacitor has leakage. To avoid such a situation diaforizoume the pulse input to the panel shown in Sch.7.13.
Figure 7.13. A ground shot with a lattice-stimulated differentiation
The diode protects the IC 555 from being excited by the positive input of the pulse front.
Example 7-5
To calculate a ground of a shot to 555, with pulse duration of 1 ms.
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Solution
In principle, we choose C = 100 nF.
Then, from Eq. (7.6.7) we have
The length of time constant must be much smaller than the duration of the pulse output. Therefore, we choose
tp
Finally, pick Rt = 10 kQ and the ratio r = RtCt find:
7.6.4 Schmitt Stimulator with IC 555
The circuit of the timer shows Sch.7.14 555 syndesmologimeno as stimulator Schmitt (Schmitt trigger, ST). The inputs of two comparators, corresponding to excitation terminals (2) and threshold (6), and associated with a polarized external voltage VCC / 2, through two equal resistors R1 and R2.
Figure 7.14. The timer 555 as a stimulator of the waveforms and Schmitt
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Because the comparator threshold, which is the terminal threshold (6), stimulated in 2VCC / 3 and the comparator trigger, the trigger pin (6), to (reference) VCC / 3, the polarization generated by the resistors R1 and R2 is the midway between these two values, so a voltage VCC / 2.
When the circuit is excited by sinusoidal signal suitable width (height) to levels exceeding the reference switches from a standard conductivity and vice versa, so the au pairs out ¬ Noumea square pulse. If R1 = R2 = R the 555 automatically polarized voltages that may have VCC values from 5 to 15 V.
