FIGURE 19-11 An inverting Schmitt trigger.

Remember that this is an inverting Schmitt trigger. As shown above, the UTP is determined by the values of . When the circuit input exceeds the UTP, the output from the circuit goes to (as shown in Figure 19-11). The values of then determine the value of the LTP. When the input makes a negative- going transition past the LTP, the output from the op-amp returns to .

The circuit in Figure 19-11 is limited to UTP and LTP values that are equal in magnitude. Like the noninverting circuit described earlier, the circuit in Figure 19-11 can be modified so that it has unequal trigger points. Such a circuit is illustrated in Figure 19.30 of the text. This circuit also uses two diodes that conduct on opposite transitions of the output. When the op-amp output is positive, conducts and determines the UTP in conjunction with . The UTP can be found using:

And when the output is low, conducts and in conjunction with determines the LTP. It can be found from

By using unequal values of and , the circuit can be designed for asymmetrical trigger points; that is, for trigger points that are not equal in magnitude.

Multivibrators are circuits that are designed to have zero, one, or two stable output states. The input/output relationships for the various multivibrators are shown in Figure 19-12.

 

FIGURE 19-12 Multivibrator input/output relationships.

The astable multivibrator is a circuit that has no stable output state, as shown in Figure 19-12a. The circuit output switches back and forth between high and low states with no input signal. It is essentially a rectangular-wave oscillator, and is often referred to as a free-running multivibrator.

The monostable multivibrator has one stable state, as shown in Figure 19-12b. The circuit output is normally in its stable state (). When an input trigger is received, the output goes high for a predetermined time and then reverts to its stable state. It then remains in this state until another trigger pulse is applied to the input. The monostable multivibrator is generally referred to as a one-shot.

The bistable multivibrator has two stable output states, as shown in Figure 19-12c. It remains in one state until it receives a trigger input, when it switches to its second stable state. It remains in this second state until it receives another trigger input, which returns the circuit to its original output state. This circuit is often referred to as a flip-flop.

(Flip-flops are covered extensively in any digital electronics course and are not covered in depth here. This section concentrates on astable and monostable multivibrators.)

555 Timers. Although these circuits were once constructed using discrete devices, they are almost exclusively constructed using ICs. One of the most common is the 555 timer.

The 555 timer is an eight-pin IC that contains a flip-flop, an inverter, and several resistors and transistors. Its internal circuitry can be represented as shown in Figure 19-13. The voltage divider created by , and sets the reference voltages for and to and , respectively (when there is no control voltage applied to pin 5 of the IC).

 

FIGURE 19-13 The 555 timer (equivalent circuit).

Here is a brief summary of 555 timer operation: If the threshold input (pin 6) exceeds the reference voltage, the output from C_A goes high (otherwise it stays low). If the trigger input (pin 2) goes more negative than the reference voltage, the output from goes high. Under normal circumstances, the comparators and flip-flop operate in one of the following input/output combinations:

The input combination shown in italics causes the flip-flop to work in an unpredictable fashion. Therefore, the combination of pin 6 = high and pin 2 = low is never allowed.

As shown in Figure 19-13, the flip-flop output is tied to pin 7 via the npn transistor. When the flip-flop output is high, pin 7 is grounded through the transistor. When the flip-flop output is low, pin 7 is open. The reset input (pin 4) is used to disable the 555 timer. If pin 4 is grounded, the output (pin 3) is held low. This is only done in special applications, so pin 4 is normally tied to. Section 19.4 of the text provides a more detailed explanation of the internal workings of the 555 timer.

The 555 timer can be wired as a one-shot with the addition of a single resistor and capacitor, as shown in Figure 19-14. Note that:

1. Pin 4 is connected to , which disables the reset circuit.
2. Pins 8 and 1 are tied to and ground, respectively, which is necessary for normal circuit operation.
3. The control voltage input (pin 5) is left open.
4. Pins 6 and 7 are tied together between R and C. This means that is applied to both of these pins.

Briefly, the circuit operates as follows: When a trigger pulse is applied to pin 2, the output (pin 3) immediately goes high. At the same time, pin 7 goes to an open and the capacitor begins to charge. Once it reaches a specific voltage (in this case 10 V), several things happen at once. Pin 3 () goes low and pin 7 is coupled to ground (via the 555 timer), which causes the capacitor to discharge.

 

FIGURE 19-14 A 555 timer one-shot and its waveforms.

The pulse width of the 555 timer output is determined by the values of R and C, as is found using:

PW = 1.1RC

The derivation of this equation is found in Section 19.4 of the text. Example 19.10 demonstrates the use of this equation.

Troubleshooting the one-shot is fairly simple. First of all, you should check the following:

1. Are the and ground connections good?
2. Is the reset input tied to ?
3. Is the trigger signal valid?
4. Is the resistor good?
5. Is the capacitor good?

If the answer to any of these questions is no, you may have found the problem. If all answers are yes, then change the IC.

Question 3 (above) refers to a valid trigger signal. There are two conditions for determining a valid trigger signal. If the control voltage input is not used (as in Figure 19-14), then a trigger voltage () is valid when:

If pin 5 has an active input, then is valid when:

Example 19.11 of the text demonstrates the use of both these equations.

There are two main causes for a one-shot to work intermittently. The first cause is the trigger signal. If a trigger signal is very close to the threshold of being valid, it may trigger the one-shot intermittently. If you have intermittent problems, the trigger signal is the first suspect. The other possible cause of intermittent operation occurs when the 555 is operated at high frequencies. This can be controlled by adding a decoupling capacitor as shown in Figure 19.41 of the text.

The 555 timer can also be wired as a free-running multivibrator, as shown in Figure 19-15. Note that there are two resistors and one capacitor. Also note that there is no input signal (which is the circuit recognition feature). The key to the operation of this circuit is the connection of the capacitor to both the trigger and threshold inputs. As the capacitor charges and discharges between the threshold and trigger voltages, a steady train of pulses is generated at the output (pin 3).

 

FIGURE 19-15 A 555 timer astable multivibrator.

The output cycle time for the free-running multivibrator is found as:

The reciprocal of this equation gives us the operating frequency:

The PW of the circuit is found using:

The duty cycle is found using:

Example 19.12 of the text demonstrates the basic analysis of a free-running multivibrator.

A voltage-controlled oscillator (VCO) can also be constructed using a 555 timer. (See Figure 19.45 of the text.) A dc control voltage (applied to pin 5) determines the frequency of the VCO output. As the control voltage increases, the output frequency decreases. The opposite is also true.

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