Single vibrator. Scheme, description

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Single vibrator. Radio - for beginners

Radio - for beginners

This is the so-called single pulse generator. With a short-term signal at the input, it generates a rectangular electric pulse of a well-defined duration (it does not depend on the duration of the input, i.e., the trigger pulse), after which it goes into standby mode and remains in this state until the next trigger signal arrives.

Often in the technical literature you can also find another name for this device - a waiting multivibrator. Now this name is used less and less.

You can see a diagram of the simplest single vibrator in Fig. 1, a. It also has two logical elements, but the first of them is used for its intended purpose - as a 2I-NOT logic element and the second as an inverter. Pushbutton switch SB1 performs the function of a trigger signal sensor.

Rice. 1 Single vibrator

In order for the generated pulses to be indicated by a DC voltmeter, an incandescent lamp or other similar relatively inertial device, the capacitance of the capacitor C1 must be at least 50 μF, and the resistance of the resistor R1-1 ... 1.5 kOhm. You can, of course, do without the SB1 switch, simulating the sensor signal by closing the piece of the mounting wire of output 1 of the first element to a common wire, however, in this case, there may sometimes be malfunctions of the device that occur due to the "bounce" of the closing contacts. Below we will return to a detailed consideration of this phenomenon and ways to deal with it.

After mounting the single vibrator and turning on the power, immediately measure the voltage at the inputs and outputs of the elements. At the input pin 2 of the DD1.1 element and the output of the DD1.2 element, it must correspond to a high level, and at the output of the first element and the inputs of the second, it must be low. Therefore, in standby mode, the first element is in the zero state, and the second element is in the one state.

Then connect a voltmeter to the output of the second element and, watching the indicator arrow, briefly close the contacts of the switch SB1. How does the meter react to this? Its arrow sharply deviates to the left almost to the zero mark of the scale, and after about 2 s it also abruptly returns to its original position. The device detects the appearance of a low-level pulse. And the LED? It shines during the impulse. Repeat the experience several times.

Connect another capacitor in parallel - with a capacity of 1000 microfarads - and repeat the experiment. The duration of the output pulse will almost triple. Replace the fixed resistor R1 with a variable one, with a resistance of about 2 kOhm (but not more than 2,2 kOhm). Now, using only this resistor, you can change the duration of the generated pulses within certain limits. But with its resistance less than 100 ohms, the one-shot will stop working.

The conclusion suggests itself: the duration of single pulses of a single vibrator will be the greater, the greater the capacitance of the timing capacitor C1 and the resistance of the resistor R1. With a small capacitance and a small resistance of the resistor, the pulses become so short that the indicators that you use are unable to respond to them.

To understand the essence of the action of a single vibrator will help the timing diagrams shown in Fig. 1b. Since in standby mode, the input pin 1 of the DD1.1 element is not connected to anything (the contacts of the pushbutton switch are open), this is equivalent to applying a high voltage level to its input. At the input of the element DD1.2 there is a low voltage level, since the voltage drop across the resistor, created by the input current of the element, keeps the input transistor of the element closed. This means that the output of this element is a high level; the same level is at the top input of element DD1.1 according to the scheme. Thus, the output of the element DD1.1 is low, the capacitor is almost discharged.

Filed on the input pin 1 triggering a low-level pulse (duration T zap on the top graph) switches the element DD1.1 to a single state. A positive voltage jump created at this moment ti (it is commonly called a positive voltage drop) at its output is transmitted through a capacitor to the input of element DD1.2 and switches it from a single state to zero. Now, at the top input of the DD1.1 element according to the circuit, there is a low level, therefore its state does not change even after the contacts SB1 are opened, i.e., after the trigger pulse has ended.

From the moment a positive voltage drop appears at the output of the DD1.1 element, the capacitor begins to charge through the resistor R1. As the capacitor charges, the voltage across the resistor decreases. As soon as it drops to the threshold, the DD1.2 element will switch to a single state, and DD1.1 to zero. Now the capacitor will quickly discharge through the output circuit of the element DD1.1 and the input DD1.2 and the device will return to standby mode.

When conducting experiments and experiments with a single vibrator, keep in mind that for its normal operation, the duration of the triggering pulse must be less than the duration of the generated output pulse.

See other articles Section Beginner radio amateur.

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