Simple frequency counter. Scheme, description

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Simple frequency counter. Radio - for beginners

Radio - for beginners

On the basis of only one K155LAZ chip, using all its logical elements, it is possible to build a relatively simple device capable of measuring the frequency of an alternating voltage from about 20 Hz to 20 kHz.

The input element of such a measuring device is a Schmitt trigger - a device that converts a sinusoidal alternating voltage supplied to its input into rectangular pulses of the same frequency. That is, it converts sinusoidal "pulses" with a gentle rise and fall into rectangular ones with a steep rise and fall. The Schmitt trigger "fires" at a certain amplitude of the input signal. If it is less than the threshold value, there will be no pulse signal at the trigger output.

Let's start with experience. Using the Schmitt trigger circuit shown in Fig. 1, a, mount the K155LAZ chip on the breadboard, turning on only two of its logical elements. Here, on the panel, place batteries GB1 and GB2, composed of two galvanic cells 332 (or 316) each, and a variable resistor R1 with a resistance of 1,5 or 2,2 kOhm (preferably with a functional characteristic of A-linear). Connect the battery leads to the resistor only for the duration of the experiments.

Turn on the power of the microcircuit and, using a DC voltmeter, set the variable resistor slider to such a position that there will be zero voltage on the left terminal of the resistor R2, which is the input of the Schmitt trigger. In this case, the element DD1.1. will be in a single state, its output will be a high-level voltage, and the element DD1.2 will be zero. This is the initial state of the elements of this trigger.

Simple frequency counter
Rice. 1 Experience with Schmitt trigger

Now connect the DC voltmeter to the output of the DD1.2 element and, carefully watching its arrow, begin to smoothly move the variable resistor slider up the circuit until it stops, and then, without stopping, in the opposite direction - to the lower output, then again to the upper etc. What does the voltmeter show? Periodic switching of the element DD1.2 from the zero state to the single state and vice versa, i.e., in other words, the appearance of positive polarity pulses at the output of the trigger.

The operation of this version of the Schmitt trigger is illustrated by graphs b and c in the same fig. 1. By moving the variable resistor slider from one extreme position to another, you simulated the supply of a sinusoidal AC voltage to the input of the trigger (Fig. 1, b) with an amplitude of up to 3 V. While the voltage of the positive half-wave of this signal was less than a certain value, which is commonly called the upper threshold (Unop1), the device kept its original state. When this threshold voltage, equal to approximately 1,7 V (at time t1), was reached, both elements switched to the opposite state and a high-level voltage appeared at the trigger output (at the output of element DD1.2). A further increase in the positive voltage at the input does not change this state of the trigger elements.

When moving the slider of the resistor R1 in the opposite direction, when the voltage at the trigger input has dropped to the lower threshold value (Unop2). equal to approximately 0,5 V (time t2), both elements have switched to their original state. At the trigger output, a high voltage level reappeared.

The negative half-wave did not change the state of the elements that form the Schmitt trigger. During this half-cycle, the internal diodes of the input circuit of the DD1.1 element open, closing the trigger input to a common wire.

At the next positive half-wave of the input alternating voltage, a second high-level pulse will form at the trigger output (moments t3 and t4). Repeat this experiment several times, and, according to the readings of the voltmeter connected to the input and output of the trigger, plot graphs characterizing its operation. They should be close to those shown in the graphs in Fig. 20. Two elements of different threshold levels are the most characteristic feature of the Schmitt trigger.

Now let's move on to studying the frequency counter. The schematic diagram of the frequency meter proposed for repetition is shown in fig. 2. Here, the logical elements DD1.1, DD1.2 and resistors R1-R3 form the already familiar Schmitt trigger, and the remaining two elements of the microcircuit form the shaper of its output pulses, the readings of the microammeter RA1 depend on the repetition rate of which. Without a shaper, the device will not give reliable results of measuring the frequency, because the duration of the pulses at the output of the trigger depends on the frequency of the input measured AC voltage.

Simple frequency counter
Rice. 2 Frequency counter

Capacitor C1 is separating. Passing a wide band of sound frequency oscillations, it blocks the path of the constant component of the signal source. The VD2 diode closes the negative half-waves of the input voltage to the common wire (it duplicates the internal diodes at the input of the DD1.1 element, so this diode can not be installed). Diode VD1 limits the amplitude of the positive half-waves received at the inputs of the element DD1.1, at the level of the supply voltage.

From the output of the Schmitt trigger (from the output of the element DD1.2) pulses of positive polarity are fed to the input of the shaper. Element DD1.3 is turned on by the inverter, and DD1.4 is used for its intended purpose - as a logical element 2I-NOT. As soon as a low-level voltage appears at the input of the shaper - at the inputs of the DD1.3 element connected together, it switches to a single state and one of the capacitors C4-C2 is charged through it and the resistor R4. As the capacitor charges, the positive voltage at the lower input of the DD1.4 element rises to a high level. But this element remains in a single state, since at its second input, as well as at the output of the Schmitt trigger, there is a low voltage level. In this mode, a small current flows through the RA1 microammeter.

As soon as a high-level voltage appears at the output of the Schmitt trigger, the DD1.4 element switches to the zero state and a significant current begins to flow through the microammeter, determined by the resistance of one of the resistors R5-R7. At the same time, the element DD1.3 switches to the zero state, and the charged capacitor of the shaper begins to discharge. After some time, the voltage on it will decrease so much that the element DD1.4 will again switch to a single state. Thus, a short low-level pulse appears at the output of the shaper (see Fig. 1, d), during which a current flows through the microammeter that is much larger than the initial one. The deflection angle of the microammeter needle is proportional to the pulse repetition rate: the higher the frequency, the larger the angle.

The duration of the pulses at the output of the shaper is determined by the duration of the discharge of the included time-setting capacitor (C2, C4 or C1.4) to the switching voltage of the element DD2. The smaller the capacitance of the capacitor, the shorter the pulse, the greater the frequency of the input signal can be measured. So, with a time-setting capacitor C0,2 with a capacity of 20 μF, the device is able to measure the oscillation frequency from approximately 200 to 3 Hz, with a capacitor C0,02 with a capacity of 200 μF - from 2000 to 4 Hz, with a capacitor C2000 with a capacity of 2 pF - from 20 to 5 kHz . When adjusting the tuning resistors R7-R1,5, the microammeter pointer is set to the end mark of the scale corresponding to the highest measured frequency of each of the subranges. The minimum level of alternating voltage, the frequency of which can be measured, is about 8 V, and the maximum is 10 ... XNUMX V.

Consider again the graphs in Fig. 1 to memorize the principle of operation of the frequency counter, and then supplement the Schmitt trigger assembled on the breadboard with the details of the input circuit and driver and test the device in action. At this time, a subrange switch is not needed - a time-setting capacitor, for example C2, can be connected directly to terminal 13 of the DD1.4 element, and one of the tuning resistors or a constant resistor with a resistance of 2,2 ... 3,3 kOhm can be connected to the microammeter circuit. Microammeter RA1 - for the current of the total deflection of the arrow 100 μA.

After completing the installation, turn on the power source and apply high-level pulses to the input of the Schmitt trigger element DD1.1. Their source can be a multivibrator according to the circuit in Fig. 10 or other similar generator. Set the pulse repetition rate to the minimum. In this case, the pointer of the PA1 microammeter should deviate sharply by a small angle, which will indicate the efficiency of the frequency meter. If the microammeter does not respond to input pulses, you will have to choose another resistor R2 of greater resistance. In general, its resistance can be in the range from 1,8 to 5,1 kOhm.

Next, apply to the input of the frequency meter (through capacitor C1) an alternating voltage of 3.. .5 V from a step-down network transformer. Now the needle of the microammeter should deviate by a larger angle than in the previous experiment. Connect another capacitor of the same or greater capacity in parallel with the timing capacitor. Now the angle of deviation of the arrow will decrease. In the same way, you can test the device on the second and third measurement subranges, but with input signals of the appropriate frequency. If you decide to include this frequency meter in your home measuring laboratory, its parts must be transferred from the breadboard to the circuit board and trimming resistors R5-R7 should be mounted on it (Fig. 22), and the board should be fixed in a box of suitable sizes. Capacitors C2-C4 can be composed of two or more capacitors each.

The appearance of the design of the frequency meter is shown in fig. 3. On its front panel, place a microammeter, a sub-range switch (for example, a biscuit PZZN or another with two sections for three positions), input sockets (XS1, XS2) or clamps.

The frequency meter scale is common for all measurement sub-ranges and is almost uniform. Therefore, it is only necessary to determine

the initial and final limits of the scale in relation to one of them - to the sub-range "20.. .200 Hz", and then adjust the frequency limits of the other two measurement sub-ranges under it. In the future, when switching the device to the "200.. .2000 Hz" subrange, the measurement result read on the scale will be multiplied by 10, and when measuring in the "2.. .20 kHz" subrange, by 100.

Simple frequency counter
Rice. (3)23 Mounting parts of the frequency counter. Rice. 4(24) Amplifier that increases the power of the frequency meter

This is the grading method. Set the SA1 switch to the measurement position in the sub-range "20 .. .200 Hz", the trimmer resistor R5 engine to the position of the highest resistance and apply a signal with a frequency of 33 Hz with a voltage of 20 .. .1,5 B. Make a mark on the scale corresponding to the angle of deflection of the microammeter pointer. Then tune the sound generator to a frequency of 2 Hz and set the instrument's pointer to the final mark of the scale with a trimming resistor R200. After that, according to the signals of the sound generator, make marks on the scale corresponding to 5, 30, 40, etc. up to 50 Hz. Later, divide these sections of the scale into 190, 2 or 5 parts.

Then switch the frequency meter to the second measurement sub-range, apply a signal with a frequency of 200 Hz to its input. In this case, the arrow of the microammeter should be set against the scale mark corresponding to the frequency of 20 Hz of the first subrange. More precisely, you can set it to this initial scale mark by selecting capacitor C3 or connecting a second (third, etc.) capacitor in parallel to it, which somewhat increases their total capacitance.

After that, apply a signal with a frequency of 200 Hz from the generator to the input of the device and set the microammeter needle to the end mark of the scale with a trimmer resistor R6. Similarly, adjust the limits of the third subrange of the measured frequency to the scale of the microammeter - 2.. .20 kHz. Perhaps the frequency measurement limits on the subbands will turn out different or you want to change them. Do this with a selection of timing capacitors C2-C4.

It is possible that you would like to increase the sensitivity of the frequency counter. In this case, the simplest frequency meter will have to be supplemented with an input signal amplifier, using, for example, a low-power n-p-n transistor or, even better, an analog K118UP1G microcircuit (Fig. 4). This microcircuit is a three-stage amplifier for video channels of television receivers, which has a large gain. Its 14-pin case is the same as that of the K155LAZ microcircuit, but it has the 7th positive power output and the 14th negative power output. With such an amplifier, the sensitivity of the frequency meter will increase to 30 ... 50 mV.

Oscillations of the measured frequency can be sinusoidal, rectangular, sawtooth - any. Through capacitor C1, they enter the input (pin 3) of the DA1 microcircuit and, after amplification from the output (pin 10 connected to pin 9), the microcircuit through capacitor C3 comes to the input of the Schmitt trigger of the frequency meter. Capacitor C2 eliminates internal negative feedback, which weakens the amplifying properties of the microcircuit.

Diodes VD1, VD2 and resistor R1 (Fig. 2) can now be removed, and in their place an analog chip DA1 and oxide capacitors can be mounted. The K118UP1G chip can be replaced with K118UP1V or K118UP1A. But in this case, the sensitivity of the frequency meter will be somewhat less.

See other articles Section Beginner radio amateur.

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