Refinement of a logical probe. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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According to the author, the design of the logical probe described in the article by B. and P. Semenovs ("Radio", 1996, No. 12, p. 34) can be simplified, while fully preserving all its capabilities. For example, it uses the K1401UD2B chip, which contains four op-amps, but only two of them are used. Meanwhile, the remaining free op amps make it possible to implement the specified functions performed in the original device by discrete diodes and a transistor.

On fig. 1 shows a schematic diagram of a modified logic probe.

The two-threshold comparator on the op-amp DA1.1 and DA1.2 is used practically unchanged, only the resistance of the divider resistors R1R2R3R5. and also R4 is chosen large. Thanks to this, the input impedance of the probe was increased from 50 to 110 kOhm.

A few words about the work of the probe. If the voltage at its input is less than 0,8 V, a high-level voltage operates at the output of the op-amp DA1.2, while segments a, d, b and f of the HG1 indicator are lit, displaying the left side of the "0" sign. When the input voltage exceeds 2,3 V, a high level will appear at the output of the op-amp DA1 1 and the point s will light up on the indicator.

Through the adder R6R7, the signals from both outputs of the comparator are fed to the repeater DA1.3. Therefore, if the voltage at the input of the probe is less than 0,8 V or more than 2,3 V, a voltage of about 1.3 V acts at the output of DA2,2, which is used to light the number "1". In this case, if the voltage at the input of the probe is low, the luminous segments b and c complete the zero image.

It is undesirable to send a signal from the DA1.3 output directly to the indicator: the brightness of the segments will be different. But if the voltage is raised to the saturation voltage of the op-amp, the need to select limiting resistors R8 - R10 will disappear and the brightness of the point of the left and right parts of the "0" sign will be the same.

You can increase the output voltage by replacing the follower with an amplifier with a gain of 1.5 ... 3, but this will require two additional resistors in the feedback circuit.

The problem is solved differently. A comparator is assembled on the op-amp DA1.4, which compares the voltage at the output of the DA1.3 repeater with the voltage taken from the resistor R3 of the divider. When the voltage at the output DA1.3 exceeds the threshold level, the comparator DA1.4 will go into a single state and the indicator will show "1". Now, if a low level signal is applied to the input, the voltage at the output of comparators DA1.2 and DA1.4 will be the same.

When a pulse signal with a frequency above 20...30 Hz acts at the probe input, the flickering of the indicator will become imperceptible, and it will display a zero and a point. The ratio of the brightness of the glow of the left and right parts of the symbol "0" will depend on the duty cycle of the input pulses.

The device is assembled on a printed circuit board, a sketch of which is shown in Fig. 2a. The arrangement of elements and jumpers is shown in fig. 2, b. It is permissible to use the OU K1401UD2A in the design, the digital indicator can be replaced by other similar ones.

After installation, the device does not require adjustment, but if necessary, it is permissible to rebuild the comparator thresholds. In this case, it should be borne in mind that the voltage at pins 3 and 6 of the DA1 microcircuit should be no more than 2 V at any voltage at the probe input (this corresponds to the Umn-3 V limit). The current consumed by the device (at IP "t -5 V) does not exceed 20 mA.

The supply voltage can be increased to 9 ... 15 V, this will make it possible to establish devices on CMOS logic chips. The resistance of resistors P8 - R10 must also be increased by 2 ... 3 times.

Repeater DA1.3 can be excluded by applying a signal from the resistors R6uR7 directly to input 5 DA1.4.

Author: A. Shitov, Ivanovo

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