Probe-oscilloscope. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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When repairing and setting up equipment somewhere far from a well-equipped laboratory, the "smaller brothers" of measuring instruments - all kinds of probes, indicators and probes - become indispensable assistants to a radio amateur. Surely, in the arsenal of any specialist there is at least one, or even several such devices. But not everyone can boast of having a small-sized oscilloscope. The imported LCD-based oscilloscopes that have recently appeared are affordable for a few at their price. And I would like to have a small-sized device that allows you to visually control the signal and at least roughly determine its shape, frequency and amplitude!

As a result of numerous experiments, a small-sized oscilloscope probe was born. Its "advantages" include small size, self-contained power supply, low power consumption, a convenient shape that allows you to adjust and repair equipment without taking your eyes off the probe. Unfortunately, due to the microcircuits used, which are not very fast, the probe turned out to be low-frequency, but it can find a lot of applications. For example, the probe has been tested in the repair and tuning of televisions, watches, tape recorders and other analog and digital devices. Like a real oscilloscope, the probe allows you to visually control the input signal and roughly determine its parameters, and also serves as an indicator of the phase wire of the network. The probe diagram is shown in Fig.1.

Fig.1. Schematic diagram of the probe (click to enlarge)

Its basis is the ALS340 LED matrix, which contains 35 LEDs - 7 rows of 5 columns.

The clock pulse generator (horizontal scan) is assembled on the elements DD1.1...DD1.3. The switch SA1 selects the required frequency range, and the resistor R3 synchronizes the signal. From the generator, the pulses are fed to the counter-decoder DD2, the outputs of which control the operation of the transistor switches. The keys alternately go through the rows of the matrix, due to which a horizontal scan is formed. Although the resolution of the matrix is ​​​​low, it is still capable of displaying a sine wave, rectangular pulses, sawtooth and other periodic signals. A signal that is near the synchronization threshold looks especially effective and "readable". Then it moves to one of the sides, which in many cases is preferable to a complete stop.

The vertical beam deflection device consists of a capacitor C1, a switch SA2 that allows you to control a constant or alternating voltage, a resistive divider R1-R4-R5, a switch SA3 that selects the required input signal range, four comparators of the DA1 chip and matching elements DD1.4, DD3. Diodes VD1, VD2 protect the comparator inputs from overloads. Resistors R6 ... R11 set the threshold voltages on the comparators, on the one hand, and on the other hand, they create a "virtual ground" necessary for the normal operation of the DA1 chip.

When there is no input signal, all comparators are turned off, and therefore the element DD1.4 is active in the matcher. In this case, the LEDs of the middle column emit, forming a zero scan line. When an input signal of positive polarity appears, comparators DA1.1, DA1.2 alternately fire, and negative - DA1.3, DA1.4. The logic of the matcher is chosen such that in case of operation of all comparators, only the outermost LEDs are lit. This made it possible to achieve an image on the screen from a chain of luminous dots that objectively convey information about the shape of the signal under study. Resistors R12...R16 - current-limiting for the LED matrix. By reducing their resistance, you can increase the brightness, but this will entail an increase in the power consumption of the probe. Elements VT8, VT9, VD3 form a voltage regulator.

In the author's version, the probe is assembled in a housing from the "Sign Logic Indicator", previously produced by our industry. Its appearance is shown in Fig.2. In it, instead of the "native" ALS324 indicator, the ALS340 matrix is ​​​​installed. Installation of parts - mixed (printed-hinged), due to its high density. The layout and dimensions of the probe are mainly determined by the switches and variable resistors used. As a case, you can use any plastic box, for example, a toothbrush case, fountain pens, pencil case. The LED matrix is ​​installed in the lower part of the housing near the metal needle-probe, the adjustment organs are also displayed there. The installation was carried out with a thin MGTF wire.

Fig.2. Mounting diagram

Power source - battery 6F22 or "Krona". Microswitches PD3-4 are used as SA9 and SA2, SA1 together with SA2 - a block of switches from an imported network adapter. Variable resistors R3, R5 - player volume controls. These elements may be different, the main thing is that they be small. Instead of ALS340, you can install AL306A, B, Zh, I. Zener diode VD3 - in a glass case. Capacitor C1 - K73-9, C2 ... C7 - ceramic, small-sized. All K561 series microcircuits are replaced by K176. Instead of K561IE8, you can use K561IE9 (taking into account the differences in the pinout). The quad OU K1401UD2 can be replaced by two dual K157UD2 by installing them on top of each other.

The probe, assembled from known good parts, starts working immediately. You may have to choose the value of R2 - for a slight overlap of adjacent ranges, and R8, R11 - for uniform operation of the positive and negative signal comparators. Working with a probe is practically no different from working with an ordinary oscilloscope.

Literature

1. Goroshkov B.I. Elements of radio electronic devices. - MRB, N1125.
2. Gutnikov VS Integrated electronics in measuring devices.
3. Parol N.V., Kaidalov S.A. Sign-synthesizing indicators and their application. - MRB, N1122.
4. Shilo V.L. Popular CMOS Chips: A Handbook.

Author: V.Rubashka, Lisichansk; Publication: cxem.net

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