Small-sized oscilloscope-probe. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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Often in the laboratory of a radio amateur there is no such necessary device as an oscilloscope. The reasons can be different - from dimensions disproportionate to the available workplace space to the high cost of such a device. Do not despair - in this article you will find recommendations for the manufacture of a very simple and inexpensive miniature device that, although it will not fully replace an oscilloscope, will to some extent facilitate the visualization of processes in electrical circuits.

We offer a simple small-sized device that can be used in the development of various crafts for the home, in car repairs, in enterprises with large magnetic fields, where the use of classic oscilloscopes is simply impossible. In it, the displayed signal is displayed on a dot matrix LED.

The schematic diagram of the device is shown in the figure. It consists of an input amplifier assembled on a transistor VT1 and an operational amplifier DA1, an ADC consisting of a chain of resistors R10-R18 and seven XOR elements DD1.1-DD1.4 and DD2.1-DD2.3, a row switch assembled on transistors VT2-VT8, a sweep generator assembled on an element D2.4 and a transistor VT9, a digit switch DD3, a synchronization unit made on diodes VD2-VD4, and a synchronization capture indicator HL1.

(click to enlarge)

The generated input signal from the output of the operational amplifier DA1 is fed to a chain of resistors and, depending on the amplitude of the input signal, causes the inclusion of one of the elements D1.1-D1.4, D2.1-D2.3, which, in turn, will open one of the keys "lines", thereby connecting one of the "lines" through resistors R26, R27 with a common wire. By changing the resistance of the resistor R26, the operating current of the switched-on LED is selected and thereby the brightness of its glow is changed. Thus, we have deployed the input signal vertically.

Horizontal sweep. The sweep generator signal from the output of the transistor VT9 is fed to the counting input CP of the DD3 microcircuit. Switch DD3 alternately sets the log level. 1 on one of the outputs 0-9 of this chip, supplying power to the anode of one of the LEDs, in the selected row and in the selected column. Thus, at a certain point in time, only one of the LEDs of the HL2-HL64 matrix is ​​lit. By changing the bias voltage at the inverting input of the operational amplifier DA1 resistor R7, you can shift the luminous point ("beam") up or down.

The operation of the synchronization node. When switch SA5 is turned on in the upper position according to the scheme "Synchronization" - "Waiting" for the pulse of discharges, having reached output 9 of the DD3 microcircuit, the sweep generator will be disabled through the VD2 diode, the DD3 switch will remain in state 9. This state is maintained until the signal the outputs of the microcircuits DD1, DD2 through the SA3 switch will not reset the DD3 counter to the log state. 0 and enable the sweep generator, thereby synchronizing it with the input signal.

Technical specifications

• Number of pseudo-graphic field positions (horizontal-vertical)......7x9
• Sensitivity, V / pos ...... 0,1 ... 1 and 1..70
• Sweep rate (8 subranges), ms/pos......0,01...33
• Synchronization ...... on the front and fall of the pulse
• Input resistance within sensitivity limits 0,1V, kOhm......300
• 1 V, MΩ......3
• Supply voltage, V ...... 12
• Consumption current, mA, no more ...... 20

input shaper. The gain of the operational amplifier DA1 is chosen so that when a voltage of 100 mV is applied, the beam in the column shifts by one line.

Analog to digital converter. It is known that the switching threshold of digital microcircuits is approximately Upit/2. The K176LP2 microcircuit has such a feature that it is not necessary to apply a log level to the inputs to switch from one state to another. 1 or log. 0 is enough for the difference between the inputs to reach several tens of millivolts. That is, if at Upit = 10 V a voltage of 5,05 V is applied to one of the inputs, and 4,95 V to the other, the element will "understand" this as a log. 1 on one input and log. 0 on the other. Chips of the K561 series do not have this property, so they will not work in this device! Based on this property, the operation of the ADC is built. When a voltage of +5 V is applied to the connection point of resistors R13 and R14, the inputs (pins 1, 2, 5, 6, 8, 9) of elements D1.1 -D1.3 will be log. 1, at the inputs of the elements D2.1 -D2.3 - log 0, at pin 12 of the input of the element D1.4 - log. 1, and at pin 13 of the input element D1.4 - log. 0. Therefore, at the output of the element D1.4 - the state of the log. 1, which opens the VT5 "string" key. If the voltage at the ADC input drops, the next lower element in the circuit will switch, if it rises, the next upper element in the circuit will switch.

Setting. It is highly desirable to choose microcircuits DD1 and DD2 from the same batch, or rather, choose resistors R10-R17 and capacitors C2-C7. With synchronization off (SA5 in the lower position according to the diagram), check the operability of the sweep generator on all ranges (collector VT9), check the cyclic appearance of the log. 1 on each of the outputs of the switch "digits" DD3. An indication of the operation of the "bits" switch can be the blinking of the HL1 LED. Set the resistor R7 to such a position that when a voltage of 100 mV is applied to the input of the device, line 1 lights up on the display, when a voltage of 200 mV is applied, line 2 and so on.

Structurally, the device is assembled on a single printed circuit board. Switches SA3, SA4 - self-made, made by printed wiring for reasons of height reduction, the rest of the switches - from imported equipment of suitable sizes, variable resistances - imported, for printed wiring.

The device is assembled in a case with dimensions of 120x80x30 mm. For these purposes, you can use the case from a pocket receiver.

When developing this device, Roman Krause's recommendations were taken into account in his publication of a description of a similar device ("Digital Oscilloscope". - Praktyczny Eektronik, 2001, No. 4, pp. 4 - 8). At the named author, the design was made using a specialized IC and a linear LED matrix.

Authors: B. Makeenko, A. Zhebrikov, Sayanogorsk, Khakassia

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