The proposed device belongs, rather, to the category of oscilloscope-graphic probes. Its capabilities allow only "by eye" to evaluate the shape and parameters of low-frequency signals. Nevertheless, due to its small size and cost-effectiveness, such an oscilloscope can be used in amateur radio practice, especially when diagnosing and repairing equipment in the field.

This development is based on a small-sized two-beam oscilloscope-multimeter, described in. Only one "beam" is left in it. The maximum sensitivity of the vertical deviation channel has been increased from 640 to 100 mV (full screen). The minimum sweep duration has been reduced from 5 to 3 ms, and when observing logic signals - up to 300 µs. Significantly reduced the dimensions of the device, its weight and current consumption.

Main technical characteristics

The oscilloscope circuit is shown in fig. 1. The investigated signal of arbitrary shape, depending on its amplitude, is fed to "Input 1" - one of the sockets 1-5, 7, 8 of connector X1, and the common wire of the source of the signal under study is connected to its socket 6. Resistors R1-R6, which set the sensitivity of the vertical deflection channel of the oscilloscope, are mounted directly on the pins of the connector sockets. Through the amplifier on the K140UD608 (DA1) op-amp, the signal is fed to pin 2 (RA0) of the microcontroller (DD1), which serves as the input of the ADC built into it. Digital counts of instantaneous signal values ​​for the time corresponding to the selected sweep duration are stored in the microcontroller's RAM and displayed on the HG1 graphic LCD in the form of an oscillogram. An LCD is used, which is controlled via the lines of the ports RB0-RB4 and RC0-RC7 of the microcontroller. When developing software, the recommendations from the article turned out to be very useful.

Variable resistor R10 is designed to shift the waveform vertically. Resistor R17 is selected to achieve the best image contrast on the indicator screen.

The oscilloscope sweep is one-shot, triggered each time the SB2 button is pressed. The duration of the sweep is changed by pressing the SB1 button. After each click, the indicator screen displays a number — the value of the selected duration.

If the sweep duration is set to 300 µs (full screen), the ADC of the microcontroller no longer has time to digitize the samples of the signal under study. At such a speed, the indicator can only observe the nature of the change in time of the logical levels of pulses applied to slot 9 of connector X1 ("Input 2" of the oscilloscope). Through the separating capacitor C1, these pulses are fed directly to the discrete input RA1 (pin 3) of the microcontroller.

The oscilloscope is assembled by surface mounting on a board (Fig. 2), placed in a case made from a fishing tackle box. The HG1 indicator is located on the housing cover. Appearance the operating device is shown in fig. 3. The third button visible in the photographs is left unconnected. It is not used with the device.

Assembly source code and firmware for the PIC16F873A microcontroller are available at .

Literature:

1. Kichigin A. Small-sized two-beam oscilloscope-multimeter. - Radio, 2004, No. 6, p. 24-26.
2. Liquid crystal module MT-12864J. - .
3. Milevsky A. Using a graphic LCD MT-12864A with a Microchip microcontroller. - Radio, 2009, No. 6, p. 28-31.

The oscilloscope is based on an ATmega32 microcontroller. Indicator graphic LCD 128 x 64 dots. The scheme of this device is very simple. One of the disadvantages of this oscilloscope is the low maximum frequency of the measured signal, for a square wave it is only 5 kHz. The program is written in C in WinAVR, in conjunction with AVRStudio 4. The graphics library was written specifically for this project.

Description:

The supply voltage of the circuit is 12 volts. From this converter output voltage we get +8.2V for IC1 and +5V for IC2 for IC3. This circuit has an input range of -2.5 volts to +2.5 volts or 0 to +5 volts depending on the position of S1(AC/DC). Using a divider, you can expand the range of measured voltages. The display contrast is adjusted using potentiometer P2. The maximum input voltage is 30 volts for DC and 24 volts for AC.

You can see a more advanced oscilloscope circuit based on the ATxmega128A3 microcontroller.

Scheme:

microcontroller firmware:

Firmware file AVR_oscilloscope.hex , when flashing, set Fuse bits of the microcontroller for clocking from an external crystal. Be sure to disable the JTAG interface.

It is difficult for any radio amateur to imagine his laboratory without such an important measuring instrument as an oscilloscope. And, indeed, without a special tool that allows you to analyze and measure the signals acting in the circuit, the repair of most modern electronic devices is impossible.

On the other hand, the cost of these devices often exceeds the budgetary possibilities of an ordinary consumer, which forces him to seek alternatives or make an oscilloscope with your own hands.

Solutions to the problem

You can refuse to buy expensive electronic products in the following cases:

• Use for these purposes built into a PC or laptop sound card(ZK);
• Making a USB oscilloscope with your own hands;
• Refinement of a regular tablet.

Each of the options listed above, which allow you to make an oscilloscope with your own hands, is not always applicable. For full-fledged work with self-assembled set-top boxes and modules, the following prerequisites must be met:

• The admissibility of certain restrictions on the measured signals (by their frequency, for example);
• Experience with complex electronic circuits;
• The ability to refine the tablet.

So, an oscilloscope from a sound card, in particular, does not allow measuring oscillatory processes with frequencies that are outside its operating range (20 Hz-20 kHz). And for the manufacture of a USB set-top box to a PC, you will need some experience in assembling and configuring complex electronic devices (as well as when connecting to a regular tablet).

Note! The option in which it is possible to make an oscilloscope from a laptop or tablet with the simplest approach comes down to the first case, which involves the use of a built-in ZK.

Consider how each of the above methods is implemented in practice.

Using ZK

To implement this method of obtaining an image, it will be necessary to make a small-sized prefix, consisting of only a few electronic components available for each. Its diagram can be found in the picture below.

The main purpose of such an electronic circuit is to ensure the safe supply of an external signal under study to the input of the built-in sound card, which has its own analog-to-digital converter (ADC). The semiconductor diodes used in it guarantee the limitation of the signal amplitude at a level of no more than 2 Volts, and the divider of resistors connected in series allows applying voltages with large amplitude values ​​to the input.

A wire with a 3.5 mm plug at the opposite end is soldered to the board with resistors and diodes from the output side, which is inserted into the ZK socket under the name "Line Input". The signal under test is applied to the input terminals.

Important! The length of the connecting cord should be as short as possible to ensure minimal signal distortion at very low measurable levels. As such a connector, it is recommended to use a two-wire wire in a copper braid (screen).

Although the frequencies passed by such a limiter are in the low frequency range, this precaution improves the quality of the transmission.

Program for obtaining oscillograms

In addition to technical equipment, before starting measurements, you should prepare the appropriate software(software). This means that you need to install one of the utilities designed specifically for obtaining an image of an oscillogram on your PC.

Thus, in just an hour or a little more, it is possible to create conditions for the study and analysis of electrical signals using a stationary PC (laptop).

Tablet refinement

Using the built-in map

In order to adapt a regular tablet for taking oscillograms, you can use the previously described method of connecting to an audio interface. In this case, certain difficulties are possible, since the tablet does not have a discrete line input for a microphone.

This problem can be solved in the following way:

• You need to take a headset from the phone, which should have a built-in microphone;
• Then you should clarify the wiring (pinout) of the input terminals on the tablet used for connection and compare it with the corresponding contacts on the headset plug;
• If they match, you can safely connect the signal source instead of the microphone, using the previously discussed prefix on diodes and resistors;
• In the end, it remains to install a special program on the tablet that can analyze the signal at the microphone input and display its graph on the screen.

Advantages this method connecting to a computer is easy to implement and cheap. Its disadvantages include a small range of measured frequencies, as well as the lack of a 100% security guarantee for the tablet.

It is possible to overcome these shortcomings through the use of special electronic set-top boxes connected via a Bluetooth module or via a Wi-Fi channel.

Homemade prefix to the Bluetooth module

Connection via Bluetooth is carried out using a separate gadget, which is a set-top box with an ADC microcontroller built into it. Due to the use of an independent information processing channel, it is possible to expand the bandwidth of transmitted frequencies up to 1 MHz; while the input signal can reach 10 volts.

Additional Information. The range of such a self-made prefix can reach 10 meters.

However, not everyone is able to assemble such a converting device at home, which significantly limits the circle of users. For all those who are not ready to independently manufacture the set-top box, it is possible to purchase a finished product, which has been available for free sale since 2010.

The above characteristics can suit a home master engaged in the repair of not very complex low-frequency equipment. More complex repairs may require professional converters with up to 100 MHz bandwidth. These opportunities can be provided by a Wi-Fi channel, since the data exchange protocol speeds in this case are incomparably higher than in Bluetooth.

Wi-Fi oscilloscopes

The option of transmitting digital data using this protocol significantly expands the bandwidth of the measuring device. Working on this principle and freely sold consoles are not inferior in their characteristics to some samples of classic oscilloscopes. However, their cost is also far from being acceptable for users with average incomes.

In conclusion, we note that, taking into account the above limitations, the Wi-Fi connection option is also suitable only for a limited circle of users. For those who decide to abandon this method, we advise you to try to assemble a digital oscilloscope that provides the same characteristics, but by connecting to a USB input.

This option is also very difficult to implement, so for those who are not completely confident in their abilities, it would be wiser to purchase a ready-made USB set-top box available for sale.

Video

This simple and cheap USB oscilloscope was designed and made just for fun. A long time ago, I had a chance to repair some kind of muddy video processor, in which the input was burned up to the ADC. ADCs turned out to be affordable and inexpensive, I bought a couple just in case, one went to replace it, and the other remained. Recently, he caught my eye and after reading the documentation for it, I decided to use it for something useful in the household. As a result, we got such a device. It cost a penny (well, about 1000 rubles), and a couple of days off. When creating, I tried to reduce the number of parts to a minimum, while maintaining the minimum functionality necessary for an oscilloscope. At first I decided that it turned out to be some kind of painfully frivolous device, however, now I constantly use it, because it turned out to be very convenient - it does not take up space on the table, it easily fits in my pocket (it is the size of a pack of cigarettes) and has quite decent characteristics:

Maximum sampling rate - 6 MHz;
- Input amplifier bandwidth - 0-16 MHz;
- Input divider - from 0.01 V/div to 10 V/div;
- Input resistance - 1 MOhm;
- Resolution - 8 bits.

circuit diagram oscilloscope is shown in Figure 1.

Fig.1 Schematic diagram of the oscilloscope

For various settings and troubleshooting in any power converters, household appliances control circuits, for studying all kinds of devices, etc., where accurate measurements and high frequencies are not required, but you just need to look at the waveform with a frequency of, say, up to a couple of megahertz - more than enough.

The S2 button is part of the hardware needed for the bootloader. If you keep it pressed when connecting the oscilloscope to USB, then the PIC will work in bootloader mode and you can update the oscilloscope firmware using the appropriate utility. As an ADC (IC3), a "television" microcircuit, TDA8708A, was used. It is quite available in all sorts of "Chip and Dip" ah and other places for obtaining parts. In fact, this is not only an ADC for a video signal, but also an input switcher, an equalizer and a white-black level limiter, etc. But all these charms are not used in this design. The ADC is very fast - the sampling rate is 30 MHz. In the circuit, it operates at a clock frequency of 12 MHz - no need for faster, because the PIC18F2550 simply cannot read data faster. And the higher the frequency - the greater the consumption of the ADC. Instead of the TDA8708A, you can use any other fast ADC with parallel data output, such as the TDA8703 or something from Analog Devices.

The clock frequency for the ADC was cunningly extracted from PIC "a - PWM was launched there with a frequency of 12 MHz and a duty cycle of 0.25. The positive polarity clock pulse passes in the Q1 PIC cycle", so that any access to port B that occurs in the Q2 cycle data ADC will be ready. The PIC core operates at a frequency of 48 MHz, received through a PLL from a 4 MHz quartz. The copy command from register to register takes 2 cycles or 8 cycles. Thus, ADC data can be stored in memory at a maximum frequency of 6 MHz using a continuous sequence MOVFF PORTB, POSTINC0 commands One PIC18F2550 RAM bank of 256 bytes is used for the data buffer.

Lower sampling rates are implemented by adding a delay between MOVFF instructions. The firmware implements the simplest synchronization on the negative or positive edge of the input signal. The cycle of collecting data into the buffer is started by a command from the PC via USB, after which this data can be read via USB. As a result, the PC receives 256 8-bit samples, which can, for example, be displayed as an image. The input circuit is simple to disgrace. The input voltage divider without any frills is made on a rotary switch. Unfortunately, it was not possible to figure out how to transfer the switch position to the PIC, therefore, in the graphic face of the oscilloscope, there are only voltage values ​​​​in relative units - scale divisions. The input signal amplifier (IC2B) operates at a gain of 10 times, the zero offset required for the ADC (it accepts a signal in the range from Vcc - 2.41V to Vcc - 1.41V) is provided by a voltage from the PIC programmable voltage reference generator (CVREF IC1, R7, R9) and a divider from the negative supply voltage (R6, R10, R8). Because there was an "extra" amplifier (IC2A) in the op-amp package, I used it as a bias voltage follower.

Do not forget about capacitive circuits for frequency compensation of the input capacitance of your op-amp and limiting diodes that are not on the circuit - you need to select capacitances in parallel with the divider resistors and resistor R1, otherwise the frequency response of the input circuit will ruin the entire bandwidth. With direct current, everything is simple - the input resistance of the op-amp and closed diodes is orders of magnitude higher than the resistance of the divider, so the divider can simply be calculated without taking into account the input resistance of the op-amp. For alternating current, it’s different - the input capacitance of the op-amp and diodes is a significant value compared to the capacitance of the divider. From the resistance of the divider and the input capacitance of the op-amp and diodes, a passive low-pass filter is obtained, which distorts the input signal.

To neutralize this effect, you need to make sure that the input capacitance of the op-amp and diodes becomes much less than the capacitance of the divider. This can be done by building a capacitive divider parallel to the resistive one. It is difficult to calculate such a divisor, because both the input capacitance of the circuit and the mounting capacitance are unknown. It's easier to pick it up.

The selection method is:
1. Put a capacitor with a capacity of approximately 1000 pF in parallel with R18.
2. Select the most sensitive limit, apply rectangular pulses with a frequency of 1 kHz and a span of several divisions of the scale to the input and select a capacitor parallel to R1 so that the rectangles on the screen look like rectangles, without peaks or blockages on the fronts.
3. Repeat the operation for each next limit, selecting capacitors in parallel with each divider resistor according to the limit.
4. Repeat the process from the beginning, and make sure that everything is in order at all limits (the capacitance of mounting capacitors may appear), and, if something is wrong, slightly correct the capacitances.

The op-amp itself is an Analog Devices AD823. The most expensive part of an oscilloscope. :) But on the other hand, the band is 16 MHz - which is quite good. And besides, this is the first of the smart ones that came across in retail for sane money.

Of course, this dual op amp can be changed to something like the LM2904 without any modifications, but then you will have to limit yourself to audio range signals. It will not pull more than 20-30 kHz.

Well, the shape of rectangular, for example, signals will be slightly distorted. But if you manage to find something like OPA2350 (38 MHz), then it will be wonderful on the contrary.

The source of the negative supply voltage for the op-amp is based on the well-known charge-pump ICL7660. Minimum binding and no inductance. The output current of -5 V, of course, is small, but we don’t need much. The power circuits of the analog part are isolated from digital interference by inductances and capacitances (L2, L3, C5, C6). The inductances came across with a nominal value of 180 uH, so I installed them. No power interference even at the most sensitive limit. The PIC firmware is loaded via USB using a bootloader that sits from the 0th address in the program memory and starts if you hold down the S2 button when turned on. So before flashing the PIC - fill in the bootloader there first - it will be easier to change the firmware.
The sources of the oscilloscope driver for kernels 2.6.X are in the archive with the firmware. There is also a console utility for checking the performance of the oscilloscope. Its source code is worth looking at to figure out how to communicate with an oscilloscope if you want to write your own software for it.
The computer program is simple and ascetic, its appearance is shown in Figures 2 and 3. Connect the oscilloscope to USB and start qoscilloscope. Requires QT4.