Cell phone - voltmeter and oscilloscope. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Cell phone - voltmeter and oscilloscope. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The article describes a set-top box for a Siemens cell phone, which allows you to see on its screen an oscillogram of a signal fed to the set-top box input with respect to the scale along the time and voltage axes. Other cellular phones that have a serial port and support Java version MIDP 2.0 can be used in a similar way.

The cell phone has become a familiar item in everyday life, and many do not suspect that this is a fairly powerful computer (some phones have processor clock speeds exceeding 100 MHz) with a color screen, keyboard, and good sound capabilities. Many phones have a serial port that is programmatically accessible from Java applications (Midlets) with support for Java (J2ME platform) and MIDP 2.O. Through this port, you can interact with various external devices, greatly expanding the standard set of functions of a cell phone. Among Siemens products, the MIDP 2.0 specification is supported by cell phones of the 65, 75 series (for example, M65, S75).

Cell phone - voltmeter and oscilloscope

The proposed attachment turns a cell phone into a kind of oscilloscope with an input impedance of 1 MΩ, a sweep rate of 0,001 ... 1 s / div and a sensitivity of 0,5 ... 50 V / div. The average value of the input signal (its constant component) is displayed in digital form

The set-top box, the circuit of which is shown in the figure, is controlled by the PIC16F688 (DD2) microcontroller, which includes ADC and serial port controller blocks. Unfortunately, the built-in ADC is rather slow, but for a low-frequency oscilloscope, its speed (the maximum sampling rate is tens of kilohertz) is quite sufficient.

Cell phone - voltmeter and oscilloscope

Unlike the signal levels adopted in the RS232 standard, the serial port of a cell phone is characterized by the usual levels for logic circuits: log. 0 - about 0 V, log. 1 - at least 3,6 V. This simplifies the pairing of the phone with the MK, allowing you to connect them directly. The information exchange rate is chosen to be 9600 baud. With a larger value, some models and instances of phones work unstably. The HL1 LED lights up when a packet is being transmitted from the microcontroller to the phone.

The set-top box is powered by the phone. Since pins 5-7 of the XS1 connector connected to the phone's system connector are connected to a common wire, the phone controller considers that a DCA-500 data cable is connected to it and supplies 1 V voltage from its battery to pin 3,6 of this connector. The negative voltage for powering the op-amp of the set-top box was obtained using the DA3 polarity converter. On a parallel voltage regulator DA1 and op-amp DA2.2, connected according to the repeater circuit, a reference voltage source of 2,5 V is made.

An electronic attenuator is assembled on the DD1 multiplexer and DA2.1 op amp, which allows you to change the sensitivity of the device depending on the code that the MK sets at the address inputs of the multiplexer (pins 9 and 10 of DD1). The multiplexer switches resistors R1-R3 in the feedback circuit of the op-amp, the resistance of which must correspond with high accuracy to that indicated in the diagram. With code 00 at the address inputs of the multiplexer, the signal applied to the XW1 connector is transmitted to the output of the op-amp DA2.1 without change. For other values of the code, the input signal is attenuated by a factor of 10, 100, or 1000 times. The last value is not used due to the insufficient dielectric strength of the resistor R4 and capacitor C1. Diodes VD1-VD4 limit the voltage at terminal 13 of the multiplexer at the level of 1,2 V (in absolute value). The cascade on the op-amp DA2.3 shifts the level of the signals arriving at the AN1 input of the MK so that the middle of the ADC scale corresponds to zero voltage at the XW1 connector.

The operation of the oscilloscope is controlled by a midlet (Java program) loaded into a cell phone in the form of a jar file. This MIDlet is responsible for controlling the operating mode, changing the scale along the time and voltage axes, and displaying information coming from the set-top box. The prefix is controlled by transferring control bytes to it. The three least significant bits of the byte contain the code that sets the ADC start frequency, and the two most significant bits contain the position of the electronic attenuator. The rest of the bits are not used. In case of receiving a control byte equal to zero, the set-top box operates in the "idle" mode, without transmitting information to the phone.

Working with a serial port in J2ME is organized through the CommConnection interface, and the port itself has the name COM0. Before sending and receiving information, you must use the Connection.open method to establish a connection. To avoid blocking the application during the exchange of information, all reads from and writes to the port are moved to a separate thread. For more information about working with the serial port of a cell phone, see the Internet publication "Using Serial on Motorola J2ME handsets" - .

The MC of the set-top box, having received the control byte, sets the specified mode of operation of the electronic attenuator, and then starts the ADC at a specified frequency and writes the results of its work to the internal buffer. When the buffer is full, the MC stops the ADC and transmits the synchronization byte to the cell phone, followed by the contents of the buffer. Having received this information, the phone displays it as a curve on the screen, calculates and displays the average voltage value.

If the oscilloscope operates in the automatic voltage scale mode (the letter A is displayed on the screen), then with an average voltage value close to zero or the maximum allowable, the phone will generate a control byte with the attenuator position code changed in the appropriate direction. But this will affect the next measurement cycle.

The oscilloscope is controlled using a cell phone joystick: moving it vertically and horizontally changes the scale of the oscillogram along the corresponding axis. Enabling automatic zoom selection and exiting the application - through the menu.

The cell phone program is installed into it like a normal Java application. It is enough to copy the osc.jar (compiled program) and osc.jad (its description) files to the java/osc directory created in the phone's memory. This is done using special software that comes with the phone. After launching the application, the phone will ask questions about allowing access to the cellular port and to the accessory. Both must be answered in the affirmative.

The prefix was assembled by surface mounting on a breadboard, a printed one was not developed. ConnectorXS1 -special for connecting to a cell phone. Headsets and chargers are equipped with such connectors. Input connector XW1 - СР-50-73Ф or imported BNC series.

Instead of the TL431 chip, you can use KR142EN19, and instead of K561KP2 - CD4052. The AD8054 op amp will replace any other low current quad op amp such as the MC3403.

Before starting work with the oscilloscope, it is necessary to achieve a zero average voltage value displayed on the phone screen with a shorted input of the set-top box with a trimming resistor R11.

Programs for the microcontroller of the set-top box and cell phone can be downloaded hence.

Author: S. Kuleshov, Kurgan; Publication: radioradar.net

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