Refinement of the multimeter MY-67. Scheme, description

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Refinement of the multimeter MY-67. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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At present, inexpensive multifunctional digital multimeters "MY-67" can be found on sale, which differ from other similar devices of the same price category (500-800 rubles) by automatic range switching in the voltage and resistance measurement mode, high accuracy, comfortable keyboard and good case design. It turned out to be a pleasant surprise that the readings on the display do not "float" when manually switching the ranges, as in some other models of automatic multimeters, i.e., if a voltage of 4 V was measured when manually switching on the 2,563 V range, then with successive pressing of the button " UP" on the display, the readings remain: 2,56 - 2,5 - 2 V, and not some other values. Among others, the multimeter of this model also has the functions of sound continuity of circuits, a switchable sound alarm for switching the type of work, and an intermittent sound signal when overloaded.

Unfortunately, the buzzer in this model works quietly, which under certain circumstances causes inconvenience. When studying the design of the device, it turned out that the sound emitter, made on an electromagnetic capsule with a winding resistance of 50 ohms, is powered by two printed tracks through two current-limiting resistors of 100 ohms from a single chip - a frameless LSI (board "MY67-6"). Studies with an oscilloscope showed that anti-phase rectangular pulses with an amplitude of 0,3 V (double peak-to-peak 0,6 V) arrive at the sound emitter. When the capsule is turned off, the signal amplitude increases by 10 times. By reducing the resistance of current-limiting resistors or connecting piezoelectric different models instead of an electromagnetic sound emitter, it was not possible to significantly increase the signal volume.

Refinement of the multimeter MY-67

When analyzing the circuit of the device, it was found that the common wire "COM" and the minus of the battery in this model are the same (as in "MY-68", "M320"), which greatly facilitates its refinement.

On both tracks, previously going to the "tweeter", there is a log. 0 when there is no audio signal, or anti-phase pulses with an amplitude of 3 V relative to the common wire when the sound should be. To increase the volume of the emitter, both of these tracks are cut in a convenient place and a simple assembly is assembled, the diagram of which is shown in the figure. The amplifier input is connected to any of the cut tracks. The polarity of the emitter is indicated on the side of the terminals and can be found out by temporarily unsoldering it. The power wires from the node must be connected to the same points where the wires from the battery are soldered.

With the value of the resistor R2 indicated on the diagram, the amplitude of the pulses on the capsule increases from 0,6 to 2 V, which gives an increase in volume by 2 ... 3 times. It is interesting to note that the current consumed in the continuity mode increases after the alteration by only 15%. You can increase the volume even more by reducing the resistance of the resistor R2.

The transistor must be taken with a small reverse collector current, any of the KT3130, KT3102, KT342, BC547, SS9014, 2SC1222 series will do. The diode is suitable for types KD512, KD522, KD103 with any letter index.

Additional parts can be carefully glued to the board from the mounting side with BF-2 glue. During long breaks in operation, the central switch in the "MY-67" device should be set to the "OFF" position, since when the device is turned off with the "POWER" button, it continues to consume a small current.

Author: A.Butov, p. Kurba, Yaroslavl region

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