Sound probe upgrade. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Sound probe upgrade. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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For continuity of contacts and multicore cables, a simple probe is usually used, which is a circuit of a current source (batteries or galvanic cells) and an indicator - an incandescent bulb. In the process of work, such a "flashing light" is also used as a flashlight. But this circuit does not allow you to check relay windings, signal lamps, and similar elements with relatively high resistance.

In search of a suitable scheme in the magazine "Radio" for 1998, No. 7, p. 37 there was an article "Sound probe" by B. and P. Semenovs. When assembling this device on a breadboard, the characteristics stated in the article were confirmed - sound signaling of measured resistances strictly from 0 to 10 Ohms. The use of sound indication in the probe allows you to speed up the ringing process without being distracted by observing the indicator readings.

A detailed check of the probe on the breadboard revealed that the current consumption in standby mode reaches approximately 22 mA. For a portable device powered by galvanic cells, which is exactly what a probe should be, this is an unacceptably large value. In this case, the main current consumer is the VD1R1 circuit, since the VD1 diode is connected in the forward direction, and the resistor R1 has a relatively low resistance - 300 ohms. However, it is not possible to change this circuit - the probe simply will not work.

In addition, in practice, using this probe only for continuity of low-resistance circuits is not beneficial. Moreover, the analysis of the probe circuit shows that there is a simple possibility to introduce a second, kilo-ohm measurement range. That is why the probe was redesigned in accordance with these considerations. Schematic diagram of the upgraded version of the sound probe is shown in the figure.

Sound probe upgrade

To obtain two measurement ranges, a dual switch SA1 is introduced. In the switch position shown in the diagram, the probe input is connected directly to the base of the transistor VT2. This range of measured resistances is in the range from 0 to 8 kOhm. Since in this case the cascade on the transistor VT1 is not involved in the work, the second pair of contacts - SA1.2 - breaks its power, including the high-current circuit R1HL1. Thus, a low current consumption of the probe in this range is achieved - only 5 mA, which is quite acceptable.

Instead of the VD1 diode, the HL1 LED is installed. In fact, with such a replacement, the electrical parameters of the R1VD1 circuit do not change. At the same time, the introduction of the HL1 LED makes it possible to visually evaluate the resistance of the measured circuit in the low-resistance range from 0 to 10 Ohms. Obviously, when measuring within these limits, the LED goes out, since the measured circuit practically closes it. In addition, when the probe is in standby mode, a constantly lit LED reminds you of the need to conserve batteries.

The low-resistance range is switched on by the SA1 switch. For visual control in the main range from 0 to 8 kOhm, a key is installed on the transistor VT3, resistors R7, R8, LED HL2. The LED ignition algorithm for resistance measurements is given in the table. The circuit for switching on the audio frequency generator has also been changed.

Sound probe upgrade

Switch SA1 can be any small. LEDs - AL307 series. Transistors - any appropriate structure. The table shows the states of the LEDs HL1, HL2 and the piezo emitter ON 1 depending on the resistance of the measured circuit and the position of the movable contact of the switch SA1.2. The probe is powered by four 316-type elements connected in series.

Author: S. Stashkov, Perm

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