Probe with sound indication. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology
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There is a well-known drawback of the avometer when the wires of the installation are ringing - you have to constantly look from the circuit under test to the arrow of the device. Another thing is the proposed probe (Fig. 1): it announces the presence of a connection with an audible signal. Moreover, it "works" only when the resistance of the tested circuit is less than 10 ohms.
The basis of the probe is a bridge of parts R1, VD1, R5, R6. A somewhat unusual differential cascade is connected to its diagonal, made on transistors VT1, VT2. This is followed by a key stage on the transistor VT3, an inverter on the element DD1.1, an audio frequency generator on the elements DD1.2 - DD1.4, a sound indicator - a piezo emitter HA1.
In the initial state, the transistor VT1 is closed, and VT2 is open. When the XP1 and XP2 probes touch a working circuit under test and a circuit with a resistance of less than 10 ohms is connected in parallel with the diode, the state of the transistors will become opposite: VT1 - open, VT2 - closed. The transistor VT3 will open, a logical 1 will appear at the output of the inverter, allowing the generator to work. The piezo emitter will emit a sound signal with a frequency of approximately 1000 Hz.
The probe does not indicate a short circuit if, in parallel with the VD1 diode, a silicon or germanium diode is connected through the probes in the forward direction, i.e., in the polarity of the VD1 diode shown in the diagram.
A probe was mounted on a printed circuit board (Fig. 2) made of one-sided foil fiberglass. Transistors can be any of the KT315 series. Resistors - MLT-0,125 or MLT-0,25, capacitor - KT, KM or another. Diode - any silicon. A piezo emitter (it is glued to the board) can be used by any of the ZP series, an emitter from Chinese telephones is also suitable (it has a plastic semicircular ledge).
The board is designed to be placed in a Tick-tock candy box. Power conductors and probe conductors are led out of the box. The probes themselves are made of knitting needles - they are cut to the desired length and inserted into plastic handles.
As a rule, the probe is ready for use immediately after connecting the power supply. If after the first turn on the sound immediately appears, you should swap the transistors VT1 and VT2. Naturally, when using a probe, the device under test must be de-energized.
When assembling the proposed device, first try not to install the transistor VT3, and apply the signal from the VT2 collector to input 9 of the DD1 microcircuit (of course, without connecting this input to the output of the DD1.1 element). The probe will probably work just fine. If you like this option, do not forget to connect inputs 1 and 2 of the unused DD1.1 element to a common wire or +5 V circuit.
Author: B. Semenov, P. Semenov, St. Petersburg
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