A simple universal probe. Scheme, description

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A simple universal probe. 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 probe circuit is shown in Fig.1. With this probe, you can check the health of diodes, transistors, cables, and also roughly determine the TTL and CMOS logic levels. Unlike those previously published, this probe is easier and more convenient to use to test pn junctions, since the probes do not need to be swapped to "ring" in both directions of the junction. The probe is capable of operating at 4,5 V, drawing about 15 mA and about 45 mA at 9 V.

Fig.1 (click to enlarge)

The device contains a multipolar pulse generator with amplification, assembled on the K176IE5 IC and a complementary pair of transistors (VT1 and VT2), as well as a registration circuit (VT3, VT4, VD1, VD2). If there is a common wire potential on the XP2 probe, VT4 opens and the green LED VD1 lights up and the positive power potential opens VT3, lighting the red LED VD2. The resistance of the resistor R6 is chosen so that when the supply voltage is 9 V and there are no signals on XP2, both LEDs go out. The generator generates pulses with a frequency of about 2 Hz and a duty cycle of 1, which are amplified in current by transistors VT1 and VT2, fed through resistor R3, which prevents current overload of VT1 and VT2, to the XP1 probe.

The following parts are used in the device: all MLT type resistors, transistors from the KT3102 (VT1, VT3) and KT3107 (VT2.VT4) series, if possible with close gains in pairs VT1-VT2 and VT3-VT4, any small-sized capacitors, AL307B LEDs - VD2 and AL307G-VD1. The probe is assembled on a printed circuit board made of one-sided foil fiberglass 1 mm thick. The dimensions of the board are 20x35 mm (Fig. 2).

Ris.2

The LEDs are mounted on the side surface of a cylindrical housing with a diameter of 25 mm and a length of 60 mm. The XP2 probe is fixed on the end surface of the housing through an insulating gasket, XP1 is a piece of insulated wire passed along with the power wires through the opposite end. The wires are equipped with "crocodiles". Capacitor C1 is soldered to the terminals DD1 directly. In the presence of an alternating voltage of very low frequency (up to 10 Hz) on XP2, the LEDs light up alternately; at a higher frequency, their glow seems to be simultaneous. The green LED indicates the presence of a logical 0, and the red LED indicates a logical 1.

Diodes are checked by connecting them to the XP1 and XP2 probes. If both LEDs are alternately lit, then the diode under test is broken, if the red LED is flashing, then the anode of the diode under test is connected to XP1, if the green LED is flashing - the cathode. Similarly check the transitions of transistors. If there is more than 1000 pF capacitance between the probes of the device, the LEDs flash alternately, but with very short flashes. The integrity of conductors is checked in the same way as diodes. The absence of a glow of both indicators indicates a break.

Author: A.M. Shary

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