Logic probe - prefix to a digital multimeter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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When setting up and repairing radio equipment, which includes logical digital microcircuits, a probe is very useful, indicating the logical levels at their outputs. If such a probe is also sound, then you will not have to be distracted by observing the readings of an indicator or a voltmeter, often used for such purposes. The scheme and design of the probe that satisfies the listed conditions are described in the proposed article.

A logic probe for checking the operation of digital microcircuits can be greatly simplified by making it an attachment to a digital multimeter with a "sound ringing" mode. These multimeters include M832, M838 and some others.

Together with a multimeter, the probe provides the following logic level indication mode when a voltage with a low logic level is applied to the input or when there is no voltage at the input, there is no sound signal. At a high voltage level at the input, a continuous sound signal sounds, but if there are pulses at the input with a voltage that changes from low to high logic level, then the sound signal will be intermittent.

The diagram of the logical probe-attachment is shown in fig. 1. Only one DD1 logic chip is used in the probe. The probe is connected to the input sockets of the digital multimeter, designed to measure resistance or voltage.

A buffer stage is assembled on the logical element DD1.1, which has a large input resistance and loads the device under study a little. On logical elements DD1.2. DD1.3 is a low-frequency pulse generator (several hertz). The fourth logical element of the microcircuit is not used, and its inputs are connected on the circuit board to a common wire. Transistor VT1 acts as an electronic key, and diodes VD1 - VD3 rectify the pulse voltage.

The probe is connected to the power circuits of the controlled device with a voltage of 3 to 15 V. Since the current consumed by the probe is very small, connecting the probe has almost no effect on the operation of the device.

The probe works like this. When a logic low voltage is present at the input, the output of the DD1.1 element will be set to a high level. At the input 1 of the element DD1.2 there will be a low level, since it is connected to a common wire through a resistor R3, and at its output it will be high. Both inputs of the DD1.3 element will be high, so its output will be set to a low level and the transistor VT1 will be closed. Since the multimeter is set to the "beep" mode, and the resistance of the closed transistor exceeds 1 kOhm, there is no sound signal.

When a voltage with a high logic level is applied to the input, a low level will remain at the input of the DD1.2 element, low at the input 8 of the DD1.3 element, and high at its output. The transistor will open, its resistance will decrease and a continuous beep will sound.

If, however, a pulsed voltage is applied to the input with alternately low and high logic levels, then the output of the DD1.1 element will also have pulses that are rectified by the diodes VD1, VD2 and VD3. In this case, high logic levels will be set at inputs 1 and 8 of elements DD1.2 and DD1.3, respectively, and the generator will start to work. In this case, the transistor will open several times per second with the generation frequency and an intermittent sound signal will sound.

The device is operational when pulses with a duration of 0,3 μs or more with a repetition rate of several kHz or more are applied to the input. The maximum pulse repetition rate is 2...3 MHz.

Structurally, the probe is made in the form of an attachment, the basis of which is a printed circuit board made of one-sided foil-coated fiberglass (Fig. 2). Pins XP4, XP5 are fixed on the board, which are inserted into the multimeter sockets (see photo). From above, it is desirable to close the board with a plastic cover.

It is permissible to use the following parts in the device, in addition to those indicated in the diagram in Fig. 1: microcircuit - K564LA7, transistor VT1 - KT312, KT315, KT3102 with any letter indices, diodes VD1 - VD3 - KD521A, KD522 with any letter indices, capacitors - K10-17a, resistors - MLT, C2-33. The device does not require adjustment.

Author: I.Nechaev, Kursk

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