Logic probe. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The probe is as simple as possible and contains a minimum number of radio components. It uses a semiconductor sign-synthesizing indicator ALS324B.

The device indicates three different states at the input: no signal (the -| sign lights up), low logic level voltage (0 is on), and high logic level voltage (1 is on). The device is powered by a 9 V direct current source ("Korund" battery). Schematic diagram of the logic probe is shown in Figure 1. Transistor VT1 acts as an electronic key. Elements DD1.1 and DD1.3 chip DD1 are used to amplify the input signal, and DD1.2 is used as a comparator. Logical information is displayed by the sign-synthesizing indicator HG1. Fixed resistors R6-R10, R12, R13 limit the current of the indicator LEDs, and the trimming resistor R3 serves to set the probe to its initial state in the absence of an input signal. Battery GB1, zener diode VD1 and trimmer resistor R11 form a stabilized DC power supply.

Let's assume that after the power is turned on, there is no signal at the probe input (probes XP1 and XP2 are not connected to the electrical circuit of the device under test). In this case, the transistor VT1 will be locked and a high logic level voltage will be set at the input 9 of the DD1.2 element. The same value will be the voltage at inputs 5 and 6 DD1.1, and therefore at output 1 DD1.3 and input 8 DD1.2. As a result, logical 10 will be set at the output 1.2 of DD0, and segments g, b and c will light up on the HG1 indicator (the last two are connected directly to the “negative” power wire through resistors R12 and R13), indicating the absence of a signal at the probe input.

If now a high logic level voltage is applied to the input, the state of the elements DD1.1 and DD1.3 will not change, but the transistor VT1 will open and logic 9 will be set at input 1.2 DD0. The element DD1.2 will switch, a logic 1 will appear at its output and the g segment of the indicator will turn off. At the same time, segments b and c continue to glow, forming the number 1.

Apply a logic low voltage to the input of the probe. Transistor VT1 will again be locked, but the elements DD1.1 and DD1.3 will switch to opposite states and logical 1 will be set at output 1.3 DD8 and input 1.2 DD0. In this case, the element DD1.2 will also switch, and at its input a logic high voltage will appear. As a result, segment g will go out, and segments a, d, e, f will light up, forming, together with the continuously burning segments b and c, the image of the number 0.

Rice. 1 Schematic diagram of a logic probe

All elements of the probe, with the exception of the GB1 battery, SA1 toggle switch and XP1 and XP2 probes, are placed on a 62x30 mm circuit board made of foil-coated getinax or fiberglass 1-2 mm thick (Fig. 2).

The following parts can be used in the device. Transistor KT601 - KT603, KT608 with any letter index. Instead of the ALS324B indicator, you can use any other indicator with separated cathodes, for example, KLTs201. Zener diode - KS156A or KS147A. Fixed resistors-VS, MLT, OMLT, S2-23, S2-33; tuning - SPZ-16. It is also permissible to use tuning resistors SP4; however, it will be necessary to slightly modify the design of the circuit board, taking into account its dimensions and pinout. Toggle switch - small-sized, for example PDM or MT1, MTD1. Probes - from an industrial measuring device.

Fig.2 Printed circuit board and layout of elements

Setting

First of all, by rotating the engine of the tuning resistor R11, select the supply voltage of the DD1 chip so that it is 5 V. Then, in the absence of an input signal, set the probe to its original state by moving the R3 engine to the extreme right position according to the diagram. At the same time, segments b and c should be lit on the indicator. Further, slowly rotating the R5 engine in the opposite direction, achieve the glow of the g segment. The probe is now ready for use.

Publication: cxem.net

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