Probe with advanced features. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Probe with advanced features. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Indicators, detectors

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For those who have to set up electronic devices assembled on TTL digital microcircuits, a logic probe, the schematic diagram of which is shown in the figure, may well come in handy. In some cases, it can even replace an oscilloscope. The device is characterized by small dimensions, low power consumption and easy handling. It is convenient to use it when searching for defects in blocks located in hard-to-reach places, especially when repairing outside the workshop.

Unlike many other designs, the probe allows you to determine and display with three multi-colored LEDs not only voltage levels 0 and 1, but also an intermediate level. The indication time of short pulses of all three levels is increased to 0,1 s, which ensures their visual observation. If desired, the increase in the duration of the pulses can be turned off, and then the brightness of the LEDs can be used to evaluate their duty cycle and squareness. To determine the number of incoming pulses, the probe is equipped with a counter for 8, indicated by three single-color LEDs.

The probe also allows "by ear" to judge the frequency of the signal under study. To do this, it has a jack for connecting a phone to the output of a frequency divider by 2 (for audio frequencies) or to the output of a frequency divider by 4096 (for high frequencies, up to 10 MHz).

(click to enlarge)

Technical specifications

Displayed voltage levels, V: zero (0)......0...0,4
intermediate ......0,4...2,4
units (1) ......2,4...5
Input resistance, kOhm, not less than......40
The duration of the studied pulses, ms, not less ...... 50
Consumed current, mA, not more ...... 70

The input device of the probe is an emitter follower transistors VT1 and VT2. Resistor R1 protects it from overload when a signal with a voltage higher than the supply voltage or pulses of negative polarity is applied. Diodes VD1 - VD3 and resistors R2 -R4 determine the thresholds of the elements DD1.1 and DD2.I. By selecting resistor R2, the lower limit of the determined level is set to 1, and by selecting resistor R3, the upper limit of level 0 is set. Elements DD1.1 and DD2.1 form a steep front and a decline in pulses. Element DD1.2 determines the intermediate level of the signal under study.

LEDs HLI - HL3 display respectively level 1, intermediate voltage and level 0. These LEDs are connected to the outputs of single vibrators on the elements DD2.2 - DD2.4. In the position of the SA1 switch shown in the diagram, the single vibrators lengthen the short input pulses supplied to them to such a duration that the glow of the LEDs is noticeable to the eye. When switch SA1 is switched to the lower position according to the scheme, the resistors R6, R9, R12 are disconnected from the common wire and level 4 is supplied to them through the resistor RI1, which puts the single vibrators into repeater mode. In this case, there is no increase in the duration of the pulses. In this position of the switch, the glow of the HL2 LED is the brighter, the longer the duration of the rise and fall of the studied pulses. If they are almost rectangular, the HL2 LED does not light up.

Since input C1 of counter DD3 is connected to the output of element DD1.1, the counter counts the number of pulses at level 1. It can count them at level 0 if you switch this counter input to the output of element DD2.1. LEDs HL4 - HL6 are connected to the outputs of the counter, each of which displays the state of its corresponding binary digit. The number of received pulses is equal to the sum of the weight coefficients of the counter outputs corresponding to each of the luminous LEDs HL4 - HL6 (respectively 1, 2 and 4 pulses). Every eight pulses, the counting cycle is repeated. The counter is reset during the switching (span) of the contacts of the SA1 switch, since only during this period of time is level 1 present at both inputs R of the counter. In order to reduce the current consumption, in the position of the switch SA1 shown in the diagram, the LEDs HL4 - HL6 do not light up.

Counters DD8 and DD3 are connected in series to output 4.1 of counter DD4.2. The total frequency division factor of the three counters is 4096. Pulses from output 1 of the DD3 counter are fed to one of the inputs (pin 2) of the DD1.4 element, and pulses from output 8 of the DD4.2 counter to one of the inputs (pin 5) of the DD1.3 element. 1.3. In the position of the SAI switch shown in the diagram, the DD1.4 element is off, and the DD1 element is on (level 1 is present at pin 1). Consequently, pulses pass to the phone with a frequency that is two times less than at the probe input. This is necessary so that the duty cycle of the pulses in the phone is equal to two, regardless of the duty cycle of the studied pulses. When the SA1.4 switch is switched to the lower position according to the diagram, the DD1.3 element closes, and the phone receives pulses from the output of the opened DD4096 element with a frequency 10 times lower than at the probe input, which allows you to listen to input pulses with a frequency of up to XNUMX MHz.

Diode VD4 protects the probe from improper connection to the power source. Capacitors C4 and C5 block impulse noise in the power circuit, they should be distributed at different points in this circuit (evenly).

The body of the probe is a pencil case from a collet pencil measuring 155x28x13 mm. All parts are mounted on a board with dimensions of 115x21x1,5 mm. Installation is carried out with MGTF-0,12 wire.

In the probe, the K555 series microcircuits can be replaced with similar ones from the K155 series, but the current consumption will increase by one and a half times. Diodes KD521V (VD1 - VD3) can be replaced with KD503, KD509, KD510, KD521, KD522 with any letter index. LEDs will fit any other, both in type and in color. Instead of the KT315G transistor, you can use KT312, KT342, KT3102, etc., and instead of KT361G - KT313, KT3107 and others, also with any letter index. Diode D310 can be replaced with D311A. Resistors - MLT, capacitors C1 - C3 - K50-6, and C4, C5 - KM-5. The SA1 switch can be any small-sized one, but it is convenient to use the PDM1-1 switch, since its design allows you to set the counter to zero by lightly pressing the handle without moving it to another extreme fixed position (when the force is removed, the handle returns to its original position).

The input probe is made of a piece (threaded) from a bicycle spoke 50 mm long, fixed on the front end of the housing with two M2 nuts. At the opposite end of the case there is a socket for a TM-2 telephone, and wires for supplying voltage from the device under study are led into the hole.

Author: Yu.Yuditsky, Gomel

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