Logic probe with high input impedance. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology
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In some cases of checking logical devices, it is necessary that the measuring device, in this case a probe, does not change the operating mode of the device under test during the measurement process. This requires a probe with a high input impedance.
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Through the resistor R1, the signal is fed to the gate of the field-effect transistor V3. Diodes V1, V2 serve as an input voltage limiter, thanks to which it is possible to connect the probe to circuits energized with 250 V. After the source follower, the signal is fed to emitter followers (transistors V4 and V5), which reduce the mutual influence of microcircuits and shift the input signal levels, which necessary for a clear operation of the logic elements D1.1 and D1.2.
With the values of the resistors R2-R5 indicated in the diagram, the threshold voltages of operation "0" and "1" are equal to 0,4 and 2,4 V, respectively. If other values "0" and "1" are selected in the device under test, you should select these resistors.
When the input voltage exceeds the threshold value "1", a logical "1.1" appears at the outputs of the elements D1.2 and D0, and the segment d on the indicator H1 lights up ("1" is displayed). If the voltage at the probe input is below the threshold voltage of logical "0", a logical "1.2" appears at the output D1, a logical "2" appears at the output D0A, and the segment f, a, b and g lights up (the sign "0" is displayed) . If the input voltage is between 0,4 and 2,4V (intermediate level), the logic "1" will be at the outputs D2.1 and D2.2 and "0" at the output D2.3. Now the segments a, b, g are lit (the P sign is displayed) The input pulses are detected by the waiting multivibrator (elements D1.4, D2.4, C5 and R13). When the input signal goes from "0" to "1" and vice versa, negative pulses are generated at the output of element D2.3. A point (segment h) is connected to the output of the waiting multivibrator. At an input pulse frequency of less than 20 Hz, the dot flashes noticeably, at frequencies greater than 20 Hz, the dot glows continuously.
If the input pulses are close to the meander, the signs "0" and "1" are lit simultaneously with the dot, and their relative brightness depends on the duty cycle of the pulses.
With a large or small duty cycle, one of these signs and a dot glow.
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