Logic probe uses two comparators
Build a logic probe whose voltage thresholds vary with your circuit's power supply voltage.
Vladimir Rentyuk, Zaporozhye, Ukraine; Edited by Paul Rako and Fran Granville -- EDN, August 25, 2011
Measurement instruments must not affect the circuits they’re measuring. A logic probe, for example, must correctly detect logic levels, and it must place no undue loads on the test circuit. The logic probe must set thresholds on automatically checking logic levels, depending on the supply voltages of the ICs it is checking. It should also not cause the checking circuit to function incorrectly.
The logic probe in a previous
Design Idea suffers from loading problems
(Reference 1). Its thresholds do
not adequately depend on the supply
voltages of the ICs it is checking.
After some analysis, you’ll find that
the circuit can have an input-current
range of 50 to 80 μA. Unfortunately,
CMOS ICs such as NXP’s HCMOS 74HC/HCT/HCU
(Reference 2) and Signetics’ LOCMOS HE4000
families have input current as low as
±1 μA. The HE4000 family of logic
ICs has input currents of ±0.1 to ±0.3
μA. Connecting the referenced logic
probe to input pins of these ICs loads
the circuit under test and distorts the
waveforms to the point at which you
may not be able to see some problems,
such as glitches, on an oscilloscope.
Logic probes also have negligible input
capacitance. Too much capacitance
attenuates at high frequencies.The circuit in Figure 1 is an alternative logic probe for ICs of the 74HCxx family, for example. It comprises comparators IC1A and IC1B. Not every comparator will work properly in this circuit. The comparator must, for example, operate with minimal supply voltages, and it must have low input leakage current. The Analog Devices AD823AR or an equivalent comparator is a good choice.
The network comprising R1 and D1,
two BAT54S Schottky diodes, protects
the logic-probe circuit from overvoltage
at its input, from ESD (electrostatic
discharge), and from signals of
negative polarity. Yellow LED3 indicates
when the logic-probe circuit and the
circuit under test start up. The yellow
LED is useful if you connect the probe
to the circuit under test with crocodile
clips. This approach ensures that
both the probe and the test circuit are
always on. D2 and resettable fuse F1, an
MF-USMF005, which has a hold current
of 0.05A and which comes in a surface-mount
package, protect the probe circuit
from improperly powering up. Tantalum
capacitor C1, in size A or B, and ceramic
capacitor C2, in size 0805, prevent the
test circuit from influencing power for
the logic probe. R1 minimizes the influence
of input capacitance on this logic
probe.The logic-level thresholds automatically depend on the supply voltages (tables 1 and 2). You can use this logic probe with other ICs, such as the 74HCU, 74HCT, or 4000 series. You can freely select the value of R6. You can also calculate the value of R5 and R4 using R5=VH/(VL/R6)–R6 and R4=VDD//(VL/R6)−R6−R5, where VDD is the supply voltage of the device, VH is the threshold for checking the high logic level for the chosen supply voltage, and VL is the threshold for checking the low logic level for the chosen supply voltage.
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Talkback
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Yes, I have used it since 1984, but it is the modern version of this useful device. My target was to give some illustration of the right design as against the incorrect way. Why you did not offer the right solution if you know it? I wanted to reform the incorrect design to suitable for using and nothing more.
Vladimir Rentyuk - 2011-1-9 06:28:29 PDT -
this solution is 30 years old...
janca - 2011-29-8 13:07:50 PDT
