Logic Probe Project

CMOS logic integrated circuits are extremely versatile devices which are used a great deal in modern electronic circuits. Like any logic circuits, those which employ CMOS devices can be difficult to test using an ordinary multi-meter as both static and pulsing voltages are involved. A voltage reading of half the supply potential could be a static
Logic Probe Project - schematic

voltage and indicative of a fault, or it could be produced by a high frequency squarewave signal giving an average voltage of half the supply voltage with no fault present in this part of the circuit. Ideally an oscilloscope should be used for testing this type of equipment, but where this is not possible for some reason a logic probe makes an excellent alternative. This simple logic probe is intended for use with CMOS logic circuits, is powered from the circuit under investigation, and has a current consumption of only around 0 to 20mA (depending on the supply voltage and logic state indicated by the probe). The logic state detected by the unit is indicated by a two-LED display: one of the display LEDs IS a multicolour type and the other is an ordinary green type. The following table shows the display obtained from various input signal conditions. When the input is pulsing at high speed it is possible that D1 will be red or green instead of yellow, and this simply indicates that the mark-space-ratio of the input signal is far from being one-to-one. D1 will be red if the input is high for the majority of the time, or green if it is predominantly low. If the input is pulsing slowly, unless the pulses are extremely brief, D1 will be seen to switch from red to green and so on in sympathy with the input signal. Separate stages are used to drive the two indicator LEDs, a dual operational amplifier being used to drive D1 and a...

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