Logic Probe
Circuit :  Andy Collinson
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This logic probe uses a single CMOS 4001 IC and can display high, low and pulsing outputs.

This logic probe is based on a single CMOS 4001 IC. This IC contains four 2-input NOR gates, all of the gates are used in this circuit. Power for the logic probe is taken from the circuit under test and because CMOS technology is used, the circuit will work with voltages from 3 to 15 Volts DC. The IC pinout is hown below:

4001 Pinout

CMOS IC's use MOSFET's, so the power terminals are commonly called VDD for Drain voltage (positive) and VSS for Source Voltage (ground terminal). In transistor circuits and logic circuits using TTL IC's the positive supply is often termed VCC which is the collector volatge. As long as Pin 14 is wired to the circuit under test positive supply and pin 7 to ground then the IC will be powered up correctly. The gates in the circuit can be used in any order e.g. Pins 1,2,3 for gate IC1a, pins 4,5,6 for gate IC1b, etc.

Output States
This logic probe can dispay four output states, High, Low, Pulsing and tri-state (or high impedance). The tri-state output is a high impedance state in which the output pin has no value (it is not at logic 0 and not at logic high, you can think of this state as infinite impedance).

IC1a (Pinout pins 1,2,3) is wired as an inverter with a 2M2 feedback resistor. With no input, i.e. probe not connected to a logic circuit, then the output of gate 1a (pin3) is fed back to the input (pins 1 and 2) via the 2M2 resistor. This gate will oscillate at a very high rate and resultant output voltage at pin 3 will be approximately half the supply voltage. The Hi and Lo logic indicator LED's are also connected to a potential divider consisting of the two 1k resistors. The voltage at the resistor junctions is also half supply voltage hence with no input, no output LED's light representing the tri-state.

A Hi or Lo logic condition at the probe input, will cause IC1a to rest in a permanent state indicated by either the Hi or Lo LED illuminating.

With a fast oscillator or clock signal input both Hi and Lo LED's will light but appear quite dim. To increase brightness, the input signal is slowed down. This is achieved by gates IC1b and IC1c which are wired as a monostable. The time constant for the monostable is determined by the 100n capacitor and 4M7 resistor. A fast input pulse now continually triggers and re-triggers, the monostable, effectively slowing the input signal. The output of the monostable is inverted by IC1d, wired as an inverter, and increasing output current to the pulsing LED.

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