CMOS-Based Logic Probe Circuit

Overview

The circuit was designed to create a handy logic probe that can show off the logic states for high, low and pulsing outputs with the use of CMOS 4001 integrated circuit.

• Logic Probe – a handheld probe utilized for troubleshooting and examining the logical states in digital circuits and typically shaped like a pen
• CMOS 4001 – a quad 2-input NOR gate integrated circuit, generally characterized by small fluctuation in voltage supply, very high impedance, outputs that can sink and source, one output can drive up to 50 inputs, high speed gate propagation time, high frequency, and low power consumption
• Light Emitting Diode (LED) – a semiconductor diode that is commonly a source of light when electric current pass through it

The framework or chassis of the usual probe consist of LEDs to indicate the logic conditions. These indications are made possible with the functions of a single CMOS 4001 integrated circuit. They are represented as High, Low, and Pulsing conditions. The purpose of a logic probe in an electronic circuit is to analyze the logic states at a particular point. In cases where the input to the probe is neither high nor low, the LEDs will not shed light because this will only trigger the LED to light up if a voltage source is touched by the tip of the probe, which makes this the main advantage of the circuit. The probe will not respond to any floating voltage which may be available on some object with static charge. This is very essential when dealing with tri-state logic IC with high impedance.

The power of the probe is derived from the supply of the logic circuit being tested. Since the CMOS IC used in the circuit is 4001, it has the capability to test logic circuits with voltages ranging from 3V to 15V. It is still safe on this range because the maximum voltage supply of this CMOS is 18V. The arrangement of IC1 is a buffer with a difference that will enable amplification of weak signal source. Since the 2M2 resistor is ideally characterized for handling pulse load, it produces a feedback that produces the probe to oscillate under the circumstances of not having an input thereby disconnecting from the gate of the circuit. This will produce around half of the voltage supply from IC1.

Each LED indicator is attached with a 1K ohm resistor that acts as a potential or voltage divider by producing an output voltage that is a portion or fraction of its input voltage. A typical logic probe uses three colors for LED indication. Red LED would indicate a high level input or logic 1; Green LED would indicate a low level input or logic 0; while Yellow or Amber LED would indicate a pulsing input wherein a high frequency or low frequency input signal is causing the LED to oscillate between Red and Green. The clock signal or fast oscillation for both high and low conditions are causing the LEDs to glow dim. This action is performed by IC1b and IC1c which function as monostable oscillator. The 4M7 resistor and 100 nF capacitor controls the time constant of oscillation. Even in the presence of a clock signal, the monostable combination is constantly retriggered causing slowness. The pulsing LED will be driven by IC1 that is configured as a buffer. On the other hand, none of these LEDs will light on a floating input. In the event of either high or low condition, IC1 will be resting in a permanent state.

The logic probe is generally useful in debugging CMOS logic circuitry as well as a general troubleshooting tool. Although it is typically used in testing and fault finding, it can also be utilized for providing assistance in electronic design or discovering how electronic equipment functions. The probe tip can be made from a piece of piano wire as well as a bronze finishing nail, a needle, the tip of a paper clip, or a piece of brass rod. In some designs, a small switch is used to hold the pulse LED lighting in the occurrence of a pulse. Also, other designs are incorporating audible tones for each logical condition. Since the logic probe is versatile and low cost, it is conveniently used in digital test instrument.

Source:www.zen22142.zen.co.uk/Circuits/Testgear/lprobe.html