Polarity Tester Pen & TTL Logic Level Tester Pen.

This polarity tester pen is slightly different because it is able to test TTL levels and shows the status on a 7 segment display using with the letters: "H" (High) for logic level "1" and "L" (Low) for logic level "0".

In this instructables I will show you how to build one.

Step 1: Operating Description

In the first picture you can see the schematic.

DESCRIPTION:

The circuit is quite simple. A TTL oscillator "Phase Shift Oscillator" (see the oscilator picture) consisting of 3 logic inverter ports plus a driver circuit for the LEDs of the 7-segments display (CA - Common Anode) to turn on the segments appropriately according to the logic level detected by the PROBE tip.

The oscillator circuit ..... does what? .... oscillates :) .... by forming a train of pulses in the output (pin 6 of the CD-4069 IC); the LEDs stay blinking when the probe tip is loose on the bench ..... The time constant of this oscillator is determined by R1, R2 and C2. Since R1 is in parallel with R2 its form an equivalent resistor Req = R1 || R2.

In our case the oscillation frequency (flashing of the display segments) is: 7Hz (see the formula to calculate frequency in the picture ).

OPERATIONAL STATUS:

1 - When the probe tip is not connected to any point in the test circuit, the display will show the letters "H" and "L" alternating according to the frequency of the oscillator.

2 - When the probe tip is connected to the low logic level "0" (Low) of a test circuit the train of pulses are cut off and the ground signal is inverted by activating the driver to form the letter "L" on the display. This part of the circuit is formed by the logic gate IC2D (inverter), T1 (driver) and diodes D7, D8 and D9 (to form the letter L in the display).

3 - When the probe tip is connected to the high logic level "1" (High) of a test circuit, the train of pulses are cut off and the positive signal is inverted twice (1 - 0 -1) by actvating the driver to form the letter "H" on the display. This part of the circuit consists of the logic gates IC2E and IC2F (inverters), T2 (driver) and diodes D2, D3, D4, D5 and D6 (to form the letter H on the display).

The working voltage can vary from 5 to 15V, it means that: you can use this pen to measure TTL/CMOS levels in digital circuits (0 to 5V), but you can also use it to measure levels in car tests and control systems for example (0 - 12V) .... it is very useful.