Homemade Logic Detector

The logic detector is a very practical small instrument in the testing and production of digital circuits. Although there are various logic test pens on the market, it is more suitable for homemade because of its simple circuit and low cost.

The logic level of a digital circuit generally has "three states", namely, high level H ("1"), low level L ("0"), and high impedance Z. By using the logic detector to detect the three-state condition, the working state of the digital circuit can be determined, as well as the state of uncertain high and low levels caused by broken wires, short circuits, poor contact, etc. on the printed circuit board, which are sometimes difficult to detect with a multimeter or oscilloscope.

The general logic levels are expressed as "1" and "0" ("H" and "L"), but in fact there are four states from the time axis as shown in the figure.

The stationary state of "H" or "L".

The states of "H" and "L" are constantly changing (clock waveform).

There is a single short "H" level pulse (positive pulse) in the "L" level, or conversely, there is a single short "L" level pulse C negative pulse.

High impedance state.

Common logic detectors can detect "H" and "L" levels of hundreds of ms, as well as clock pulses, but single pulses of hundreds of μs to tens of ns require a storage oscilloscope or logic analyzer to detect.

The design goal of the logic detector produced this time is to add the detection function of positive and negative pulses on the basis of high and low level detection. The detector circuit is shown in Figure 1. The whole circuit uses a six-phase inverter 74HC04 and four light-emitting diodes.

Parts a and b in Figure 1 are monostable multivibrators added to detect positive and negative pulses, which delay a single pulse of tens of ns to a level that can be distinguished by the human eye. The principle is to charge the 1000p capacitor with an input pulse, and the charged charge will maintain an "H" period of time, thereby lighting up the LED. LED1 is used to indicate a negative pulse, and LED2 indicates a positive pulse; when the input is a clock pulse, both LEDs will flash continuously. LED3 is used for low-level indication, and LED4 is used for high-level indication.

Since a pull-up resistor (22K) is provided in the input circuit, it can correspond to all TTL levels.

This logic detector does not have its own power supply, but obtains +5V power from the circuit under test. It is best to purchase a dedicated finished test rod (with a telescopic hook) for power and signal input. If an alligator clip or a multimeter test rod is used, it is easy to cause a short circuit in the circuit under test.

This simple detector can be used to confirm the CS terminal signal when debugging a microcontroller, to confirm the presence or absence of a sensor output signal, or to confirm false triggering of a gate circuit due to external interference.