Homemade logic detector, Logic Level Tester

Homemade logic detector, Logic Level Tester Keywords: logic detection circuit diagram The logical detector is a very practical small instrument in the test and production of the digital circuit. Although there are various logic test pens, it is simple, the cost is very low, and it is more suitable for homemade. The logic level of the digital circuit is generally "three state", that is, high level h ("1"), low level L ("0"), high impedance Z three states. Using the logical detector to detect the three-state conditions, it can be determined that the operating state of the digital circuit, the high and low levels of disconnection, short circuit, and poor contact, etc. on the printed circuit board cannot be determined, and these faults sometimes use The multimeter or oscilloscope is difficult to detect. --like logic appears as "1" and "0" ("h" and "L"), but in fact, there are four states from the time axis. "H" or "L" rest status. "H" and "L" changing state (clock waveform). There is a single short "H" level pulse (positive pulse) in the "L" level, or in contrast, there is a single short "L" level pulse C negative pulse exists. High impedance state. Common logic detectors can detect "H" and "L" levels of hundreds of MS, and clock pulses, but with a single pulse of hundreds of μs to dozens of NS, it is necessary to detect with a storage oscilloscope or logic analyzer. The design goal of the logical detector this time is to increase 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 entire circuit uses a six-inverter 74HC04 and four light-emitting diodes. The A and B portion in Fig. 1 are a single velocity of a single pulse delay in the human eye to a single pulse of dozens of NS to detect positive and negative pulses. The principle is to charge the 1000P capacitance with the input pulse, and the charge that is charged will maintain a "H" time so that the LED lights. LED1 is used to indicate negative pulses, and LED2 indicates a positive pulse; the two LEDs will be continuously shiny when the clock pulse is input. LED3 is used for low instructions, and LED4 is used for high level indication. Since the pull-up resistor (22K) is provided at the input circuit, all TTL levels can be correspondingly. This logic detector does not have a power supply, and the + 5V power supply is obtained from the measured circuit. The input of the power supply and signal is best to purchase a dedicated finished product test rod (with telescopic hook), if the crocodile clip or multimeter test rod is used, it is easy to cause short circuit of the measured circuit. This simple detector confirms the CS terminal signal when debugging the single-chip microcomputer, or confirms whether or not the sensor output signal can function, or confirm that the external interference can be played. Read more