Digital Circuit Production 08-Logic Pen Experiment

1. Circuit simulation and principle explanation

      The op amp is a LM358 dual op amp, and the NAND gate is a 74HC00 quad two-input NAND gate. In the simulation diagram, a switch is used to switch the input state of the red test lead, and the black test lead is grounded by default. When the red test lead is connected to VCC2, the red light lights up, indicating high level; when the red test lead is connected to ground, the blue light lights up, indicating low level. The diode can set the high and low level cut-off voltage. As shown in the simulation diagram, the forward input of the first op amp above is 2.984V after being stepped down by three diodes. When the voltage of the red test lead is higher than 3V, the op amp is formed The comparator circuit outputs a low level, and then the logic red light lights up through the following NAND gate circuit; the low-level threshold voltage is limited by a diode at the bottom. When the voltage is lower than the diode voltage drop (the voltage in the figure is 0.672V), the blue light lights up when the measured potential of the red test lead is lower than this parameter.

2. Precautions