As shown in Figure 555 is a simple logic circuit test leads. Test pen 555 as the core, it will take a Schmitt trigger, the logic state of the test for digital circuit. 2,6 555
feet and then, when the input signal level is 0, 555 set, LED2 light; when the input is 1, LED1 lights. For example, when Vdd 4.5V, the measurement TTL circuit, adjust RP1, so 5 feet potential 2.4V, the Schmitt trigger R end (6 feet) trigger level 2.4V, S end (2 feet ) level is 1.2V. When the input level is higher than 2.4V, LED1 lights; below 1.2V, LED2 light. This circuit is used to test the operating voltage of -24V PMOS circuit.