LOGIC PROBE SUMMARY
Logic Probe is one of the basic troubleshooting tools for common electronic logic circuits (TTL circuits). Almost all video game circuit boards are built using TTL chips. These chips will occasionally fail and the Logic Probe is a tool that affords us a look into what may be failing, and it s a very handy measuring device used to determine output logic levels. The 555 timer IC is used in this circuit. The 555 timer IC uses power supply voltage from about 5 volts up to 18 volts. The two power supply connections are the red and the black. The red lead goes to the positive of the supply while the black goes to the ground. The input enters from pin 6 and 2 of the 555 IC. If the voltage is LOW, the bottom LED lights. If the input voltage is HIGH, the top LED lights. When the input probe is disconnected, both LEDs light. Note that pin 3 of the 555 IC always goes to the opposite logic level of the input. Therefore, if the input is HIGH, the output of the 555 IC, which is pin 3, goes LOW. This in turn activates and lights the top LED (the HIGH indicator).
Logic probes derive their operating power from the PCB under test. Care should be taken in attaching the clips to safe voltages; you may want to first use a Volt Meter (Digital Multi Meter) to verify the hook up points have good voltages. The four diodes protect the IC from reverse polarity. Capacitor C1 prevents transient voltage from affecting the logic probe when the black lead is not connected. Pin 5 of the 555 IC is grounded through resistor R1. The logic probe will indicate a HIGH for 2V or greater, and a LOW for 0.8V or less.
To use this logic probe, follow the procedure.
1. Connect red power lead to +5V of circuit under test.
2. Connect black lead to ground of circuit under test.
3. Touch probe tip to point in digital circuit to be tested.
4. One or the other LED indicator should light. If both light, the tip is disconnected from the circuit or the point is in the undefined region between HIGH and LOW.
Note that the voltage range between +0.8V and +2.4V is undefined. Any steady voltage state in this undefined range should light both LED’s on the logic probe & is indicative of a problem such as overloaded/ weak output or missing pull-up resistor.