Part 1 of Essay
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 ....

Part 2 of Essay
.... 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. ....