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