DAY 4:

Bread Board Logic Probe:

Today we make a very useful tool for electronics troubleshooting: The Logic Probe. The logic probe 'examines' the logic state at a particular point in an electronic circuit.' Basically, it can quickly gauge if the circuit connection is at high or low voltage. We use a 2N3904 transistor switch. It has 3 prongs--collector, base, emitter. Collector goes to positive, base goes to probe , and emitter to LED and common. It's fairly simple and cheap circuit to build.

Perf Board Logic Probe:

Once we have a breadboard model of the logic probe, we 'perfboard' the circuitry. It's quite straightforward, just the task of soldering can be challenging because there are so many ways of making and connecting logic probe circuitry. See below.

Transistors Switching:

The 2N3904 is a micro switch, a very popular transistor that can switch the flow of electricity, quite sensitive and versatile. It's a simple circuit that can incorporate the the finger tip as the push button. If you lick your finger and touch the circuit, the LED amplifies brightly as more current flows versus a dry finger tip. No touching with BOTH hand because it will create a complete circuit that utilizes your body to conduct the electricity. OUCH!!!