Trobleshooting Logic Probe
How the transistor logic probe works.
There are several variations of logic probes on the boards. On the 3 transistor versions they basically work the same. A transistor has 3 leads labelled E, B, C which represents the Emitter, Base and Collector of the transistor. In simple terms for the sake of illustration on the probe, think of the Base as a valve in a water supply line with the supply side the Collector, and the faucet the Emitter. With no Base current, there is no current flow through the Collector - Emitter leg. As the Base current increases the current flow through the Collector to the Emitter increases, like opening a water valve. At some point increasing the current in the Base stops increasing current in the Collector-Emitter leg.
With 5V and common (negative or ground of the 5V source) supplied to the few components that make up the probe and just one resistor coming from the base to the probe wire and it left open, there is no current flow through the Base, thus no current flow in the Collector - Emitter leg, thus the LED doesn't illuminate.
In Figure above,the circuit is shown in the two states, lo and hi. As you can see in the illustration there is no difference between the probe in open air and logic lo. If your were to supply a variable DC voltage to the probe, you would see the LED start to illuminate somewhere above 2.7 volts and continue to brighten as you increased it to 5V. This is the weak point in this circuit and can be simply corrected by the addition of a single resistor.
This above Figure shows the addition of the resistor R1. This allows visual indication of an open or indeterminate logic state. The additional resistor provides a low bias current to T1, thus the transistor acts like a valve that is open just a fraction. That results in the led having a slight illumination. The downfall of this circuit is that each 2N3904 will have a different DC gain. If you grabbed a hand full of 2n3904'S and installed them in the same circuit one at a time, you would find that there is a variance of brightness of the LED. because of that you may need to vary the value of the 150K ohm resistor to get the level of illumination a recognizable difference in the open state from lo or hi. If the LED is to bright, increase the size of the 150K. If it is to dim decrease it.
This picture below is how you would wire, the blue lines represent wires and you are looking at a top side perspective.