Logic Probe - Maker Factory ELECTRONICS EXPERIMENTS BOX Manual

2.88 Logic probe

Circuit 187: Active TTL CMOS probe with LED display
Setup information
Install the two transistors, the four diodes and the four resistors as shown in Fig. 2.187 The seven jumper
wires come next. Check the placement on the board again and only then connect the battery.
Experiment
If the level is between 0V and +2V at the tip of the probe, the red LED is lit, at a level between +4V and +5V,
the green LED is lit.
The circuit diagram for this experiment is shown in Chap. 3, Fig. 3.187.
212
Components required:
2 x 2N3904 transis-
tors, 2 x 1N4148 silicon
diodes, 1 x green LED,
1 x red LED, 3 x 1 kΩ
resistors, 1 x 10 kΩ
resistor, 1 x breadboard,
1 x 9V battery with clip
connector
Fig. 2.187: Active TTL CMOS
probe with LED display