Instructions

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Circuit 188: Active TTL CMOS probe with LED display
Setup information
Install the two transistors, the four diodes and the five resistors as shown in Fig. 2.188. The seven jumper
wires come next. Last but not least connect the moving-coil instrument. Check the placement on the board
again and only then connect the battery.
Components required:
2 x 2N3904 transis-
tors, 2 x 1N4148 silicon
diodes, 3 x 1 kΩ resis-
tors, 1 x 2.2 kΩ resis-
tor, 1 x 10 kΩ resistor,
1 x breadboard, 1 x 9V
battery with clip con-
nector
Fig. 2.188: Active TTL CMOS
probe with moving-coil
instrument
Experiment
The level at the tip of the probe is shown conveniently on the moving-coil instrument. No needle movement
means low, full-scale deflection means high.
The circuit diagram for this experiment is shown in Chap. 3, Fig. 3.188.