Instructions

122
Circuit 97: A needle fidgets in waltz time
Setup information
Install the two transistors, the LED, the four resistors and the three electrolytic capacitors as shown in
Fig. 2.97 Then connect the moving-coil instrument. Finally, insert the 13 jumper wires. Check the placement
on the board again and only then connect the battery.
Components required:
2 x 2N3904 transistors,
1 x red LED, 2 x 220 kΩ
resistors, 1 x 4.7 kΩ resis-
tor, 1 x 1 kΩ resistor,
1 x 100 μF electrolytic
capacitor, 2 x 10 μF
electrolytic capacitors,
1 x moving-coil instrument,
1 x breadboard, 1 x 9V bat-
tery with clip connector
Fig. 2.97: A needle fidgets in
waltz time
Experiment
If everything is wired correctly, the slow beat starts with a short delay. The red LED will go on in intervals
and the needle of the moving-coil instrument will swing full-scale.
The circuit diagram for this experiment is shown in Chap. 3, Fig. 3.97.