Instructions

144
Circuit 119: RF watchdog with moving-coil instrument.
Setup information
Install the two transistors, the diode, the eight resistors and the three ceramic capacitors as shown in
Fig. 2.119 Connect the moving-coil instrument. Finally, insert the ten jumper wires. Check the placement on
the board again and only then connect the battery.
Components required:
1 x 2N3904 transistor,
1 x 2N3906 transistor,
1 x BAT 85 Schottky
diode, 4 x 1 kΩ resistors,
1 x 2.2 kΩ resistor,
1 x 4.7 kΩ resistor,
2 x 10 kΩ resistors,
1 x 10 nF ceramic
capacitor, 2x 100 μF
electrolytic capacitors,
1 x moving-coil instru-
ment, 1 x breadboard,
1 x 9V battery with clip
connector
Fig. 2.119: RF watchdog with
moving-coil instrument.
Experiment
You cannot smell electromagnetic radiation. Use this circuit to detect it within the range of 100 kHz up to 1
GHz. Normally the needle of the moving-coil meter shows no or only a weak deflection. As soon as radio
waves are received, the needle shows a greater deflection, which is analogue to the strength of the RF field.
The circuit diagram for this experiment is shown in Chap. 3, Fig. 3.119.