Maker Factory ELECTRONICS EXPERIMENTS BOX Manual page 142

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.
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.
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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.