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Circuit 20: A bell with a memory
You can connect the circuit to a standard bell button and a gong.
Setup information
Install the two transistors as shown in Fig. 2.20. Then install the green LED. Now it is time for the six resis-
tors. Finally insert the five jumpers and the jumper acting as the Off switch.
It is important that you are familiar with the relevant safety guidelines according to the VDE and know how
to observe them. If this is the case you can now connect the circuit to the bell button and the gong.
Check the placement on the board again and only then connect the battery.
Experiment
If everything is wired correctly, both "switches" are open, which means that the bell button is not pressed
and one end of the jumper wire is not inserted. Press the bell button briefly. The gong sounds and the green
LED light is steady on. When releasing the bell button, the diode will continue to glow.
The circuit (called bistable or two-state flip-flop) will remember that at some point the bell button was
activated; for example, when you were not home. This will tell you that you had at least one visitor while
you were out. You can reset the circuit, by momentarily "close" the Off switch. This will turn the green LED
off. The circuit is on standby.
The circuit diagram for this experiment is shown in Chap. 3, Fig. 3.20.
Components required:
2 x 2N3904 transistors,
1 x green LED, 3 x 1 kΩ
resistors, 1 x 2.2 kΩ resis-
tor, 2 x 100 kΩ resistors,
1 x breadboard, 1 x 9V bat-
tery with clip connector
Fig. 2.20: A bell with a memory
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