45
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.
Components required:
2 x 2N3904 transistors,
1 x green LED, 3 x 1 k
Ω
resistors, 1 x 2.2 k
Ω
resis-
tor, 2 x 100 k
Ω
resistors,
1 x breadboard, 1 x 9V bat-
tery with clip connector
Fig. 2.20:
A bell with a memory
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.
Summary of Contents for ELECTRONICS EXPERIMENTS BOX
Page 4: ......
Page 265: ...267 Fig 3 56 Cir cuit 56 Audio limiter Fig 3 57 Circuit 57 Amplifier for a storm detector...
Page 269: ...271 Fig 3 64 Circuit 64 Stabilizer for 1 9 V Fig 3 65 Circuit 65 Stabilizer for 2 5 V...
Page 275: ...277 Fig 3 80 Circuit 80 Quiztimer 30 seconds Fig 3 81 Circuit 81 Handy diode tester...
Page 315: ...317 Fig 3 179 Circuit 179 HF generator 150 kHz Fig 3 180 Circuit 180 HF generator 420 kHz...
