Maker Factory ELECTRONICS EXPERIMENTS BOX Manual page 51

Circuit 28: 20 seconds switch-off delay
Setup information
Install the two transistors and the two diodes as shown in Fig. 2.28. Then install the four resistors and the
electrolytic capacitor. The seven jumpers come next.
The jumper acting as the switch (shown dashed) is installed last. Only one end of the jumper wire is con-
nected, the other is open. Check the placement on the board again and only then connect the battery.
Experiment
Even if everything is wired correctly, nothing will happen at first; the red LED remains dark. But if you close
the switch, that is, you insert the jumper wire for a moment, the LED will go on. But when you open the
switch, the LED will not turn off immediately but will glow for more 20 seconds. This kind of circuit is used
in staircase lighting. The circuit is triggered by a push button, and turns on the stairwell light. After a con-
venient delay time of 1 minute, for example, the light is switched off again.
The circuit diagram for this experiment is shown in Chap. 3, Fig. 3.28.
Components required:
2 x 2N3904 transistors,
1 x red LED, 1 x 1N4148
silicon diode, 2 x 1 kΩ
resistors, 2 x 100 kΩ
resistors, 1 x 100 μF
electrolytic capacitor,
1 x breadboard, 1 x 9V
battery with clip con-
nector
Fig. 2.28: 20 seconds switch-
off delay
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