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2.56 Sensor toggle switch (finger)
Circuit 121: Sensor toggle switch with LED
Setup information
Install the three transistors, the diode and the six resistors as shown in Fig. 2.121. Finally insert the eight
jumpers and the two jumpers used as switches (shown dashed). For your guidance we have labelled the
"switches" with "Tab On" and "Tab Off" accordingly. 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 one end of our two jumper
wires is not connected. "Close" the jumper marked Tab on for a moment. The red LED is steady lit. If you
"open" the Tab On switch again, the diode will continue to glow. The circuit is so responsive that it is
enough to bridge the two bare wire ends of the "switch" with a grease-free finger. The red LED is accordingly
switched off again by the Tab off switch.
The circuit (called bistable or two-state flip-flop) will remember that at some point the Tab off switch was
momentarily activated. You can reset the circuit by momentarily "closing" the Tab off switch. In both cases,
the touch of your finger is enough. This will turn the red LED off again.
This is a fundamental building block in modern digital electronics. A bistable flip-flop stores a piece of infor-
mation for an unlimited time. It knows two stable states, namely On and Off (digital technology calls these
states high and low or 1 and 0). Each of these states can be triggered by an external control signal. Quartz
watches, mobile phones and in particular Tablets and PCs have these basic circuits in large numbers.
The circuit diagram for this experiment is shown in Chap. 3, Fig. 3.121.
146
Components required:
2 x 2N3904 transistors,
1 x 2N3906 transistor,
1 x red LED, 1 x 1 kΩ
resistor, 1 x 10 kΩ resis-
tor, 1 x 47 kΩ resistor,
1 x 100 kΩ resistor,
2 x 220 kΩ resis-
tors, 1 x breadboard,
1 x battery, 9V battery
with clip connector
Fig. 2.121: Sensor toggle switch
with LED
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