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Circuit 122: Sensor toggle switch with LED
Setup information
Install the three transistors and the six resistors as shown in Fig. 2.122. Finally, insert the seven jumper wires
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 moving-coil instrument
shows a full-scale deflection. If you "open" the Tab On switch again, the deflection stays the same. The
circuit is so responsive that it is enough to bridge the two bare wire ends of the "switch" with a grease-free
finger. Accordingly, you can reset the needle deflection to zero 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. Now the needle of the moving-coil meter returns to zero.
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.
Components required:
2 x 2N3904 transistors,
1 x 2N3906 transistor,
1 x 1N4148 silicon diode,
1 x 1 kΩ resistor,
1 x 10 kΩ resistor,
1 x 47 kΩ resistor,
1 x 100 kΩ resistor,
2 x 220 kΩ resistors,
1 x breadboard, 1 x 9V bat-
tery with clip connector
Fig. 2.122: Sensor toggle switch
with moving-coil instrument
147
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