146
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.
Components required:
2 x 2N3904 transistors,
1 x 2N3906 transistor,
1 x red LED, 1 x 1 k
Ω
resistor, 1 x 10 k
Ω
resis-
tor, 1 x 47 k
Ω
resistor,
1 x 100 k
Ω
resistor,
2 x 220 k
Ω
resis-
tors, 1 x breadboard,
1 x battery, 9V battery
with clip connector
Fig. 2.121:
Sensor toggle switch
with LED
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.
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...
