Instructions

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2.7 Binary memory
Circuit 19: An electronic brain cell
Setup information
Install the two transistors as shown in Fig. 2.19. Then install the green LED. Now it is time for the six resis-
tors. Finally insert the five jumpers and the two jumpers which will function as switches (shown in dashed
lines to identify them easily). For your guidance we have labelled the “switches” with “On” and “Off”
accordingly. 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 kΩ
resistors, 1 x 2.2 kΩ resis-
tor, 2 x 100 kΩ resistors,
1 x breadboard, 1 x 9V bat-
tery with clip connector
Fig. 2.19: An electronic brain cell
Experiment
If everything is wired correctly, both “switches” are open, which means that one end of our two jumper
wires is not connected. “Close” or insert the jumper wire marked On for a moment. The green LED light is
steady on. When you “open” the On switch again, the diode will continue to glow.
The circuit (called bistable or two-state flip-flop) will remember that at some point the On “switch” was
activated. You can reset the circuit, by momentarily “close” the Off switch. This will turn the green LED off
again. This is a fundamental building block used in modern digital electronics. A bistable flip-flop stores a
piece of information for an unlimited time. It knows two stable states, namely On and Off (digital technol-
ogy 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. 3.19.