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4QD-TEC
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Just to prove that transistors can also be useful in logic situations; this is a data latch and indicator which also is a data switch and has a bidirectional input/output port which can be read or switched.
I should warn that I never actually made it up - so if you find problems with it you'd best let me know!
If the second transistor is off, a small currrent will flow via the LED, 12K and 220K to make Tr1 conduct, keeping Tr2 off. At the same time, the 4n7 charges up. Now, if the switch is closed, the current pulse of the 4n7 discharging should bring Tr1 out of saturation (as it has only limited base drive) and cause Tr2 to turn on. The LED lights and as Tr2 is on, no current flows into Tr1's base, so the circuit has changed state.
The circuit's state can be read from the voltage at the input and if the input is forced high or low,the LED will be forced on or off.
Of course you could use relay instead of the LED.
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