User Manual

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Required parts: 9V battery*, 3 x1K resistor (brown black red gold), 10K resistor (brown black
orange gold), red LED, 2 x push-button, 1x BC547 transistor, 1x BC557 transistor, 5 x wire
jumper
How it works: The “START” button will
light up the LED; it will remain on when
the pushbutton is released. To switch off
the LED, press the “STOP” button.
T1 and T2 are in state of rest (OFF, no cur-
rent). By pressing the “START” button a
current flows via R4 through the LED. At
the same time the base of T2 is pulled low
(was high via R1). Since the value of R3 is
much lower than R1 the voltage on the
base of T2 drops making it conduct and
via the collector of T2 and R2. T1 also
starts to conduct. From this point on, both transistors keep each other in conducting state,
even when the “START” button is released. Pressing the “STOP” button will end the current flow
towards the base of T1 and it will stop conducting. The circuit is interrupted and T2 will also stop