Project 7: Start-stop circuit.
Control a LED using 2 pushbuttons
Required parts: 9Vbattery*, 3x1Kresistor (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 current). By pressing the "START" but-, ton 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 IR2. T1 also starts to conduct. From this point on, both transistors keep each other in con-ducting 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 inter-rupted and T2 will also stop conducting. The LED will turn off.