Project 10: Wateralarm
Make a liquid level trigger an alarm
Required parts: 9V battery, 10K resistor (brown black orange gold), 470K resistor (yellow purple yellow gold), buzzer, 2x BC547 transistor, 2 wires
How it works: The 2 sensor wires must be placed in a tank at a certain distance (e.g. use a drinking cup). Fill up the tank with a conducting liquid (e.g. water) until the level reaches * both sensor wires. A small current will flow via R2 towards the base of . T2. The base is protected against interférence with a resistor R1.
T1 S T2 are configured as a Darlington switch hence only a very small current is needed to make T1 conduct and activate the alarm sound.