Velleman EDU01 - Manual (Page 29)

www.vellemanprojects.com Pag. 29
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
interference with a resistor R1.
T1 & T2 are configured as a Darlington switch hence only a very small current is needed
to make T1 conduct and activate the alarm sound.