Water level alarm

Posted on Dec 21, 2012

The circuit draws so little current that the shelf-line of the battery is the limiting factor. The only current drawn is the leakage of the transistor. The circuit is shown in the form of a water level alarm but by using different forms of probe can act as a rain alarm or shorting alarm; anything from zero to about 1M between the probes will trigger it. Q1 acts as a switch which applies current to the unijunction relaxation oscillator Q2

Water level alarm
Click here to download the full size of the above Circuit.

Alarm signal frequency is controlled by values and ratios of C1/R2, Pulses switch Q3 on and off, applying a signal to the speaker. Almost any NPN silicon transistor can be used for Q1 and Q3 and almost any unijunction for Q2.

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