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
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.