This circuit can be used to detect the high and low resistance of a speaker. Which can be indicated by the tone heard on the loudspeaker being tested. When the speaker being tested has a low impedance the resulting tone will be low. conversely, if the loudspeaker impedance is high then a high tone will sound too.
This circuit will automatically turn off the power supply source to IC 555 when it detects resistance above 200k ohms. This means that resistance of this magnitude is the same as when the circuit is not used. This circuit is very practical if operated using batteries.