Soil water shortage prompt circuit

The circuit of the low-voltage soil water shortage informer is as shown in the figure. The humidity detection circuit is composed of a
3kHz square wave signal generated by a multivibrator composed of VT1, VT2, etc. , and is passed through the load compensation network composed of current amplification VT3 and VT4 to provide detection of the soil
resistance current. The current passing through the soil resistance is AC, and the DC component has been blocked by C4, so the two probes will not suffer from electrochemical corrosion
. This square wave signal also causes the buzzer BZ to emit a 3kHz prompt sound when the soil is dry. The normal square wave
signal     of soil moisture is added to the base of VT5 through the soil resistance between El and E2. Potentiometer RP is used to adjust detection sensitivity. When the soil moisture is normal, the soil resistance value between the probes is very small, and the 3kHz signal produces a large voltage drop at both ends of (RP+R7). Its positive and negative half cycles keep VT6 in the cut-off state, so the buzzer BZ is not working. During the positive half cycle, VT5 is saturated and conducts, causing G to discharge quickly, and VT6 is cut off; during the negative half cycle, VT5 is cut off, and G begins to charge. The charging time constant is     t= Rs×G.     This time constant is relatively large, and the voltage at both ends of C has not yet risen. Before it is high enough to turn on VT6, the positive half cycle arrives again, turning on VT5 , and c5 quickly discharges, resulting in VT6 always remaining off. The diode VD is turned on during the half cycle of the signal level to protect the base emitter junction of VT5 from reverse breakdown.     When the soil is dry, due to the increase in resistance between the two probes, the voltage applied to the base of VT5 decreases. If it is not small enough to turn on VT5 , then VT5 is cut off, VT6 is saturated and turned on, and the oscillation signal emits a prompt sound through the buzzer.     The 6 transistor model is BC550C, available with 9014 and 3DG6 tubes. BZ can use a small 1.5V piezoelectric buzzer, fixed with tape at the top of the circuit board near VT1 and VT2. The circuit is powered by a 1.5V dry battery, and the quiescent current of the buzzer is o. 1-0. 2mA. Both probes are made of 15cm long qilmm enameled wire. One end of the probe is stripped of paint and soldered to points El and E2 respectively. The other end is stripped of paint and 4cm long and is coated with solder to prevent oxidation after being inserted into the soil . The two probes should be straight and of equal length.