Simple low/high voltage cut circuit

Here is a circuit of simple low/high voltage cut circuit.Various safety circuit (low/high voltage cut) using IC.But in this circuit we are using two transistor for this circuit.

As we saw that the entire circuit is build using two transistor and very few of other components.Transistors are used to drive the relay.And also transistor T1 and T2 used to cut the supply in high and low voltage respectively.Where the variable resistors VR1 and VR2 is used to adjust the high and low voltages.As we know that when zener diode is connected to emitter of transistor then it get back bias voltage. The variable resistor VR1 and VR2 is so adjusted that it does not connect the transistor T2 and T1 in high and low voltage respectively. The load is connected through relay RL1.

PARTS LIST

Resistors
R1, R4 = 4.7 KΩ
R2, R3 = 220 Ω
VR1 = 10 KΩ
VR2 = 10 KΩ
Semiconductors
T1, T2 = BC148
ZD1, ZD2 = 5.6V
Miscellaneous
RL1 = 18V/500Ω