This circuit allows the load to be connected to the input voltage only if the input voltage is between 10-15V. This type of protection is useful for circuits like linear regulators where too low a voltage makes the regulator useless and too high a voltage can set the pass transistor on fire. It could have other applications too.
When the input voltage is between 10-15V the 2 op amps' outputs go high, which turns on the PMOS. The BJTs and 10k resistor form an AND logic gate. When both op amps go high, the BJTs turn ON and the output of the AND gate is equal to logic 0 and the PMOS turns on. When either of the op amps output 0V, the corresponding BJT(s) turn(s) off and the output point is pulled high by the 10k pullup.
Note:
The odd values of resistance (2k & 5k) can be obtained using potentiometers.
Op amp output is set to 19V (20V - 1V dropout). And real world op amps are not magically powered by a fixed voltage.
A zener may be needed to protect the gate of the MOSFET from over-voltage.
MOSFET used: IRF4905 http://everycircuit.com/circuit/5282567524450304
