An NPN_Output of a sensor

It looks like it was intended to be a pulse-limiter.

When the pulse is applied, it turns on Q1 through R19 and pulls the output low, energizing the load. Meanwhile, C9 charges up through R18 and turns on Q5 after a short delay, which turns off Q1. The Q1 output will pulse for the width of the R18/C9 timing, assuming the input pulse is wider.

R20 discharges the capacitor between pulses. D5 protects the transistors from flyback pulses from an inductive load.

I don't really know what the purpose of Q4 is except to provide a soft-off trickle current through the load when the pulse-limiter kicks in.

What kind of sensor is it, and what is the intended load?
Maybe Q4 provides some hysteresis for the pulse-limiting switch-off event. The rising charge on C9 starts turning on Q5 which starts turning off Q1, but it will happen relatively slowly. As the Q1 collector voltage rises as it is turning off, Q4 will conduct this voltage level right onto the capacitor and force Q5 to shut off more rapidly and therefore Q1 to shut off more rapidly.

I'm sure someone will model this in PSPICE.