Is it possible to use a PIR detector to switch something on and keep it on without a microprocessor?
If the PIR detector output can activate an electromechanical relay (such via an open collector transistor) then, one of the spare contacts in the relay can be used to short out the open collector transistor and keep the relay self-activated.
Alternatively, If the PIR detector has a voltage output then it can be diode-orred with the spare relay contact in order to keep the relay activated when the PIR signal falls away.
Another contact on the relay can switch 12 volts to whatever load is required.