Hi Walid,
You can't make a voltage regulator without a voltage reference like a zener diode or a voltage divided from a stable reference voltage. Since you are using a PNP transistor, input and output voltages in my descriptions are negative:
1) If the 10V is stable, then the 2N5401 can be used as an emitter-follower voltage reducer with the 10V divided down to about 8.2V with a couple of resistors. Since you need 1.8V across one resistor and 8.2V across the other, their ratio must be 1.8:8.2. Use 1.8k and 8.2k. With a lw load current, the emitter's voltage will be about 7.5V.
2) You can use the reverse-biased base-emitter junction of a silicon transistor as a 6V to 7V zener diode when it has avalanche breakdown. If the voltage is too low, the transistor's forward-biased base-collector diode can be added in series with it for about 0.7V more. A forward-biased silicon diode can also be added in series for about 0.7V more. Like a zener diode, the current must be limited with a resistor in series. ;D
