Dmitry Kiryashov wrote:

> I don't understand why you using PNP transistor ?
> Up to me it isn't necessary. (motor return directly
> to base of NPN or trough some diode or resistor)

  It's based on the premise that the base-emitter junction of a
transistor carrying a certain collector current is the best analog
(match) of teh base-emitter junction of another transistor carrying the
same current (even if one is PNP and the other NPN).  That is why two
22k resistors are used.

  The same effect may in all probability be obtained by returning the
collector of the PNP to its base, and this being the case, a matching
NPN transistor could be used with its base and collector joined.  That
would turn the circuit into a typical current mirror.

 +5V o--+-------+---+
        |       |   |
        <       <   <
    22k <   22k <   < 4k7
        <       <   <
        |       |   |
 Pic o--+       |   |
        |       |   |
         \|     |   |
     NPN  |-----+   |
         v|     |   |
        /       |   |
       |        |   |
       +--------(---+
       |        +--+----+
       <           |    |
   47R <            \|  |
       <       NPN   |--+
       |            v|
       |           /
 Gnd o-+---/\/\/\-+----o motor return
           Rsense

--
  Cheers,
        Paul B.