🚀 go-pugleaf

Winfield Hill wrote...
>
> The circuit can be easily scaled to handle many LEDs.  One possibility
> is to operate Q1 at higher currents, scaling R1 to R4 appropriately. 
> Another is to add a buffer transistor between Q1 and the bank of current-
> sink transistors.  We can do this because the Vbe reference is adjustable
> and can easily accommodate another Vbe drop. 
>
>=    1.69V               power supply
>=    ,----+-- R3 --+---- 3.4V to 6V or more
>=    |    |  1.8k  |                            multiple 30mA
>=    |    R4      R6                            white LED driver
>=   R2b   62      27                            using 2n4401
>=   68    |        |       white    white
>=    |    | 1.63V  |       LED a    LED b ...
>=    |    |        C         |        |
>=   R2a   +----- B           | 30mA   | 30mA
>=   3.3k  |    Q2  E         |        |
>=    |    C        | 0.96V   C Q3a    C Q3b..
>=    +--B          +------ B ------ B ------- many many more...
>=    |    E Q1     |         E        E
>=    R1   |        R5        |        | <- 0.20V
>=   2.2k  |       1.0k      R7a      R7b
>=    |    |        |         |        |
>=    '----+--------+---------+--------+--- etc
>=                           all 6.8 ohms
>
> As before R4 cancels changes in Vbe-Q1 with supply voltage.  If you
> desire, R2 can be trimmed for exactly 30mA with your transistors,
> this worked out to 3.368k for Spice's 2n4401 parts.  R6 protects Q2
> by limiting the transistor-array base drive to 100mA, which should
> be enough for perhaps 500 LEDs.  :>)

 Or better yet, use a trusty LM358 (miniDIP version of the 324) for
 a simple yet accurate low-dropout multiple-LED current source.

=                             power supply
=    ,-----------------+---- 3.3V to 6V or more
=    |                 |                          multiple 30mA
=    R3                |       white    white     white LED driver
=   22k                |       LED a    LED b
=    |           LM358 |        | 30mA   | 30mA
=    |                _|        |        |
=    +-- R2 --+------|+ \       C Q1a    C Q1b..
=    |  36k   |      |   >--- B ------ B ------- many many more...
=    |        |   ,--|-_/       E        E
=    | LM385- |   |    |        |        | 2n4401
=  \_|_1.2   R1   '----|--------+        |
=   /_\     4.7k       |        |        | <- 0.14V
=    |        |        |       R5a      R5b
=    |        |        |        |        |
=    '--------+--------+--------+--------+--- etc
=                               all 4.7 ohms

 The LM385-1.2 is one of my favorite parts, a jelly-bean reference
 IC that should be in every parts cabinet.  If more than 50 LEDs
 are driven, Q2-style buffer can be added as in the previous design.

 Thanks,
    - Win