>> Hadn't factored in the internal protection diodes, 
>> thanks.
>> I need to do this with 8 outputs, and only one OC output. 
>> I started with a
>> 2 transistor circuit (1 NPN + 1 PNP), but thought I could 
>> simplify it this
>> way.
>
> Yeah, or add a ULN2003/2803 ?

The ULN280x/ULN200x products are well priced and extremely 
useful. They come in SOIC and DIP (at least).

    http://www.st.com/stonline/products/literature/ds/1536/uln2805a.pdf
    http://focus.ti.com/lit/ds/symlink/ulq2004a.pdf

A few points to note:

- They are "darlington" output so the output can never 
saturate - you get at least 1 x Vbe drop to ground. 
Typically this is 0.6V at about zero output current and will 
rise with Iout.

- They do not have amazingly high capabilitty outputs. The 
data sheet shown the ULN200x as having Vout=1V typical at 
200 mA and a rather undesirable 1.5V at 500 mA. Actual Vout 
varies with input drive but the above is at a forced beta of 
10 ie Iin = 500 uA, and it's worse at lower drives. YMMV but 
may not.

- The ULN280x are no better.

- While the ULN2003 / ULN2803 are the most common the others 
in the series can be very useful. The ULN2x04 may work well 
with no input resistor and 5V drive. Examine the spec sheets 
carefully to see if a particular one meets your need.

- At under $1 in 1's and under $US0.30  1000 up it is hard 
to match the price using discretes, although discrete 
drivers will allow you achieve a better spec for a similar 
price with care.

- The suggested 2 transistor driver allows grounded load and 
no polarity inversion. The ULN2x0x give high side connected 
load and arguably polarity inversion, depending which supply 
your brain/frame of reference is wired to.





        Russell McMahon

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