On 10-Jan-02 Roman Black wrote:
> jim wrote:
>>
>> Well then, physics must work differently in my part of  Texas than
>> anywhere else, because this very circuit works well.  I can't explain it
>> beyond what I've already stated.
>>
>>  And I agree about the PNP transistor.   I also stated this a few emails
>> back.
>
>
> Sorry Jim, I can't see how your circuit works
> either. :o) You say it turns on whenever the
> base voltage is 0.7v greater than the emitter,
> but the emitter is supplying the PIC power! How
> can the PIC drive the base at Vcc+0.7v???
>
> Using the PNP is viable on the ON state as the
> PIC can sink to the base and turn it on. However
> it will be OUT OF SPEC in the OFF state as the
> base is at high potential of input voltage-0.7v.

Infact this can cause the transistor not to turn off completly.  Easily fixed
by putting a diode (to drop another 0.7v) in series with the base and the base
resistor to the PIC pin.


Peter

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