Rawle Watson wrote:

<SNIP>

> Both the PIC and the transistor switch operate as they should , when
> connected independently.

> However, the following is observed when a logic probe is connected in circuit.
> The transistor remains on all the time once it is switched on by the pic
> even though you can hear a pulse at the end of the timing period on RB4.

It sounds to me like it might be thermal runaway.  Try putting a small resistor
(even as low as an ohm) between the emitter and ground.

Michael

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The strong do what they have to do and
the weak accept what they have to accept.
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