Hi,
The advantage of the single pnp output transistor that Dave chose is that it's base resistor is simply grounded by the open collector output of pin 7. All you really have to be aware of with pin 7 is Ic max and Vce Max. Couldn’t be any simpler.
(maybe a pull up resistor for the base of the pnp, but it seems not to need it )
Mr Smiley
Ok, here's a perhaps stupid question for you. What would be required to use an NPN transistor?
You have three options if you want to use a npn transistor,
1/ Calculate the required base resistor and drive it from the original Dave Cad drawing from pin 3 which if i remember correctly is a totem pole output.
2/ Calculate the required base resistor for the npn power transistor, connect that between pin 7 and Vcc, then connect the power transistor base to pin 7, problem with that is that when the power transistor is on; when pin 7 is open you draw Ib through the power transistor. And when pin 7 is on, pulled low, you still draw Ib, but this time it's just dumped to ground and wasted.
3/ A pnp driving a npn using pin 7. A pull up resistor from pin 7 to Vcc ( say 10K) a calculated base resistor from pin 7 to the base of a pnp transistor, the collector of the pnp connected to the base of the npn power transistor through a calculated base resistor for the power transistor and possibly a pull down resistor from the base of the power transistor to ground.