I'm on my phone, looking at this circuit and i can't help wonder why he used a pnp, could if not simply be replaced by moving the resistor to ground via the emitter that way it would give a high signal for an npn?
No, if you want to use a npn, add a third stage with an npn transistor.
The 220uF should last for some time, so there should be a high gain to drive the light.
The way the npn and pnp are used is to multiply the gain.
The voltage is 3V, that is another reason for the npn + pnp circuit, since it will work with a very low voltage and doesn't waste a lot of current.
2 npn's will amplify (a darlington pair).....
I'll build two circuits... this one and a 2nd npn with the resistor placed on the low side of the first transistor to give a high on a 2nd npn.
A darlington will amplify, I didn't understand that from your first post.
The capacitor voltage has to be 1.2V for a darlington and only 0.6V for a single npn transistor. The circuit is designed to work as long as possible with a low battery.
Here's the changes i made..
I've not emulated or built it.... but considering how long it just took me to use ms paint to edit that thing to do that circuit, i'd have been better of emulating the circuit..
ohhh right, it's all about energy conservation ...
The idea was to use 2 npns, but if it's purely down to the voltage drop (which in turn would lower the current draw (lower voltage) )
screw it, use an LED!
Darlingtons will waste 0.8 to 1.1 volts depending on current level, using common-emitter
stages like this wastes perhaps 0.15V so the bulb isn't dim. Darlingtons are a poor choice
when the load current is fixed since they waste voltage/power, but a good choice when the load
is variable/unknown as they are trivial to bias for a wide range of current (without wasting bias
current).
A discrete pair of NPN/PNP driving a 1A load might have 50mA of bias to the second stage
to guarantee saturation, which will flow even if the load is changed to one of 10mA (in other
words the circuit bias is tuned to the load current).
A darlington driving a load of 1A takes perhaps 1mA of bias which is negligible even if the
load is changed to 10mA... The same circuit will drive 5mA or 1A...