Note that I had to add a second PNP to get the required current gain.
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Just for learning purposes: have you added the smaller transistor before the BC327 in emitter-follower setup so the current drawn by its base is not wasted?

I'm slowly understanding how transistors work...

 

Ok so it's a P channel MOSFET? My local electrical parts place only had very low current such as 20mA or 7A + models. Will a high amperage model require more power to switch? Would the 33k resistor have to be changed to match the MOSFET?...

What are the part numbers of the PMOS/PFETs you can get locally? The 7A one will work provided that it Vds rating is >= |20V|. No schematic changes are needed.

 

Just for learning purposes: have you added the smaller transistor before the BC327 in emitter-follower setup so the current drawn by its base is not wasted?...

Refer to the schematic with the two PNPs:

With the panel illuminated, R1 (33K) wastes a bit of power, but the voltage at V(panel+) is ~13V. When the panel goes dark, R1 pulls the voltage at V(panel+) close to ground because D2 is reverse-biased. However, R1 can only sink a few hundred uA of current. In order to switch 300mA of load current at Q2, we need enough current gain so that a few hundred uA can switch 300mA. One transistor alone cant do that; the compound connection of two in a pseudo-Darlington connection can. The base current of Q2 goes to ground through R4.

 

Thank you for the explination.

The MOSFET model number is IRF9540N, that's a 23A model as the 7A model I've just realised costs $24.95 a piece...

 

Thank you for the explination.

The MOSFET model number is IRF9540N, that's a 23A model as the 7A model I've just realised costs $24.95 a piece...

Here is the data sheet. A bit of overkill, but it will work in the PFET circuit without mods. Pay attention to the Drain and Source connections...

 

I've built the double PNP circuit. It worked in the test board but once I soldered it up it wouldn't work. I then replaced to second transistor (the one powering the LEDs) and it worked. I put the batteries and circuit in a box and it wasn't working. Replaced that transistor again and now it's working again. The first two that are now dead were bc328 's, now I've got a bc327.

Any idea what may have killed both of them? Would shorting one of the pins momentarily kill it?

 

Would shorting one of the pins momentarily kill it?

That's usually a recipe for disaster . Semiconductors can die in nanoseconds.

 

I've built the double PNP circuit. It worked in the test board but once I soldered it up it wouldn't work. I then replaced to second transistor (the one powering the LEDs) and it worked. I put the batteries and circuit in a box and it wasn't working. Replaced that transistor again and now it's working again. The first two that are now dead were bc328 's, now I've got a bc327.

Any idea what may have killed both of them? Would shorting one of the pins momentarily kill it?

Any chance that your LED assembly is drawing more current than you think? Or could there be a large inrush current (capacitive charging) when the LED assembly is first turned on? You observed the correct CBE pin order?

 

Any chance that your LED assembly is drawing more current than you think? Or could there be a large inrush current (capacitive charging) when the LED assembly is first turned on? You observed the correct CBE pin order?

I'm only using a digital multimeter so I can't see if the PWM controller is spiking with current draw. I checked the suppliers datasheet before using the transistors so the pin order should be correct. I think as I was cutting off the extra long pins from the back of the board I may have shorted the transistor as I soldered the battery in with no switch.. I think it best I add some switches to this circuit.

 

It might be time to consider battery high V cutoff & low V disconnect.

 

Hi again, thanks for all the help. I've been looking at the MOSFET circuit and wanting to build it now I have the two PNP circuit working.
I've been looking at P channel FETs on Element14 (http://au.element14.com/vishay-formerly-i-r/irf9z14pbf/mosfet-p-60v-6-7a-to-220/dp/8648662) and I'm not entirely sure if this is a good pick or not. It's obviously over kill, but it's one of the smallest TO-220 ones I could find. Would it be alright? Would a lower amperage model waste less power? Also is the "MURS120" just a standard diode, would a 1N4001 work in its place?