Here's the updated schematic. Does anyone know what the PNP base resistor value should be? I'm coming up with 3.2K, but I'm not quite sure if that is correct.

 

The base resistor must go to ground (negative)

And you were told that the transistor is redundant.

I would think the base resistor would need to be about 220 ohms.

EDIT:
Incorrect schematic removed.

 

The base resistor must go to ground (negative)

And you were told that the transistor is redundant.

How is it redundant? Here is the scenario... Battery gets charged all day. Night comes and you decide to flip the switch and light up the cockpit. Well, after a few drinks and some great conversation you get tired and head into the cabin to sleep... completely forgetting to flip the light switch off. Thankfully, when the sun rises the next morning the PNP transitor has cuttoff the LEDs and the solar panel continues to charge the battery.

 

The way you have it configured the LEDs will light whether the battery is charging or not.

The PNP will be on because it is connected to the battery.

 

The way you have it configured the LEDs will light whether the battery is charging or not.

The PNP will be on because it is connected to the battery.

If the PNP base is receiving the proper amount of power from the solar panel, how will the collector make a connection to the emitter?

 

Ok, so maybe I'm misunderstanding something here, so where does the PNP get its negative from to turn on?

 

Ok, so maybe I'm misunderstanding something here, so where does the PNP get its negative from to turn on?

Ahhh... I see your point. There’s no negative connection with the switch set to the off position. Perhaps a light sensor would make more sense...

 

Actually, I was referring to the fact that the transistor has no negative bias to turn it on.

 

Actually, I was referring to the fact that the transistor has no negative bias to turn it on.

Wouldn’t there be a negative bias on the opposite side of the LED’s if the switch was removed?

 

Yes, but a PNP needs a negative bias at the base to conduct from emitter to collector.

 

Yes, but a PNP needs a negative bias at the base to conduct from emitter to collector.

When it’s sunny, the output of the solar cell is high at the transistors base, which opens the transistor and switches off the LEDs.
When it gets dark, the solar cells voltage drops to zero, the current flows out the transistors base and through the solar cell to ground, this closes the transistor letting the current flow through the LEDs.

 

Your total LED current is 150mA. When a transistor is used as a switch, the rule of thumb is to assume a current gain of 10, to ensure the transistor switches on hard. Therefore the required base current will be 15mA, hence the base resistor will be (3.7-.7)V/15mA = 200Ω. Use 220Ω (as ES suggested in post #22).
What is the maximum voltage output of the solar cell? There is a risk that it could cause reverse breakdown of the transistor base junction unless the voltage is clamped (with, say, a zener diode from base to ground).

 

When it’s sunny, the output of the solar cell is high at the transistors base, which opens the transistor and switches off the LEDs.
When it gets dark, the solar cells voltage drops to zero, the current flows out the transistors base and through the solar cell to ground, this closes the transistor letting the current flow through the LEDs.

Thanks, I completely overlooked that path.

 

Your total LED current is 150mA. When a transistor is used as a switch, the rule of thumb is to assume a current gain of 10, to ensure the transistor switches on hard. Therefore the required base current will be 15mA, hence the base resistor will be (3.7-.7)V/15mA = 200Ω. Use 220Ω (as ES suggested in post #22).
What is the maximum voltage output of the solar cell? There is a risk that it could cause reverse breakdown of the transistor base junction unless the voltage is clamped (with, say, a zener diode from base to ground).

On a real sunny day I’ve seen the voltage go up to 6V. Manufacturer states that it is a 5V panel.

 

If 6V is the real maximum then a zener is unnecessary.

 

If 6V is the real maximum then a zener is unnecessary.

Alec_t, thanks for all the great info. Just out of curiosity, at what voltage would you begin to consider using a zener diode?

 

Alec_t, thanks for all the great info. Just out of curiosity, at what voltage would you begin to consider using a zener diode?

You can have zener's work down to close to 1V, but you need to also be aware that zeners tend to require at least 10mA in order to function. Also, are you preventing too much current from flowing into your LEDs? a 1-ohm resistor won't protect them.

I agree, you learn more by doing, but so far, you seem to be letting others kind of build it for you. But, you have what's most important, the desire, and you are asking questions, so keep going. But I *urge* you to get this book:

Understanding Basic Electronics
(Publication No. 159 of the Radio Amateur's Library) 1st Ed.
Author: Larry D. Wolfgang

ISBN-13: 978-0872593985
ISBN-10: 0872593983

It is a very fun text, and will answer literally all the questions you have, with examples and problems. Not only will it give you a means to learn in an organized way, it will give you a valuable reference you can go back to, again and again.

 

at what voltage would you begin to consider using a zener diode?

You don't want the base-emitter junction to breakdown. For the transistor you're using, and just about every one I've ever checked, that's 5V.

Even small reverse currents can kill beta.

 

Thanks dl324, the specs clearly state that the base max voltage for transistor 2N4403 is indeed 5V. As suggested, I've added a zener diode (1N4732) to the circuit which has a breakdown voltage of 4.7V. Here's an updated schematic. If anyone sees any issues, please let me know. Thank you all for your help!

 

An alternative to using a zener would be to connect a normal silicon diode in series with the 220Ω resistor. This would have the advantage of not wasting solar cell power in the zener.