This question is probably Electronics 101. I am not sure if questions this basic should be asked in this forum. If there is another site that is more appropriate, please point the way.


Attached is a solar path light schematic that I'm having a hard time understanding. In this circuit during daylight hours, the solar cell is charging the batteries and the LED is turned off. When the solar cell is no longer providing current, the LED comes on with the batteries providing current.

What I don't understand is why the transistor Q2 does not come on (and it shouldn't because we don't want the LEDs on during daylight hours) when the solar cell is providing current. I don't see why there's not enough current through Q2's emitter and base to turn that transistor on. Isn't there current through the emitter, base, through R3 and back to the solar panel?

Now don't get me wrong, I'm not saying the circuit is not correct. I am ONLY saying I don't understand why it works. Here is the website's explanation which I'm having trouble grasping:

Apparently I need to be spoon-fed this explanation because I still do not see why "this prevents current from flowing into the base of transistor Q2.

Attached Images

schematic1led.gif (31.6 KB, 37 views)

 

the key here is the diode.

Q1 Q2 are PNP transistors.

In order to turn on a PNP transistor you need a LOW at the Base.

When there is no sunlight the base of Q1 is LOW since, NO battery voltage is allowed pass the diode. The base Q2 is also LOW via Q1. Now the LED is on since Q2 is on allowing current flow.

When there is sunlight the base of Q1 is Hi, sunlight causes the Solarpanel to produce 4.5V. Which causes the base of Q1 to be Hi, Q1 is off. Now the base Q2 is high due to R3 and q2 is off aswell, therefore stopping current flow throught the led. 3.9V is applied to the batteries to charge.

Anything else ? did I miss something ?

 

No they are not! They are NPN.

You missed EVERYTHING.

The solar panel turns on Q1 through R1 when there is light. Then Q1 turns off Q2 and the LED. The diode passes current from the solar panel to charge the battery.

When it is dark then Q1 is off which allows Q2 to be turned on through R3. Then Q2 lights the LED.
The diode blocks the battery from turning on Q1.

The LED doesn't have a current-limiting resistor in series so I wonder what stops it from burning out.

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Uncle $crooge

 

Oh god! How could you miss this one?

Some of the symbols are a little wonky...

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MechTronics

World of Warcraft is a disease

 

A few years ago I traced out the circuit of my garden lights and they use the exactly same circuit. I guess the LED survives due to the fact that the two NICADs have a nominal voltage of 2.4V and they are using a yellow LED with a Vf of around 2.3V or so. Couple that with the fact that the transistor will not fully saturate, and the LED lives to glow another night.

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Sorry but I am a complete noob to electronics.

Why during daylight does the solar cell not turn on Q2? It seems like the current passes through the emitter and base of Q2 and then through resistor R3 and on through the diode. I don't understand how Q1 has anything to do with this (though I know it must).

Sorry guys I know this is something simple, elementary, and fundamental. I just don't know a better place to ask the question.

 

When Q1 is turned on by the solar cell then it shorts the base to the emitter of Q2 so Q2 doesn't have any base viltage or base current and therefore it is off.

When there is no light then Q1 is off and the current in R3 turns on Q2.

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Uncle $crooge

 

Audioguru has explained it well. Maybe you need a picture:

Attached Images

SolarLight.GIF (25.4 KB, 20 views)

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@audioguru
Thank you for putting it into easy to understand language.

@kchriste
A picture is worth a thousand words. Thank you for taking the time to annotate that diagram. It gives me more information to learn and understand what is going on in what is probably a simple circuit to most (but definitely not to me).

 

Thanks to both audioguru and kchriste for making it so clear.