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Topic: Solarbotics- Schematic question (Read 781 times) previous topic - next topic


If this question does not belong in this thread then i apologize.

Ok so heres my question:

First off im basing this all off of this link (not really important): http://www.streettech.com/modules.php?op=modload&name=News&file=article&sid=1375

the schematic im referring to is: http://www.streettech.com/storypics/BEAMSEVAR.jpg

the paragraph on that page reads:

"The solar cell charges the main capacitor until the voltage is high enough for the FLED to start flashing. When the FLED flashes, current flows through the FLED and the base of the PNP transistor and it turns on. Now current passes through the PNP into the base of the NPN transistor and it turns on. When the NPN turns on the collector which is connected to the motor and the 2.2K resistor goes low (to GND)...."

Now can someone explain how a pnp transistor works? from what Ive played with using the arduino, if i make a signal HIGH into the base the pnp transistor then it doesn't work. If i make it LOW then it turns on and current flows from the collector to the emitter. So doesn't that contradict the above paragraph where the PNP is turned on by giving it a current?

my understanding of basic transistors is:
NPN< apply current to the base and it 'connects' the collector and emitter
PNP< apply current to the base and it cuts off the  collector emmiter connection.

Now clearly this isnt the case for PNP, because if i just put 5 volts to collector and the emmitter to ground ( and a led in the circuit) then nothing happens until the base is give a "LOW" signal.  

Im utterly confused, for those who actually read through this and comment i thank you in advance.


Until there is enough voltage to trigger the FLED in that schematic, the base of the PNP cannot be pulled to ground.

It gets a little interesting because of how the PNP feeds the NPN, but essentially until the voltage builds, the PNP base has a connection through the motor to positive voltage, hence why it will not turn on.  (Until the NPN turns on, voltage flows through the motor, but no current)

When current flows through the FLED, you are sinking current to ground, which is going to pull the base of the PNP towards ground, since there is now a connection (Minus the voltage drop through the FLED, but it's now a connection)

This now allows current to flow through the PNP and into the base of the NPN.

The motor that did not previously have a path to ground is provided one through the NPN, and as such will turn on.

Since that connection point is now ground from the motor, it also becomes an even stronger ground for the PNP transistor (Latching it on)

The motor will function until there is not enough voltage present to drive either transistor, and the entire circuit resets.


first off thanks

second of all: how does the current in the base get pulled to ground, i thought a current went through it (through the base and through the... emmitter?) thus turning it off.  is current getting pulled away from it? or is there no current going through the base at all?


From: Wikipedia

A small current leaving the base in common-emitter mode is amplified in the collector output. In other terms, a PNP transistor is "on" when its base is pulled low relative to the emitter.

I tend to think of it like this:

A PNP transistor supplies current in a circuit.  Pull the base low and current is sourced through the PNP and into your load, which is connected to ground.

A NPN transistor sinks current in a circuit.  Pull the base high and current is allowed to sink through the NPN into ground.


Another way to think of it is that both transistors work the same current flowing in the base causes current to flow across the collector / emitter leads.
BUT an NPN transistor uses conventional current that is current that flows from +ve to -ve.
where as
a PNP uses electron flow current this flows from -ve to +ve.

If you think about it the name emitter means it is spraying out electrons, the name collector means it is gathering up electrons and as electrons flow from - to + the PNP transistor make most sense.
In the early days there was only PNP transistors and schematics were drawn with the -ve rail at the top and +ve rail as ground. Cars used to have a "positive earth" or ground for the same reason.

Reason - Early experimenters just took a guess at the flow direction of electricity (long before it was known that electrons were responsible) and they got it wrong. We live with that mistake still.

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