http://www.discovercircuits.com/PDF-FILES/SHADOWDET1.pdf
In the circuit given in the url, can anyone check for me how is C4 connected?I use a 9V battery,should i connect its negative straight to the ground or to the C4 before it goes to ground?This circuit looks simple but i find it confusing
Thank you very much mr mike,Another question i would like to know …are all pin photodiodes the same?can i use a pin photodiode of a different type for this circuit?

I am making a circuit that will have a 9v battery with the positive lead connected to a 7805 5v regulator. What transistor (npn) can I use that will act as an and gate ( what transistor will switch instead of amplify at 5v)? After that it is going to a four bit adder; if a charge were added to only the base, or only the collector, would a small charge be passed through that could trigger the adder.

I am confused how you can tell if transistor amplifies or switches at a certain voltage.

Does anyone know a transistor sold at electronic goldmine that would fit my needs.

If you find any transistor, could you tell me what resistor I would need for it (if at all) and link to it. If there are many that would work could you link to the cheapest one.
Any transistor can act as a switch or as an amplifier. "Yes but at different conditions, I want one that will act as a switch under my conditions."

What is driving the transistor? What does the regulator have to do with that? "The battery is driving both the collector and base (after switches) the regulator drops the battery down to five volts. I said 9v to 5v instead of 5v because 7805 might have an on board resistor, then you would know the voltage and amperage."

"fit my needs"? you need to tell us you need.
"A transistor that works as a switch at 5 volts, does not leak enough energy to trigger an adder, npn; hopefully is sold at electronics goldmine and cheap/small."

Please don't answer with questions justto get points.

I’m trying to build my first electronic circuit, and I have a couple questions about it.

In this circuit:

http://img242.imageshack.us/my.php?image=fs9pv77fjd365×2mediumqa4.jpg

is the ground just a path back to the negative pole of the 9v battery? If so, does it matter the order in which I connect the grounds back to the negative pole? If not, what exactly is it?

My google research made it sound like people use the term ground ambiguously.

Thanks

Here is a question on my homework that is very puzzling:
So you have a 9V battery that is connected to 4 light bulbs. First is a parallel circuit with a 50w and 25w bulb. Next is a single 40w bulb, and last is a 25w bulb. What is the current that flows through these, and how does the voltage change?