Simple Electronics

[extropalopakettle]

First, I don't know much about it. I'm hoping someone else does. What I'd like to do here is start out slow and have an open discussion about basic electronic, electonic components and circuits. Things like capacitors, inductors (wire coils, I think), resistors and maybe trannsistors. No chips - too high level. Then, what can you build with them?

For starters, I'd like to build a simple (as simple as possible) circuit that I can hook a battery and speaker to, and produce a tone. I've found plans on the web, but they use chips. I don't want to use chips, as they contain many components within them, and I'm trying to go for simplest possible solutions.

Next, I'd like to be able to vary the frequency of the tone.

Then, I'd like to have the tone shift automatically from high to low, kind of like a siren.

Any takers on helping out?

Maybe first just cover basic properties of capacitors, transistors, inductors, etc.

[Ghost Post]

i remember building such a circut in "basic electricity" which was a semi-required class in Junior High. i might still it.

[MacadamiamaN]

Perhaps taking apart an old alarm system from 50 years ago might help.

[Bicho the Inhaler]

I'd like to see high-pass and low-pass filters explained. I've never seen those explained to my satisfaction.

I can help you out with capacitors:

A capacitor is something that stores a charge. It's ability to store charge is indicated by a quantity called capacitance, abbreviated C.

Q = C*V

Q is the charge stored, and V is the voltage difference on the terminals of the capacitor.

Figure one: a simple circuit with a capacitor.

code:

---

           Capacitor


            ||


 -----------||---------


|           ||         |


|                      |


|                      |


|        | |           |


 -------||||-----------


      +  | |  -


         Battery

---

The physics of a capacitor are extremely simple. The most basic kind is the parallel plate capacitor. It's just two plates of some good conductor very close and parallel to each other.

Figure 2: close-up of a capacitor.

code:

---

        to positive terminal


          of battery


             |  |


------------------------------


|                            | plate 1, positively charged


|+  +  +  +  +  +  +  +  +  +|


------------------------------





------------------------------


|-  -  -  -  -  -  -  -  -  -|


|                            | plate 2, negatively charged


------------------------------


             |  |


           to negative terminal


         of battery

---

The picture above shows a parallel-plate capacitor "in action." Based on figure 2, it appears that the circuit in figure 1 is an open circuit. There's no path for current to flow between the terminals of the battery. There's a gap between the plates of the capacitor! However, it's not quite a useless piece of junk. If I apply a voltage difference across a capacitor, as in figures 1 and 2, it will in fact cause some charges to move. That's because the plates of the capacitor are so close together that a positive charge on the top plate can induce a negative charge on the bottom plate by electrostatic attraction of opposite charges and vice versa. (The charges on the two plates of the capacitor are always equal in magnitude and opposite in sign.) This is a stable configuration, and applying the voltage will cause this to occur. A large voltage will cause more charge to be induced. Hence Q = C*V.

Actually, what happens is that as the charges accumulate on the plates of the capacitor, the capacitor starts to produce its own voltage. You can see that this acts in opposition to the battery:

code:

---

           Capacitor


       +         -              + charge => + voltage


    charge  || charge           - charge => - voltage


 -----------||---------


|           ||         |


|                      |


|                      |


|        | |           |


 -------||||-----------


      +  | |  -


         Battery


         





Conceptually:








         +  | | - 


 ----------||||--------


|           | |        |


|                      |


|                      |


|        | |           |


 -------||||-----------


      +  | | -

---

When the voltage produced by the capacitor equals the voltage of the battery, the circuit is in electrostatic equilibrium. No charges move, and the capacitor stops accumulating charges. So in short, the amount of charge Q stored by a capacitor equals the amount of charge necessary for it to produce a voltage difference equal to that of the applied voltage, V.

Using that explanation, you can derive an expression for the capacitance C of a parallel plate capacitor in terms of the area of the plates and the distance between them, but I won't do it for brevity's sake.

I could also do transistors, which are very cool, but I'm going to get off the computer now, so I'll give someone else a go at it.

[Bicho the Inhaler]

By the way, that explanation applies only to capacitors in DC voltage. I would very much like to see an explanation of AC circuits.

[extropalopakettle]

I'd also like to have a better understanding of high-pass and low-pass filters. Particularly, most stereo speakers have several different size speakers inside, each which produces a certain range of frequencies best. And, they have a crossover circuit, which splits the audio signal into high and low frequencies at some point, sending high frequencies to one speaker, low frequencies to another. Of course, once you can do a two way split, you can do multiway.

My understanding is also that capacitors act as frequency depenndent resitors in AC circuits. That is (I think), they let AC above a certain frequency pass through, while appearing as resistors to lower frequencies.

[Lucky Wizard]

Transistors Explained

If this is not detailed enough for you, I'll let Bicho handle it, because I don't know much about them other than what's described there.

I also don't know inductors.

Resistors, though... I can explain them.

No wire is perfect. In every wire, there is resistance that dampens current. The effect of this is that a battery is needed to produce current. The current is determined by Ohm's Law. If you consider the electric potential on one end of the wire, and the potential on the other end, the difference between the two is the voltage V across the wire. If you know the resistance R of the wire, you can determine the current I across the wire by using the formula I=V/R.

Since the resistance of a wire is (usually -- there are some useful exceptions) constant, and the output of a battery is usually constant, the current is what gets determined.

So how does this come into play? Well, sometimes you want to deliberately create resistance. In which case, you substitute the wire with another component -- a resistor, which is specifically made to create resistance. This is useful for several things. For instance, ever wondered how windshield wipers work? There's a circuit similar to this:

code:

---

   -------  -----/\/\/\/\----|


   |       /                 |


   |                         |


   |       |                 |


  ---      |                 |


   -       |                 |


  ---      |                 |


   -       |                 |


   |       |                 |


   |       |                 |


   |       |       | |       |


   --------|-------| |-------|


                   | |

---

That jagged thing at the top is a resistor. In addition, you'll also see the battery and a capacitor. The capacitor has no charge because the circuit is not complete. This is called an RC circuit. Note the switch at the top. If you use it to connect the end leading to the battery to the end leading to the resistor and capacitor, a current immediately starts flowing through the resistor and into the capacitor. At the very beginning, these things are true:

• The current is very high because charge is trying to get to the capacitor to equal the battery's voltage
  
• There is no charge in the capacitor
  
• Because of the lack of charge, it also has no voltage
  
• Because the current through the resistor is so high, the resistor has high electric potential. In fact, that's where the difference between the battery's voltage and the capacitor's voltage is -- it's in the resistor.

Then things change. Charge reaches the capacitor, so the current slows down, the capacitor increases in voltage, and the resistor's potential difference decreases to match the difference between the battery and the capacitor -- and also the new product of current and resistance. This process continues indefinitely. After infinite time, the current will be zero, the capacitor will be charged up, and all the electric potential will have shifted from the resistor to the capacitor. Of course, it's never going to reach that, but eventually it will be close enough. (Incidentally, the product of a resistance and a capacitance is equal to a time. By multiplying resistance and capacitance for that particular circuit, you get a time called the time constant; if at some point we know that the resistor's potential is V, then after the time constant has passed once, the potential will be approximately 37 percent of V.)

When it's close enough to equilibrium, we move the switch -- now the battery's on the open end of the circuit, and the end with a resistor and a capacitor is now connected to the end with nothing on it. The result? There's no longer anything driving the charge in any direction, so charge on the capacitor immediately starts to redistribute itself equally. This causes the current to get very high again -- BTW, the current is going "backwards", so it's considered negative -- and the resistor's potential difference changes to be a negative number equal to the product of the new current and the resistance. In fact, throughout this part of the cycle, the resistor's potential difference is equal to -1 times the capacitor's voltage. But because current is causing the charges to leave the capacitor, the voltage of the capacitor is also decreasing, as is the current. As the current decreases, the resistor's potential difference gets closer to zero, so the sum of the resistor's potential difference and the capacitor's voltage remains zero. After infinite time, all the charge has been redistributed, the capacitor is uncharged, the potential on both capacitor and resistor is zero, and there is no current. But if we flip the switch again when it's close enough, then the process starts again. In fact, we can create a periodic cycle by automating the flipping of the switch.

How does this come into play in windshield wipers? Well, if we add an extra component to measure the circuit throughout this cycle, then we can trigger something -- in this case, wipers moving -- to happen when a certain treshold is reached.

And resistors have tons of other uses, though the only one I'm very familiar with is RC circuits.