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Carbon composition resistors are THE WORST THINGS TO USE in an audiophile amp as their "absolute value" tolerances range from 10-20%. Plus, their "absolute resistance" values are very susceptible to heating and cooling effects (i.e., they get warm and their resistance changes — sometimes dramatically!).

Instead of using wire-wounds, use metal-film resistors. Their tolerances are 5% or less, they're very stable overall, and aren't as susceptible to temperature effects like carbons, AND they're much cheaper than wire-wounds.

I made some mods to your circuit. With this circuit, Q1 will go directly to the load. With the load there, Q2 will turn on, it's collector will go low. The output of U1-D will be high, so U1-B and U1-C will invert the other input. If the load is shorted, Q2 will be off and the input to U1-D will be high - it's output low. So the inputs to U1-B and U1-C will be low - this will force the outputs high and turns off Q1.

You might try your digital camera, just turn it on so you can watch the display and point it at your IR source. Many digital cameras were able to do this until they discovered it would allow "seeing" underneath someone's clothing. If your new camera doesn't show any IR try an older camera or pick up an older one at a yard sale. Take some batteries with you & an IR LED you can test it with.

Alonso, all you need to see if IR LEDs are emitting is your humble camcorder or other CCD-based video recording device.  Simply kill ambient room illumination (lighting, close drapes, etc.), activate your IR source(s), then use your camcorder in "night mode" to see the IR illumination.

In fact, you can use this method to determine if your IR remotes are working or not - just point the remote at the camcorder lens and press a remote button.  You'll see the emitter end of the remote glowing/flashing in the camcorder's viewer if it's working.

Hope this helps.

Regarding Alonso’s question about needing to see infrared light, most digital cameras are sensitive to IR light, and it can be seen on the EV.

Most digital cameras are infrared (IR) sensitive. The simple test is to point an IR remote control at the camera and watch the LCD screen. For the "dead spots" or shadow tests, turn all visible lights off, then turn on your IR source. Look around the living room with the digital camera (I recommend lowering the screen brightness), and see where there is no illumination with the IR source.

I've found that my digital camera can see the IR LEDs and have used it for that purpose. Just point at the LED and look at the viewscreen.

I have had very good luck using “regular” soldering techniques, with the correct solder and flux. Set the component in place, solder pins on diagonally opposite corners. If it's not straight enough, straighten it out while there are only two pins soldered.

The link I've included shows the result from using a toaster oven, with modifications, to solder a whole board at one time. There are a number of video's on You Tube showing how to convert a toaster oven. The options are to manipulate the temperature profile manually, or you can use a thermocouple and a small computer such as an Arduino to automate the temperature profile.

I'm still doing the soldering manually.

Unless the webcam's PCB has an explicitly-labeled location called video or similar on the PCB, you'll have to use an oscilloscope to locate the raw video feed to the USB encoder chip. If there doesn't appear to be a raw video signal anywhere on the PCB, then you're out of luck.

Otherwise, once you locate that raw video point, simply get a length of shielded cable (the thinner the better) terminated with an RCA jack and (carefully) solder the center conductor to the raw video point on the PCB and the shield to a power ground location on the PCB. Insulate the shield to prevent unwanted shorts and keep it as short as possible.

Carve a notch in the webcam's body to feed the cable through, close up the webcam, and plug it into an active USB port. You should now be able to connect a shielded cable to the new RCA jack and run it to the YELLOW JACK of your video monitor and see what the camera is picking up.

Making the move to leadless soldering is a great idea, given the health concerns of lead toxiciciy — especially if you have children in the house or your workshop. It's important to note that the rosin used as a flux in solder can be just as problematic as the lead. You can end up with an asthma-like condition if you don't work in a well-ventillated area.

There are a few options for higher performance processors that are moderately compatible with basic Arduinos. My personal favorite is the Maple from Leaflabs.com. Some others are the Arduino Due, and the ChipKIT boards from Digilent. Unfortunately, all of these boards run at 3.3V, which often means changes to external hardware are needed. The Maple and ChipKIT boards have several 5V input compatible digital pins, and 5V analog input is possible using two external resistors.

All three boards will require some modification to anything but the most basic code. The Maple will require the most changes, but also has the best access to hardware features (timers, high-speed ADC, etc) due to those changes. The Due will require the fewest changes, and has the best support for standard libraries. The Maple and Due have the best documentation. If you like the look of these boards, make sure to check that libraries you use will be supported.

If 5V is a must, you may have to consider switching development platforms. Both Atmel AVR and Microchip PIC microcontrollers are available that run at much higher speeds and have better peripheral sets while still using 5V. I'm afraid I don't know as much about the AVR line, but PIC18F, 24F, and 30F are all quite powerful controllers. If you decide to go this route, be warned that the learning curve is very steep at the start. Don't forget to compare performance by MIPS (million instructions per second), not clock speed.

Another 5V solution is to use a second Arduino! Using two board lets you do twice the work at the same speed. You can communicate between them using serial or SPI/I2C with the wire library. This can be a lot of work, but it also may be the simplest solution to some problems. You may also want to simply see if you can improve your coding abilities.

Most people underestimate what can be accomplished with a basic Arduino or microcontroller. For instance, if your code has delay()s of more than 1ms, there is probably a faster way to run the same code. Unfortunately, the Arduino libraries are not really designed for the highest performance. For example, the only interrupts that are available using the core libraries are for pin changes. Many common programming techniques to improve performance rely on timer, serial, and ADC interrupts. If your code isn't proprietary, don't be afraid to post it to the Arduino forums to ask if there is a faster way to do the same thing. If you already use the best practices, they will also be able to point you toward an upgrade that fits most closely with what you need to accomplish.

Here's a nice free circuit simulator: www.falstad.com/circuit/

Linear Technology makes a nice free SPICE program for when you graduate to more advanced design methodologies.
www.linear.com/designtools/software/#LTspice

To further your education, Digilent has some nice online instruction for analog at
[url=http://www.digilentinc.com/Classroom/RealAnalog/]http://www.digilentinc.com/Classroom/RealAnalog/[/url]
[url=http://www.digilentinc.com/Products/Detail.cfm?NavPath=2,842,1018&Prod=ANALOG-DISCOVERY]http://www.digilentinc.com/Products/Detail.cfm?NavPath=2,842,1018&Prod=ANALOG-DISCOVERY[/url]

Also Analog Devices
[url=http://www.analog.com/en/university/topic.html]http://www.analog.com/en/university/topic.html[/url].

For digital try [url=http://www.digilentinc.com/classroom/realdigital/]http://www.digilentinc.com/classroom/realdigital/[/url]
[url=http://www.digilentinc.com/classroom/Electronics101/index.cfm]http://www.digilentinc.com/classroom/Electronics101/index.cfm[/url].

As a side note, as to textbooks, I'm impressed by [url=http://www.ntspress.com/publications/circuits-second-edition/]http://www.ntspress.com/publications/circuits-second-edition/[/url]. Unfortunately, it’s a bit spendy.

Here’s a site on basic car audio electronics which is very engaging: [url=http://www.bcae1.com/]http://www.bcae1.com/[/url]. This one is geared towards the car audio enthusiast who wants to learn about and repair his equipment: [url=http://www.bcae1.com/repairbasicsforbcae1/repairbasics.htm]http://www.bcae1.com/repairbasicsforbcae1/repairbasics.htm[/url].

You ask how to get "a positive and a negative part of a sine wave from a circuit that runs on a nine volt battery." It is not clear how you are going to use that output, so I'll presume this is to satisfy your curiosity (or perhaps to help on your homework). The circuit suggested is not efficient, and would not make a good power supply.

First, a 9-volt battery produces direct current, DC, with a steady amplitude of 9 volts (gradually decreasing as it is drained). One simple circuit that produces a fair approximation of a sine wave from DC is a phase shift oscillator, shown below on the left side, (adapted from [url=http://www.learnabout-electronics.org]http://www.learnabout-electronics.org[/url]). The sine wave can be observed between the Out and the 0V test-points.

Second, the sine wave must be separated into positive- and negative-going signals, which is done by the half-wave bridge rectifier. The two half-sine waves can be observed between the + or - test-points and the 0V test-point. The waveforms are shown below, (adapted from [url=http://macao.communications.museum]http://macao.communications.museum[/url]). The negative half-wave should look like the second graph flipped upside-down.
 

If this did not answer your question, you might provide some more information as to how the signals would be used.

First off, neither a capacitor or inductor is added to an antenna in order to change the length, they are added to match an impedance mismatch. In effect, this is LIKE changing the length IF changing the length would match the impedance of the antenna to that of the radio & feed-line. What you are really doing is counter acting the mismatch with the equal and opposite type impedance to make an impedance matching circuit, or a fixed antenna tuner, so-to-speak.

Many terms used by hams are misleading in that they really don't do what the term is saying they do. An antenna tuner doesn't tune the antenna, it transforms a complex reactance of the feed line and antenna into a resistance so that the radio can pass all its power to the antenna. The word "complex" is used because a mismatched antenna will look like a resistor and either a capacitor or an inductor. The radio wants to "see" just a simple resistor, a perfectly matched antenna to the impedance of the radio's antenna port.

All these terms may have come into being because someone somewhere along the line said, "Oh that's like changing the length of my dipole to match the impedance of my radio" and so the idea of "changing the length" stuck. I don't know how the term came about but it creates the idea that we're changing the length when we're really matching the over-all antenna and circuit resistance to that of the radio, and that's what we're after, a matched circuit so we can put all our radio energy onto the antenna and into the air.

Try reading, An Approach To Antenna Tuning by Lloyd Butler VK5BR, it explains this in more detail: http://users.tpg.com.au/users/ldbutler/Approach_Ant_Tuning.htm

An antenna becomes resonant when the energy that is racing down the wire hits the open end and is reflected back to the sending end and the transit time is equal to the time of one cycle of frequency. The open end has high voltage and low current (there can’t be any current at the open end) and the sending end has high current and low voltage. Adding inductance at the sending end will lower the resonant frequency thus making the wire appear longer. Adding capacitance to the open end will also reduce the resonant frequency. Since radiation occurs from the wire, adding these L and C elements will reduce the antenna efficiency because it is shorter than it would be if the L or C were not added.