Superpositioned

The schematic

Download the schematic

The oscillator

The first section of the circuit is an oscillator based on a flip-flop. Clock and D are both grounded while Reset is tied high. Hence, the output Q will only be high if Set (node 6) is high. When the output is low, the transistor Q1* is cutoff. This allows *node 6 to be charged with a delay relating to the system of impedances R1*, *R2, R3*, and *C3. Once the voltage at node 6 triggers Set, the output changes to high and Q1* is opened. *Node 6 then discharges out through the capacitor. Once node 6 is low enough, Set is no longer triggered and the output is automatically reset (because R is tied high) to low and the process is repeated.

The screen capture below shows node 6 charging and discharging as the blue trace. The yellow trace is the output at node 1. You can see that the output turns high when node 6 reaches the switching threshold of the flip-flop (about 1.8 volts). Right afterwards it spikes up due to feedback through C2*, but quickly starts discharging. The oscillator switches off when *node 6 returns below the 1.8volt switching voltage. Feedback through C2* draws *node 6 to ground before the process repeats itself.

In order to change the period of oscillation, adjust the value at C3*. If you would like to make the pulses longer, adjust *C2. The circuit works best right where it is at, though.

The delay and ‘sensor’

The output of the oscillator is divided down two paths. The time constants of the two delays are nearly equal and can be adjusted with the sensitivity potentiometer. The path to node 11 is the Clock input of the flip-flop, and the path to node 9 determines if there is an alarm or not.

In the capture above, node 9 high than the the clock. Hence, the flip-flop stays high when the leading clock edge triggers it to lock. When the doorknob is touched, your body absorbs some of the charge and node 9 charges slower. This can be seen in the capture below. When the clock edge rises, node 9 is not high yet and low value is locked into the flip-flop.

The alarm

The designer uses an audible buzzer in order to relay the alarm. This is also my intent for the circuit, but I use a LED in my photos because you cannot see sound. They are both attached to the inverting output of the second op-amp (Q-bar) because it is high when the alarm is triggered.

There is an endless number of uses for this circuit, but I will just name a few crazy ideas:

1. Using the intended buzzer for your hotel or dorm room. (This is a bit more impressive than the old sock trick.)
2. Connecting the output to a relay that triggers the doorbell for you house. Just make sure to put it in parallel with your standard doorbell switch. That way you can still hear the Fed-Ex man. (This one has a major cool factor when someone opens your door.)
3. Tying the output into a security or home automation system. You could have the lights turn on as soon as you touch the door handle to scare the dog away from laying on the door.

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Affordable Programmers

The AVR is well known for how simple it is to program. To start, you only need a few resistors and a parallel port. This will burn your code onto the microchip, but if you plan on pursuing larger projects you will need a more complex programmer with ISP support. This will enable the extra features and debugging support.

1. The ultra low-cost AVR programmer uses an LPT port and a few resistors, but requires special software.
2. A simple serial programmer for Linux uses several resistors and two LEDs. Programming is done with uisp.
3. More complex schematics for ISP programmers that can be programmed via AVR Studio or AVR-Dude.
4. A usb programmer that supports ISP.

A free development environment

AVR Studio is available directly from Atmel at no cost. It has an integrated simulator and programming software. However, there is only an assembler available. You will have to purchase more software for high level languages.

The gcc-toolkit

If you want to use C/C++, then the gcc-toolkit is available for the AVR, free. WinAVR comes loaded with a gcc, binutils, the avrdude programmer, simulavr, and more. On Linux, you will need to download the packages for your specific distribution or compile your own cross-compiler.

A great community

AVR Freaks is a site dedicated to the AVR line of microcontrollers. There is a large archive of tutorials and a very helpful forum.

Standing waves in a microwave

The waves in a microwave oven are standing waves. These waves are stationary in space with an amplitude changing over time.

   

With this demonstration, it is obvious that particular sections of the chips are heated more than others. In fact, these locations are located half of the wave’s length apart.

The physics of waves

We now know the frequency of the microwave and can presumably measure the length of the wave, but how are they related to the speed of light? Simple. Electromagnetic waves propagate through free space (like that in a microwave) at the speed of light. Therefore, their length is related directly to the speed of light by λ = c / f where λ is the wavelength, c is the speed of light, and f is the frequency of the microwave. Solving for the speed of light, c = λ * f.

Where do the chocolate chips come in?

Chocolate chips are perfect for measuring the distance between melted spots. The heat does not spread as quickly through them because they are not uniform. This means the melted spots will be smaller and you will have more time to measure before they all start to melt.

It is hard to tell from the photos, but there were distinct melting spots almost exactly 6cm apart. Remember, this is only half of the wavelength, so λ = 12cm. Plugging all the known variables into our equation, we get c = 12x10^-2 * 2.5x10⁹ = 3x10⁸. Not bad! The true speed of light is 2.9987x10⁸.

Notes if you replicate this experiment

• The chocolate chips only take 20-30 seconds to melt. The longer you have them in, the bigger the melted spots will be and the less time you will have to measure.
• This will not work in a microwave with a spinning carousel. In fact, the microwave spins to counteract these effects. Usually, you can just flip the carousel upside down to stop it from spinning. (Thanks Ryan)
• If you plan on putting the chips back in the bag, simply refrigerate them. Freezing causes them to stick to the plate.
• You can microwave anything that melts. (Cheese or a chocolate bar) However, chips work particularly well.

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Electrical losses in the flashlight

In this flashlight, each LED/resistor combination consumes 4.5volts at 30mA or about 135mWatts. The ballast resistor alone consumes 1.1volts at 30mA or about 33mWatts. Therefore, 25% of the power being drained from the batteries is lost in the resistor of each LED chain!!

Battery life

The ‘C’ size batteries in the PVC flashlight have a capacity of around 4 500mAh. If you build your flashlight with seven LEDs, there is a constant current drain of 7 * 30mA or 210mA. Dividing this into our battery capacity, it becomes obvious this flashlight will only stay lit for about a day. This is a far cry from the 50-100 hours claimed by commercial flashlights running on smaller ‘AA’ batteries.

Cutting your losses

As mentioned above, the ballast resistors are wasting 25% of our battery power. Electronic Design proposed a simple circuit to resolve this in a recent article. The front end of their circuit draws less than a milliamp of extra current.

The circuit is best described in two parts: one, the boost circuit function of Q1 and Q2, and two, the control circuit of Q3 and JFET1. Assume Q1 is off. With the battery voltage slightly above Q2’s VVB, a positive Q2 base current [iB = (battery voltage VBE)/RJET1] would flow. Q2 turns on, which switches inductor L1 to ground.

The end result is a 23volt pulse (as shown in Figure 2) across the series of ultra-bright LEDs. At 278kHz, the human eye cannot distinguish the difference between these pulses and a constantly lit LED. This saves even more battery power.

As the battery voltage decreases, the pulses become further apart. The brightness remains indistinguishable until the voltage falls near 2volts. (The circuit does not function well below 2volts) I doubt the PVC flashlight has this efficiency near the end of its life.

The extended battery life

According to Electronic Design, this circuit consumes an current equivalent to about 17mA. Powered by our ‘C’ cell in the PVC, this circuit could run for 265 hours! This is ten times the original PVC design.

Another advantage to LEDs is their operating temperature. Most available today can be function for hours and remain cool to the touch. Directionality is another key property. They only emit light over a relatively small angle. This can be advantageous for reading lights, but hinders performance when attempting to light an entire room.

If the advantages of LED lighting interest you, Myths Busted, LED Lighting is an excellent article by a researcher in outdoor lighting solutions. (And the source of most my information.)

Powering your LEDs with a DC source

Warning: Driving your LEDs with too much current will permanently disable them.

If you attach LEDs directly to an unlimited power source, they naturally draw enough current to blow themselves out. Therefore, the driving current must be limited with a resistor. The relationship as described by Ohm’s Law, is V = I*R where V is the voltage over the resistor, I is the driving current, and R is the limiting resistor. Two example circuits are shown below. These particular LEDs are rated at 25mA and are powered by a 12 volt regulated supply.

Each white LED gives a voltage drop of 3.6 volts. As an example for a 12 volt light, you can run a maximum of 3 white LEDs in series at full power (3.6 x 3 = 10.8 volts drop). Subtract this from your supply voltage of 12 volts to get the additional voltage that must be dropped (in this case, 12 - 10.8 = 1.2 volts of additional drop needed). In this case, 1.2 volts of additional drop / .025 amps (25 ma) = 48 ohms… resistors are rated in watts. So in this case, 1.2 volts x .025 amps = 0.03 watts. A 1/4 watt resistor would work fine.

The tutorial above also explains how to construct a 12 volt voltage regulator from any 12+ volt DC source. Voltage regulation is highly recommended because large shifts in your driving voltage can cause the driving current to increase and burn out your LEDs!

If you are in doubt of your calculations, use one of the many LED resistor calculators.

Using an AC source to drive LEDs

It is also possible to convert an AC source to DC. My favorite way to accomplish this is with a bridge rectifier as shown below. As an added bonus, R is easily calculated using the method above.

In operation, a DC voltage of around 170 is produced from the bridge rectifier and 50uF capacitor. The capacitor value is not critical and can be anything from 20uF or more… To find the resistor value and wattage, multiply the number of LEDs by the individual LED voltage. Then subtract this number from 170 and divide the result by the desired current (usually 20 milliamps).

Dimming your LEDs (with PWM)

Using pulse width modulation (PWM) to dim your LEDs is extremely important! Simply decreasing the input voltage yields unreliable results and potentially reduces their life-span. Pulse width modulation basically pulses the source voltage on and off so quickly that your eye is unable to distinguish the difference.

PWM can be implemented using a variety of methods. The simplest is switching on/off the output of a microcontroller. There are also several circuits that implement PWM. My favorite method uses two comparators. The details are excruciatingly painful and may deserve their own article someday.

The first example uses the standard op-amp oscillator circuit to generate a triangular waveform which is level-shifted and fed to a comparator (e.g. LM339) to give the PWM waveform.

Purchasing LEDs

LEDs are available all over the Internet. Recommending a single source or particular LEDs is hard as prices, projects, and the LEDs themselves may vary. If you are planning on starting an LED lighting project, it is best to do some research. For smaller applications, like a reading light, it may be more cost-effective to skip out on the top-of-the-line and buy a few more (relatively) cheaper LEDs. If you want to build a moonlight for your fish-tank, then you don’t need all those lumens blinding your fish!

Digg! this article…

1. In the first scenario, the piano is suspended above our stick figure with some amount of stored energy. Because the piano is suspended, it poses no danger to our stick figure.
2. In the second scenario, the piano is released, and the stored energy is converted to motion. The piano is now potentially lethal.

Similarly, voltage is of no danger unless your body completes a circuit (if you are touching the ground) and a charge is able to flow through your body.

Effects of current on your body

There are several factors that influence current’s effect on your body. First of all, everyone has a unique chemistry so the effects are different for everyone. In fact, the small difference between a man’s body and a woman’s body cause significant variations. Secondly, the frequency of the current plays a critical role. The human body’s nervous system and muscles communicate using a frequency of 50-60hz making us extremely susceptible to current at this frequency. High frequencies and direct current are not as harmful.

The tabulated effects of current on the human body:

BODILY EFFECT     DIRECT CURRENT (DC)    60 Hz AC     10 kHz AC
--------------------------------------------------------------- 
Slight sensation     Men = 1.0 mA         0.4 mA        7 mA 
felt at hand(s)    Women = 0.6 mA         0.3 mA        5 mA 
--------------------------------------------------------------- 
Threshold of         Men = 5.2 mA         1.1 mA       12 mA 
perception         Women = 3.5 mA         0.7 mA        8 mA 
--------------------------------------------------------------- 
Painful, but          Men = 62 mA           9 mA       55 mA 
voluntary muscle    Women = 41 mA           6 mA       37 mA 
control maintained                                           
--------------------------------------------------------------- 
Painful, unable       Men = 76 mA          16 mA       75 mA 
to let go of wires  Women = 51 mA        10.5 mA       50 mA 
--------------------------------------------------------------- 
Severe pain,          Men = 90 mA          23 mA       94 mA 
difficulty          Women = 60 mA          15 mA       63 mA 
breathing                                                    
--------------------------------------------------------------- 
Possible heart        Men = 500 mA        100 mA             
fibrillation        Women = 500 mA        100 mA             
after 3 seconds                                              
---------------------------------------------------------------

Ohm’s law (V = I*R)

Ohm’s law, voltage = current * resistance, is the deadly relationship between current and voltage. Basically, if you connect a charged source to ground, the current through you is going to be directly proportional to the sources voltage. Therefore, a high voltage differential causes high current. The amount of current is indirectly dependent on your resistance.

The human resistance factor

Unfortunately, the human body makes a horrible resistor. Because we are composed mostly of water inside, the majority of our resistance is in our skin. Personally, my hand-to-hand resistance is about 540 k-ohm. Wetting down my fingers reduces my skins resistance and drops this number down to around 60 k-ohm. This is why you never want to be wet around electricity. Nine times more current is able to flow through my wet skin at the same voltage!

Why birds can stand on power lines

On simple property of electricity holds it ground in the example. Current prefers the path of least resistance. It would rather flow through the wire than the bird, which has a relatively high resistance compared to a power line. However, a charge always wants to obtain and equilibrium. If an object connects a charged line to a source with less charge (say the ground), then this becomes the easiest path for the source to discharge itself.

How an electric fence works

While I am writing, I should dispel another rumor. Most people assume an electric fence has current flowing from one end of the fence to the other when in fact it is charged uniformly from both ends. There is no significant current flow until you complete a circuit to the ground. In fact, the unit that powers an electric fence is sold as a ‘fence charger.’

This is a common misconception because common electric fences have a pulsating voltage. There is in fact a small current flow as the fence is charged and discharged, but the flow is not from one end of the fence to the other. It simply in and out of the fence.

Creating a battery from pennies

In order to turn pennies into batteries, another electrode and an electrolyte are needed. In this case, dimes (zinc) are used as the positive electrodes and salt water is used an electrolyte. Copper wire, galvanized nails, and lemon juice are also popular and cheaper solutions. Such a battery produces a differential of about 0.5 volts.

Finding ample power for an LED

Unfortunately, this battery is not enough to light an LED. In order to string eight of these cells in series, an ice cube tray is used. Metal paperclips hang the pennies and dimes into the electrolyte banks. Because the paperclips are conductive, the eight cells are automatically connected in series forming a more powerful battery. This provides a differential of about two volts.

As you may notice, 0.5 volts * 8 != 2 volts. Not all of the banks produced a reliable voltage. In fact, one bank seemed to be working against me.

Lighting a LED with pennies

Generally, LEDs require a resistor to prevent excessive current flow from blowing them out. This project does not require a resistor because the battery simply cannot provide that amount of current.

Connecting the short end of the LED to the penny and the long end to the dime lights up the LED! Everything works as planned. The penny batteries provide about 110 micro-amps of current in series. At two volts, this is only about 220 micro-watts of power!

It does in fact ‘cost’ pennies to power an LED.

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SigGEN: A Linux signal generator

SigGEN is a fairly advanced signal generator designed specifically for Linux. It is able to generate sine, cosine, triangle, sawtooth, square, and pulse waves as well as white noise and frequency sweeps. It also supports separate waves on each stereo channel and setting phase differences between them.

Digital Signal Generator (Windows)

I was also able to find a Windows-based solution more advanced than E-DSP’s. It is able to generate sine, square, and triangle waves as well as chirp signals and noise.

Reproduced wave accuracy

Modern computers have no problem accurately calculating a reproducing small signals. Therefore, any limitations will lie in the sound cards specs. Most modern sound cards have a 16-bit digital to analog converter (DAC), and 24-bit DACs are becoming more popular. Even 16 bits of accuracy is far better than the 10 bits common on most embedded microcontrollers.

The major limiting factor in wave reproduction is the sound cards sampling frequency. It is limited to 48kHz. Hence, any frequencies near or above 24kHz are extremely hard (if not impossible) to reproduce without additional hardware. This limit is rather low and limits the generator usefulness as a high frequency signal generator.

Maximum deliverable voltage

I am sure that different sound cards are capable of delivering different voltages. My Sound Blaster Live! card was able to deliver four volts as seen above.

Maximum current and power

A sound card is also limited in the amount of power it can deliver. Once a voltage source reaches its power limit, it begins acting as a constant current source. It simply delivers less voltage to compensate for the limited current.

In order to test the sound card’s limit, a potentiometer is connected between the generated signal and ground. The resistance is then decreased until the current peaks and the voltage starts to decrease.

Once we find the point of maximum power, the potentiometer’s resistance is measured, and Ohm’s Law is used to find the current delivered. In this case, the resistance is 24.8K ohms. V = I*R tells us 0.16mA is being delivered. P = V*I = 0.6mW maximum power.

This is not a lot of current or power. This is why E-DSP reccommends an amplifier.

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