After the overwhelming response to the Hackit we posted about automated hard drive destruction last fall, we finally decided to test out some thermite hard drive destruction ourselves. This has been done on The Screen Savers but they did not show up close results of the platters. So, aluminum and black iron oxide were procured through eBay, and until it arrived we watched some YouTube videos that showed a lot of fire and no real results. We decided to see what it would take to completely obliterate a drive.

With the amount of personal data stored on your computer, we all understand the importance of destroying the data that is stored on the platters of a hard drive before disposing of it. There are many ways to destroy a hard drive; software, physical disassembly, drills, hammers, magnets/electromagnets, and acid, but none are quite as outrageous and dangerous as thermite. That’s what we’re going to do here today. Follow along for pictures and videos of the results.

A couple different methods of containing the thermite above the hard drive were tried and we quickly found the best way is a clay flower pot with the drip tray for a lid. An Altoids tin was also tried, but it burned up to quickly. Molding a cement container was also attempted. Since thermite is extremely hard to ignite, sparklers that were left over from the 4th of July were used, and offered a very reliable method of ignition.

Our goal was to completely destroy the drive while it was still in the computer case. The theoretical application is to destroy the disk at a moments notice so it won’t fall into the wrong hands. After testing multiple methods, placing about 1 pound of thermite in a clay flower pot and lighting from the drain hole in the bottom yielded the best results. This could easily be placed in the 5.25″ bays above the drive.

A thermite reaction is a process in which the correct mixture of metallic fuels are combined with a metal oxidizer and ignited. Ignition itself requires extremely high temperatures, but once ignited, thermite supplies its own source of oxygen. It can potentially burn underwater when mixed properly. Thermite is usually used to weld railroad ties together.

The most common thermite is “black or blue iron oxide (Fe3O4), produced by oxidizing iron in an oxygen-rich environment under high heat” and Aluminum(Al). Red iron(III) oxide (Fe2O3), commonly known as rust, can also be used. There are many chemicals that can make thermite; the mixtures used to make thermite therefore vary, causing confusing and changing mixture ratios.

Since the oxidation of one substance involves the reduction of another, this type of reaction is often called redox reaction. In the following balanced reaction, 8Al + 3fe3O4 = 4Al2O3 + 9Fe + Heat. The element Al is oxidized, but Fe is reduced. This reaction is also called a displacement reaction because Al displaces Fe in the oxide. Because of the nature of this reaction, the correct ratio of substances is important to ensure the optimum amounts of fuel (aluminum) and oxygen (iron oxide) within the mixture. Thermite is very safe to handle because of the high ignition temperatures required, sparklers were used in this instance, however magnesium ribbon can also be used. We think an electric pyrogen igniter would be a far better choice for ignition, instead of unreliable methods.

There are two important aspects to ensure a successful reaction. Thorough/even mixing and smallest possible powder particle size. If thermite is not adequately mixed, it may be difficult to ignite or maintain the reaction. One problem when mixing thermite is the difference in weight between the aluminum and the iron oxide. This causes them to separate out rendering the thermite useless. The process used here with great success was five minutes in a rock tumbler. Powder particle size is measured with a measurement called mesh. Passing the powder through a mesh will determine the largest particle size, this reaction performs best with the smallest obtainable mesh size. The mesh size for aluminum was 1200 mesh and black iron oxide was 300 mesh.

The total enthalpy or heat content released is -3.677 kJoule per gram of Fe3O4/AL thermite. The ratio of Fe3O4 to aluminum powder by weight is about 3.22 to 1, according to the reaction’s stoichiometry. The reaction photographed was 200 grams of Aluminum and 644 grams of black iron oxide yielding 2368 kJoules of heat. This was more than was required to adequately destroy the hard drive, a smaller amount could have been used, and still destroyed the platters. It would have even been better controlled, or better yet contained within the computer case. What fun is that?

Using thermite to destroy a hard drive is a very violent and destructive process. Great care should be taken as the molten metal can splash and sputter for a long distance.

The reaction begins to sputter.

The thermite has just contacted the hard drive.

Things are really hot now!

Most of the reaction is completed.

The molten thermite, platters and most of the aluminum frame from the hard drive in the bottom of the case.

Above are the molten hard drive platters destroyed with 844 grams of thermite. It takes about this much thermite contained directly above the drive to get the job done, if it is not you will just get a superficial fire.

Over all the destruction of the drive and platters was accomplished in all cases in a matter of seconds. This is by far a guaranteed method of destroying data in a time of need. We’re pretty sure this will prevent most forensic data recovery methods.

Below is a video of Brainiac using thermite to burn cars and trying to stop the reaction with liquid nitrogen.

Here’s the directors cut of the thermite video which contains 4 extra minutes:

Finally, please do not try this.

The basic design for the RGB keypad came from [JMG]‘s Arduino based Monome clone. He used an Arduino, and multiplexed RGB LEDs with some digital potentiometers to create a color mixing keypad. Since we couldn’t fit the complete 4×4 keypad into a standard 2 gang wall box, we chopped the design down to a 2×4 matrix. This cuts down significantly on the cost to build the keypad and makes the code that much easier to digest.

To build your own RGB keypad, you’ll need the following:

• An electric door strike (Smarthome.com)
• A locking door handle (Any hardware store)
• An Arduino or compatible clone (Sparkfun, adafruit and others)
• 1 TIP120 transistor
• 1 1N4001 diode
• 10 1N4148 diodes
• 4 2n2222 transistors
• 1 Monome style keypad (Sparkfun Electronics)
• 1 Keypad PC board (Sparkfun Electronics)
• 8 RGB LEDs (Sparkfun Electronics)
• 1 7805 voltage regulator
• 4 100 ohm resistors
• 2 150 ohm resistors
• 8 1 kohm resistors

To reliably lock and unlock the door, we ordered an electric door strike. We scored this one as an open box item from Smarthome.com. It’s a 12 Volt DC unit designed just for Schlage commercial door locks. The edge of the strike is slightly recessed from the mounting plate, so it might not work with certain locks. It features a thinner body than the non-recessed version, which will allow us to cut a smaller but deeper hole in the door frame. Without power, the strike stays locked, keeping the locking door shut. When 12 volts is applied to the coil, the strike releases, allowing the door to be pulled open. For the prototype build, you don’t have to purchase a strike just yet; you can use a LED and a resistor to indicate the door lock state for testing your code.

The keypad is actually built from two separate circuits that physically overlap. The input circuit is a simple keypad matrix. To read each button push, the Arduino brings one keypad input line high and checks the voltage of the four output lines in order. The diodes on the PC board prevent feedback across the rows and columns.

The RGB LEDs are lit via a completely separate set of circuits. Each row of like colored LEDs is brightness controlled by a digital potentiometer. The digital pot works just like a normal pot, but it’s digitally controlled by the Arduino. Meanwhile, each column of LEDs is activated by a separate transistor. By quickly changing the resistance and stepping through the columns, each LED will appear to be individually controlled.

The door strike circuit is pretty simple. Since it contains a coil, we’ll treat it like the coil of a stepper motor and use a TIP120 transistor to supply the power. When power is removed from a coil, the collapsing magnetic field creates a current within the coil. To keep the TIP120 from burning out, we’ll add a diode to handle the surge created by the field breakdown.

update: [Triffid] pointed out that the diode is better placed in parallel with the coil to handle the transient surge. He’s correct, but the circuit here has operated perfectly for several months, so you’ll be fine either way.

The traces for the buttons looked a bit challenging to etch at home, so we ordered this PC board that Sparkfun produces for their keypads. Sparkfun helpfully provides the layout for these keys in their eagle library, so you can make your own PCB if you prefer. For reliability, you’ll probably want to have it commercially produced. The board wasn’t really designed to break apart, but after a review of the traces and vias we decided that we could get away with trimming a couple of rows from the board.

We carefully split the board down the middle with a band saw. If you look closely, you can see where some of the vias were actually cut in half. (A paper cutter might work in a pinch) Don’t forget to put on a mask to keep the dust out of your lungs.

Cutting the button pad is much easier. The pads have pre-scored lines that just need a quick swipe of a sharp knife or scissors to separate them.

The new shorter PCB only needs a few parts: some 1N4148 diodes and the RGB LEDs. The silkscreen on the board indicates the direction and position of diodes and LEDs.

Once you solder on the 1n4148 diodes, cut them as close to the PC board as you can. Flat head cutters like these work extremely well. The keypad will sit on this side of the board and we want to make sure that it can sit as flat as possible.

Install the LEDs in the orientation indicated by the silk screen. Carefully push them down into the board until they’re inserted just like this. If you let them stick up too high, they’ll interfere with the keypad buttons being pushed.

Once you’ve soldered all the LEDs in place, clip them flush as well. Then you’ll need to add some cable to jumper from the keypad to the interface board we’ll build. We used some old CAT-5 wiring. Since each axis of the board has eight pins, it’s perfect for the application.

Each RGB LED has three LEDs inside the package. They share a common terminal and have a single separate lead coming out. Because they have different characteristics – that is brightness, current and voltage requirements, we spent some time testing out various combinations. We even murdered a couple of innocent $2 LEDs just for you. Hey, the other two colors are still usable…

After some experimentation, we managed to find the right combination to create some fairly white light. The requirements will vary between manufacturers, but for the Sparkfun LEDs we found that a pair of 100 ohm resistors and a single 150 ohm resistor blended the red, green and blue fairly well.

The color combination was hard on the eyes until we put the keypad over the LED to double check our findings. In real life, you can see some blending lines from the offset of each LED, but it still looks great.

The circuit has plenty of components, but it’s pretty easy to build. We’ll break everything up by section to keep things easy. You can download the all of the schematics, Eagle project files, and code for the Arduino here.

The digital pot has six outputs. Each of these will power a row of red, green or blue LEDs, via a color matching resistor. The digital potentiometer wiring comes directly from this how-to. You can read it if you need more information, or use our quick version:

• Connect AD5206 pins 3, 6, 10, 13, 16, 21 and 24 to 5v.
• Connect pins 1, 4, 9, 12, 15, 18, 19, and 22 to ground.
• Connect pot pin 5 to Arduino pin 10
• Connect pot pin 7 to Arduino pin 11
• Connect pot pin 8 to Arduino pin 13

Grab four 100 ohm resistors and two 150 ohm resistors. Place them in the breadboard in a row with each end in a separate bus. (Across the center of the board is easiest) Connect the six LED leads from the keypad to one end of each resistor – reds get the 150′s and blue and green into the 100′s. Here’s the connection order we used.

• RED3 to a 150 ohm resistor to pot pin 14
• GREEN3 to a 100 ohm resistor to pot pin 11
• BLUE3 to a 100 ohm resistor to pot pin 2
• RED4 to a 150 ohm resistor to pot pin 23
• GREEN4 to a 100 ohm resistor to pot pin 20
• BLUE4 to a 100 ohm resistor to pot pin 17

To ground the LED busses, we’ll be using four 2N2222 transistors. The Arduino will trigger each transistor individually through a 1Kohm resistor. The collector of each transistor connects to a ground line from the keypad. The emitter of each transistor is connected to the ground. The four transistor select lines connect to Arduino pins 0, 1, 2, and 3. Yes, they’re marked Analog in, but it doesn’t matter.

The keypad switch matrix is connected in four columns and two rows. Each of the four columns gets a pull-down resistor. We used 1Kohm resistors for R11, R12, R13, and R14; one lead connects to the columns and the other is grounded.

Arduino pins 2 and 3 should connect to the two ungrounded lines, which are marked SWITCH3 and SWITCH4 on the PC board (5 and 6 on the schematic).

Arduino pins 6, 7, 8, and 9 should connect to the four output lines marked SWT-GND1, SWT-GND2, SWT-GND3, and SWT-GND4 (1-4 on the schematic).

The final version of the board takes a 12VDC input to drive the door lock. We added a 7805 to drop the 12V down to 5V for the Arduino. You don’t need it for the prototype version unless you want to test the striker. The Arduino has an on-board regulator, but 7805′s are cheap and it helps reduce the load on the Arduino’s built in regulator. For code development, we just connected an LED with a resistor to the output line that will control the door lock.

With everything wired in the prototyping board, it’s time to test things out. With any luck, you’ll soon be rewarded by the pulsing, glowing sight of several RGB LEDs under your tender digits.

Programming the Arduino is a snap. Just download the software for your OS here. Now follow the Getting Started guide to get the Arduino software talking to the Arduino board. Once you’ve enjoyed the blinking LED demo, come back here and get your keypad rolling.

Once you’ve set up and tested your Arduino, it’s time to test out your prototype. Download the button_test code from here. Paste it into a new sketch and upload it to the Arduino. Click the serial console button and you should start seeing dots accumulating in the window. If you press a button on the pad, the Arduino should print a message to the console and toggle the lock output state.

Once your buttons are tested, you’ll probably want to try out your LEDs. Grab the RGB_light_fade routine from the same page and upload it to your Arduino. You should get treated to a nice little light show. This is our favorite demo because it really shows off the color mixing capabilities of the digital potentiometer.

With your LEDs and buttons working, you can grab the row_entry_pad_meffect lock code from the same place and upload it. Now the keypad should start flashing blue buttons while it’s idle. On key presses, the keys will change colors. By entering the correct color code, the pad will flash green and unlock the door for 10 seconds. If you go over the limit counter, it will flash red for 30 seconds.

Next time we’ll show you how to make the permanent version of the keypad, walk through the code for the Arduino, make the PC board, cut a custom wall plate, and install the lock strike.

We covered many of [Jason Rollette]‘s personal projects in the past and are happy to welcome him as our newest Hack-A-Day contributor.

The electronics industry has shifted to lead free compliance, but most hobbyists haven’t even considered the personal impact of using lead. Today’s How-To will cover what it takes to switch from tin/lead solder to completely lead free. Our previous posts Introduction to soldering and the follow-up still apply to lead free. You may have never considered switching to lead free before, but we hope to help you make an informed decision.

The reason we are even talking about this is because of the Restriction of Hazardous Substances directive. RoHS was adopted in February 2003 by the European Union; all electronics sold there must comply. The substances restricted are: lead, mercury, cadmium, chromium VI also known as hexavalent chromium plating, and pbb, pbde flame retardants used in plastics. We will focus on lead. Solder joints must contain less than 0.1% lead to be compliant. RoHS is not currently required in the US, but California RoHS, effective September 2003, had a compliance deadline of January 2007.

Although the electronics industry has been directly targeted it only accounts for a small percentage of the lead used in manufacturing. The battery industry consumes nearly 80% of manufactured lead. Tire wheel weights also account for a larger percentage.

There are some good reasons for the hobbyist to convert to lead free solder, both personal safety and environmental. Lead poisoning can occur when lead enters the body through inhalation, ingestion or dermal contact such as direct contact to mouth, nose, eyes, and skin lesions. Even if you keep using tin/lead solder, wash your hands before and after you do anything. Most poisoning cases are from lead building up over time. The main environmental issue is lead leaching into drinking water or watersheds when disposed of improperly.

There are many different varieties of lead free solder. Two alloys seem be the most popular: SAC305 contains 96.5% tin, 3% silver, and 0.5% copper and melts at ~217C, SN100 contains 99.3% tin, 0.6% copper, and some nickel and silver and melts at ~228C. Choose whatever alloy and brand you feel is appropriate for you. Compare those alloys to traditional 63% tin and 37% lead melts that melts at ~183C. The higher melting point is part of what makes lead free harder to work with. We use SAC305 with a “no clean” flux core.

In addition to picking an alloy, you also need to decide on the diameter. We recommend for through hole .032″,.020″ and .015″ for SMT. Choosing the right diameter solder is very important to success. A finished joint viewed from the side should look like a little Hershey’s kiss, not an inflated balloon. The correct diameter solder makes this much easier to control.

A high wattage soldering iron with temperature adjustment is the best choice for precision work. Get a soldering iron with several sizes of replaceable tips for different soldering applications. Having a good iron makes soldering with lead free easier because of the higher melting point. It also helps to keep the dwell time as short as possible reducing damage to components caused by excessive heat.

Choosing the correct size tip for the job is perhaps the most important part for a new person learning to solder. Lead-free is less forgiving and the right tip will go a long way in preventing defects.

Using a tip that is too small will take longer, abuse the tip, and will not efficiently transfer heat to the joint. A small tip will seem too cold or too slow.

Using a tip that is too large will damage the circuit board, over heat and damage the parts, and burn off the flux too soon causing a bad joint.

Use tips that are designed for lead-free. Tips designed for lead free will have the longest life. The iron temperature should be set to 700-800F. Do not use pressure to compensate for lack of wetting and heat transfer; this will cause damage to the circuit board. Heat transfer is optimized by providing the best contact area.

Differences from tin/lead soldering

Tin/lead solder should not be mixed with lead free solder because it decreases the strength of the joint. Lead free parts can be used with tin/lead solder. You should try ordering all parts, ICs, resistors, caps, proto boards, etc. lead free even if you are not using lead free solder yet. This will ease the conversion for you in the future. Consumables such as flux and tip cleaner should be certified for lead free soldering. Flux will need to withstand higher temperatures and longer dwell times, and some tip cleaners have tin/lead solder in them that could contaminate the lead free solder joints.

Not all fluxes are capable of sustaining high soldering temperatures. Flux charring, called “black tip syndrome”, occurs when thermally incapable fluxes turn the tip black and make re-tinning nearly impossible. Heat transfer is severely reduced when this happens. Buying compatible flux is key.

As we’ve said many times: you’ll need to set your solder iron slightly higher temperature than you are used to. You will also notice that slightly longer dwell times are needed because of higher melting points. Wetting or spread is also a little slower when compared to tin/lead. The resulting lead free solder joints will appear slightly grainy and dull compared to shiny tin/lead.

The iron must be kept clean and fully coated with the solder alloy, otherwise at the higher temps oxidation can occur. Solder tips will need to be cleaned and tinned more frequently. Use a wet sponge for cleaning and keep your tip tinned by adding a small amount of solder. In general, all tips will have a reduced life when using lead free alloys.

Lead free solder is more prone to solder bridges (shorts). Tin whisker growth is also possible with high tin alloys, but is poorly understood.

The temperatures required make lead free soldering a different experience, but not out of reach. If you can solder with tin/lead solder, you can solder lead free. If you are having problems soldering, maybe some of the tips here will help. Remember, soldering takes practice; solder, desolder, resolder some old circuit boards from computers or consumer electronics before attempting a project. Otherwise you can’t expect a perfect outcome. Knowledge, practice and experience will provide the consistency required for excellent hand soldering results. Please share any experiences you have working lead free.

Old computer mice are being abandoned in droves. They’re tossed out because of dirt, obsolescence, or for being entirely too beige. Anyone who has a computer usually has more than one mouse and you can get them for pennies, if not free just for asking. Fortunately for the discriminating (read: cheap) hacker, these little widgets are chock-full of project parts. Today’s How-To will dissect a computer mouse, extract the useful parts, and give some ideas about how to use them.

Here we have a standard PS2 mouse; a USB mouse will look pretty much the same. We’ll talk about optical mice later on.

Lets open it up. First, take out the mouse ball. Then there will be one or more screws on the bottom side that need to come out. Screws are sometimes hidden under the rubber pads.

The first thing to notice is the cable connector. Most mice have a very convenient plug for the cable, instead of soldering it onto the board. That’s the first usable part: a 4 (or 6) conductor cable with a nice plug on one end. The socket can be removed from the PCB and used in other projects.

With the cord out of the way, we can start pulling out parts. First thing that pops off is the mouse wheel. Yes, it’s just a big rubber wheel. If you’re annoyed by the mouse wheel’s clicking noise, you can silence it. There are at least two micro-switches and usually a third one under the mouse wheel which can be used for bump sensors or buttons.

Next thing to take out are the two plastic slotted discs. They’re the encoder wheels. They turn whenever the mouse moves and interrupt an IR beam, producing pulses. It used to be that precision encoder wheels were pretty expensive, but not anymore. Every single (non-optical) mouse comes with a pair built in.

On either side of the encoder wheel are some little boxes. One side is an IR emitter, and the other side is a pair of IR detectors. Sometimes the emitters and detectors will be one complete unit. A pair of detectors is used because with 2 detectors slightly offset, it generates quadrature encoding, so that mouse knows the direction of rotation.

These IR emitters and detectors are fairly modular pieces when it comes to hacking and can be removed with some careful desoldering. It is a good idea to use a multimeter to see what kind of voltage is going to them before removing them. They are probably TTL parts, because mice are powered by 5v, but sometimes they are an even lower voltage. The emitter/detector pair by itself can be interfaced directly. Those parts alone can be used for a proximity sensor or for a line following robot.

Add an encoder wheel and this sensor setup has lots of uses. From a tachometer, to a wind speed indicator, to sensing a robot’s speed, this will do it. The quadrature output (with some decoding) can be used to measure distance, direction, and speed.

One tricky thing about using the encoder with the IR sensor is getting the emitter to line up with the detector(s). No problem: instead of desoldering the parts from the PCB, use a rotary tool to cut off the little piece of PCB that has the sensors on it (already lined up). Wires can then be soldered to the pins/traces on the other side. If holding the encoder wheel in place is a problem, a hunk of the mouse casing can be used with it. Be sure to leave enough PCB to keep it stable on the mouse casing.

This is a picture of an encoder wheel with quadrature from a PS2 mouse on a small robot. Rather than make custom parts, the mouse is carved up and the whole assembly is fitted to the motor axis.

What’s left to salvage on the mouse? The sensors for the mouse wheel. Depending on the model, the mouse wheel movement may be sensed with switches or a low resolution encoder, suitable for measuring slow rotation.

The final bit is the mouse controller chip. At first thought it seems pretty useless: it’s a single purpose part made specifically to run a computer mouse and nothing else. However, if your controller chip isn’t an anonymous blob, the datasheet could be very enlightening. Octopart can help find datasheets based on the chip’s part number. This (PDF) is a typical datasheet for a mouse controller.

This specific part does a number of useful things:

• decodes the quadrature input from the encoders
• keeps a running count of the number of encoder pulses in a set of registers
• filters out jitter
• debounces the micro-switches and keeps track of their state
• packetizes the information and sends it up the cord.

For a PS2 mouse, getting at these features is not too difficult. The PS2 protocol is pretty friendly, even for microcontrollers. Here’s an example of interfacing a microcontroller with a mouse. For USB mice, check the controller’s datasheet to see if it can do both PS2 and USB; with older mice there’s a decent chance it does. While pure USB is not very easy to talk to with a microcontroller, it’s really easy to hook to a PC (with the right software).

We promised a look at optical mice. Optical mice do not have any rotary encoders. There are still some switches and a low-res encoder for the mouse wheel. What an optical mouse has in it is a mini-camera, lights, and lenses. The light is probably a fairly standard LED, although they tend to be bright, and the lenses are purpose built. However, it is possible to interface directly to the mini-camera or use it as a really poor scanner.

We hope this How-To has sparked some ideas and that your obsolete mouse won’t seem as useless anymore.

This week’s How-To comes from our newest contributor: Logan Williams.

This simple guide will show you how to build a digital synthesizer that generates and manipulates square waves. Your synthesizer will have one oscillator, which produces a variable pitch controlled by a potentiometer, as well as an LFO which modulates that pitch at a variable frequency. The part count for this project is quite low, and it can be built for under $20.

Finding the Parts

The first step in building this digital synthesizer is to procure the parts that you will need. Most of these can be bought at RadioShack, but RadioShack’s prices are often much more expensive than ordering online. All of the parts for this project can be purchased at Jameco, Digi-Key, or Mouser. We’ve provided Jameco part numbers below. If you don’t mind waiting, this is the best way to order parts.

Not Pictured

Tools

Note: The potentiometers and audio jack must be either taped or soldered to 22 AWG solid core wire. Soldering is highly recommended, as it produces a more secure connection.

Creating an oscillator

Before we can begin with the digital synthesizer, we must generate the correct voltage. Most of you will be familiar with using a 7805 5V voltage regulator. It is very simple; connect the +9V from the battery to the left hand pin, ground the middle pin, and the right hand is +5V.

The most basic circuit in any synthesizer is the oscillator. A square wave oscillator constantly alternates between two voltages, in this case +5V and 0V. We have a logic inverter to create this, which operates quite simply; if it is given +5V in (a logic 1), it give
s 0V out
(a logic 0) and if it is given a logic 0, it gives a logic 1 as output. When the input and output are connected together, it will oscillate rapidly between those two values: a 0 goes in, comes out as a 1, goes in, comes out as a 0, and so on.

The problem is that it oscillates much too fast. A resistor capacitor (RC) delay circuit can be added to slow it down. This forces the output current to charge the capacitor before it can pass through to the input. The resulting brief delay slows the oscillations to audible frequencies.

To build the oscillator, assemble the schematic below on a breadboard.

When done, the oscillator should look something like this:

Connect one side of the audio jack to 0V and the other side to the output, and it will sound like this:

Controlling the oscillator

We can make things more interesting by allowing the user to change the frequency. We replace the constant resistor R1 with a potentiometer, such as the 100K R2. This is a simple change to do, and is reflected in this altered schematic.

Now the oscillator sounds like this:

Much more interesting. Try playing an actual song, if you dare.

Duty cycle adjustment

We can add some basic timbre control to make the oscillator more interesting. The duty cycle of a square wave is how long it spends at logic 1 vs. at logic 0. For example, a wave that spends 1 ms at +5V and 1ms at 0V per cycle would have a 50% duty cycle. 1.5 ms at +5V and 0.5 ms at 0V would be a 75% duty cycle. To adjust the wave’s duty cycle, we can add another potentiometer and diode to the circuit. When the input is high and the output is low, current will be able to flow through both potentiometers, decreasing the amount of time it takes to charge the capacitor, and increasing the duty cycle.

It should sound like this when completed:

Creating an LFO

A low-frequency oscillator (LFO) is an oscillator that oscillates very slowly, from 1 to 100 cycles per second. We will use an LFO to alternate the pitch of our oscillator between two different frequencies. This can be used for siren like sound effects, timbre control, or musical sequences.

The circuit to control the LFO is slightly more complex than the ones we have used before. Because it uses a capacitor with 10x the capacitance, and a potentiometer with 10x the resistance, the oscillations are 100x slower than our first oscillator. The LFO connects to the gate of the IRF 510 MOSFET transistor. When the output of the LFO is +5V, the transistor connects its source and drain pins. With these pins connected, current can flow through the second potentiometer, increasing the pitch. When the LFO returns to 0V, the potentiometer is disconnected, and the pitch drops back to its original level.

There are quite a number of sounds that can be produced with the LFO, such as this:

and this:

Conclusion

You have now made your own simple digital synthesizer. Keep experimenting with different control methods. The frequency is adjusted with just resistance, so almost anything can be used for an input. Try a photocell, or a flex sensor. Try combining the LFO and the duty cycle adjustment. Try using it to actually make music! We’d love to see what you come up with.

If you want to keep your money, I’d avoid RadioShack as much as possible. When you’re stuck because you can’t find a freaking 10kohm resistor, it’s fine, but the markup on their low quality parts is insane-their clearance prices aren’t too bad. As much as people bag on RadioShack, just remember that nobody else bothers to sell electronic parts in the middle of nowhere.

Shipping can eat your project budget quicker than anything else. It’s the reason that I’m sometimes willing to pay $1 for $.05 in resistors at RadioShack. When I’m buying parts, I try to buy from a single supplier if possible to maximize my parts budget.

Stocking up on parts in bulk can help make projects extra affordable later on. Buying a quantity of resistors, capacitors, PNP and NPN transistors and a decent supply of linear voltage regulators will save you a fair amount of money later on. I love it when I can build a $30 project for the cost of a proto-board and an odd capacitor.

Just about every electronics component manufacturer will provide free samples on request. That’s right – free. It’s usually just a matter of creating an account on the manufacturers web site and selecting the components you need. As a rule, I don’t mention when I’ve sampled parts for a project. Seriously, they’ll get a little suspicious if 100 people suddenly sample the exact same parts. Samples aren’t limited to semiconductor companies. [ladyada] has a nice list of sample providers, including enclosures and connectors.

Digi-Key carries just about every part you can think of. These guys prefer to sell large quantities, but they’re happy to take small orders. Orders are shipped out fast, but they’ll tack on an extra $5 fee if your order is under $25. Even if you’re not ordering, they usually have data sheets linked for every part they carry online. If you’re wiling to jump through the hoops, you can even create a parts order that’s linkable from your website – it can make it much easier to share a project with others, but I usually find that a few part numbers get deprecated as time goes by. Oh, and they’ll send you a massive parts catalog that’s handy for parts hunting and brain storming.

Mouser Electronics is one of my favorite suppliers these days. They have reasonable shipping options and are fast with UPS ground orders showing up at my house within two days. Their inventory isn’t always the best, but substitution parts are usually easy to find because the online catalog links to web enabled PDF pages from their print catalog. It makes cross-referencing very easy. Like Digi-Key, they’ll send you a massive parts catalog to shove under your monitor.

Futurlec is a great place to get incredibly cheap parts, but you won’t be seeing your order for about a month (ok, so my last order showed up after three weeks). I suggest stocking up on connector headers and resistors. I haven’t had to make a late night resistor run in a couple of years thanks to these guys.

Sparkfun electronics is like a candy shop for Hack-A-Day readers. They carry higher end parts like GPS units, GMRS modules and micro controller programmers. Pricing varies a bit, but I can always find something interesting there. If you prefer professionally made PC Boards, they even put together an inexpensive PC Board service.

ebay is one of my favorite places to shop for parts. It’s a great place to buy brand new $150 stepper motors for $20 or hunt down funky, hackable hardware. People certainly try to sell single components, but it’s usually not worth the effort.

Salvaging parts is the absolute cheapest method for parts shopping. Thanks to custom ICs and SMD parts, newer electronics don’t normally have much in the way of salvage value. On the other hand, older hardware is a great source for parts. My current favorite salvage source is the dot matrix printer. They’re easy to take apart, have nice power supplies, and they’re loaded with quality heat sinks, wiring, connectors, hardened steel rods, and stepper motors. The bigger the dot matrix printer, the bigger the stepper motor. If you can score a few of the same model, you’ll end up with a few matched sets. People hate throwing them away, so they’re easy to get for free.

Lack of availability can be a problem, but obsolete parts are another way to keep costs down. Originally, the UCN5804B stepper driver I used for the cutting board CNC machine cost about $16 each. Now that they’re deprecated, they can be had for about $5 each.

Grab bags are another good way to save money. They’re usually full of loose parts that’ll have to be identified, but they’re cheap. The guys over at uchobby put together a nice how-to on sorting them.

Thanks to the movement from mail order to internet suppliers, the parts company scene is huge. There are loads of production part and surplus companies around. Here’s a quick list of shops that’ll probably be useful if you’re looking for parts.

Digi-Key electronics
Mouser electronics
Futurlec
Sparkfun electronics
eBay
All Electronics
BG Micro
American Science & Surplus
Goldmine Electronics
MPJA Online
MCM Electronics
Parts Express

Got a favorite shop? Drop the link in the comments and I’ll add it to the list.