The Evolution of Oscillations

The laptop I’m using, found for 50 bucks in the junk bins of Akihabara has a CPU that runs at 2.53GHz. Two billion five hundred and thirty million times every second electrons systematically briefly pulse. To the human mind this is unimaginable, yet two hundred years ago humanity had no knowledge of electrical oscillations at all.

There were clear natural sources of oscillation of course, the sun perhaps the clearest of all. The Pythagoreans first proposed that the earth’s rotation caused the suns daily cycle. Their system was more esoteric and complex than the truth as we now know it and included a postulated Counter-Earth, lying unseen behind a central fire. Regardless of the errors their theory contained, a central link was made between rotation and oscillation.

And rotational motion was exploited in early electrical oscillators. Both alternators, similar to those in use today, and more esoteric devices like the interrupter. Developed by Charles Page in 1838, the interrupter used rocking or rotational motion to dip a wire into a mercury bath periodically breaking a circuit to produce a simple oscillation.

As we progressed toward industrial electrical generators, alternating current became common. But higher and higher frequencies were also required for radio transmitters. The first transmitters had used spark gaps. These simple transmitters used a DC supply to charge a capacitor until it reached the breakdown voltage of the gap between two pieces of wire. The electricity then ionized the air molecules in the gap. Thus allowing current to flow, quickly discharging the capacitor. The capacitor charged again, allowing the process to repeat.

An Alexanderson Alternator

As you can see and hear in the video above spark gaps produce a noisy, far from sinusoidal output. So for more efficient oscillations, engineers again resorted to rotation.

The Alexanderson alternator uses a wheel on which hundreds of slots are cut. This wheel is placed between two coils. One coil, powered by a direct current, produces a magnetic field inducing a current in the second. The slotted disc, periodically cutting this field, produces an alternating current. Alexanderson alternators were used to generate frequencies of 15 to 30 KHz, mostly for naval applications. Amazingly one Alexanderson alternator remained in service until 1996, and is still kept in working condition.

A similar principal was used in the Hammond organ. You may not know the name, but you’ll recognize the sound of this early electronic instrument:

The Hammond organ used a series of tone wheels and pickups. The pickups consist of a coil and magnet. In order to produce a tone the pickup is pushed toward a rotating wheel which has bumps on its surface. These are similar to the slots of the Alexanderson Alternator, and effectively modulate the field between the magnet and the coil to produce a tone.

Amplifying the Oscillation

The operation of a tank circuit (from wikipedia)

So far we have purely relied on electromechanical techniques, however amplification is key to all modern oscillators, for which of course you require active devices. The simplest of these uses an inductor and capacitor to form a tank circuit. In a tank circuit energy sloshes back and forth between an inductor and capacitor. Without amplification, losses will cause the oscillation to quickly die out. However by introducing amplification (such as in the Colpitts oscillator) the process can be kept going indefinitely.

Oscillator stability is important in many applications such as radio transmission. Better oscillators allow transmissions to be packed more closely on the spectrum without fear that they might drift and overlap. So the quest for better, more stable oscillators continued. Thus the crystal oscillator was discovered, and productionized. This was a monumental effort.

Producing Crystal Oscillators

The video below shows a typical process used in the 1940s for the production of crystal oscillators:

Natural quartz crystals mined in Brazil were shipped to the US, and processed. I counted a total of 13 non-trivial machining/etching steps and 16 measurement steps (including rigorous quality control). Many of these quite advanced, such as the alignment of the crystal under an X-Ray using a technique similar to X-Ray crystalography.

These days our crystal oscillator production process is more advanced. Since the 1970s crystal oscillators have been fabricated in a photolithographic process. In order to further stabilize the crystal additional techniques such as temperature compensation (TCXO) or operating the crystal at a temperature controlled by the use of a heating element (OCXO) have been employed. For most applications this has proved accurate enough… Not accurate enough however for the timenuts.

Timenuts Use Atoms

Typical timenut wearing atomic wristwatch

For timenuts there is no “accurate enough”. These hackers strive to create the most accurate timing systems they can, which all of course rely on the most accurate oscillator they can devise.

Many timenuts rely on atomic clocks to make their measurements. Atomic clocks are an order of magnitude more precise than even the best temperature controlled crystal oscillators.

Bill Hammack has a great video describing the operation of a cesium beam oscillator. The fundamental process is shown in the image below. The crux is that cesium gas exists in two energy states, which can be separated under a magnetic field. The low energy atoms are exposed to a radiation source, the wavelength of which is determined by a crystal oscillator. Only a wavelength of exactly 9,192,631,770Hz will convert the low energy cesium atoms to the high energy form. The high energy atoms are directed toward a detector, the output of which is used to discipline the crystal oscillator, such that if the frequency of the oscillator drifts and the cesium atoms are no longer directed toward the detector its output is nudged toward the correct value. Thus a basic physical constant is used to calibrate the atomic clock.

The basic operating principle of a cesium atomic clock

While cesium standards are the most accurate oscillators known, Rubidium oscillators (another “atomic” clock) also provide an accurate and relatively cheap option for many timenuts. The price of these oscillators has been driven down due to volume production for the telecoms industry (they are key to GSM and other mobile radio systems) and they are now readily available on eBay.

With accurate time pieces in hand timenuts have performed a number of interesting experiments. To my mind the most interesting of these is measuring time differences due to relativistic effects. As is the case with one timenut who took his family and a car full of atomic clocks up Mt. Rainier for the weekend. When he returned he was able to measure a 20 nanosecond difference between the clocks he took on the trip and those he left at home. This time dilation effect was almost exactly as predicted by the theory of relativity. An impressive result and an amazing family outing!

It’s amazing to think that when Einstein proposed the theory of special relatively in 1905, even primitive crystal oscillators would not have been available. Spark gap, and Alexanderson alternators would still have been in everyday use. I doubt he could imagine that one day the fruits of his theory would be confirmed by one man, on a road trip with his kids as a weekend hobby project. Hackers of the world, rejoice.

DIY portable charger/generator


[Glenn] from The Back Shed has built a lot of windmills and stationary generators over the years, but recently decided to try his hand at building something a bit more portable.

The charger is based of a relatively simple design, employing a 5.2 HP Kubota 4 stroke motor and a 12v car alternator to provide power. While you might be inclined to point out that his charger does exactly what an alternator and motor are built to do, there was a bit more to it than simply slapping the two parts together.

A laser cut adapter plate holds the motor and alternator together, but once [Glenn] wrapped things up and gave the motor a spin, he realized that he was driving the alternator backwards. This would eventually cause the alternator to overheat since the cooling fan was running the wrong way. He removed the fan and reversed the fins with a hammer so that he could get the cooling he needed without having to reinstall the alternator in the opposite orientation.

The whole kit was mounted on a hand truck for portability, and [Glenn] says that the charger/generator only needs to run about 5 minutes before a dead battery has enough juice to crank an engine.

[via HackedGadgets]

Automotive battery voltage monitor


[Rajendra’s] car had just about all the bells, whistles, and gauges he could dream of, but he thought it was missing one important item. In an age where cars are heavily reliant on intricate electrical systems, he felt that he should have some way of monitoring the car’s battery and charging system.

To keep tabs on his car’s electrical system, he built a simple device that allows him to monitor the battery’s instantaneous voltage when the car is powered off, as well as the charging voltage across the battery when the car is running. A PIC16F1827 runs the show, using a simple voltage divider network to step the input voltage down to an acceptable level for use with the PIC’s A/D conversion channel. The resultant measurements are output to a four digit 7 segment display, mounted on the front of the device.

He says that the voltage monitor works quite well, and we’re sure he feels a lot better about the health of his car’s charging system. For anyone interested in keeping closer tabs on their car, he has a circuit diagram as well as code available on his site.

Rodent-based power generation

Your hamster lives to good life, with food delivery and a maid service that cleans up after him. [DanF] helped to brighten up this hamster’s life even more by improving its exercise equipment and giving it a small night-light as well. This project adds a low RPM alternator to the hamster wheel.

The first part of the process was to reduce energy lost to friction by fitting the wheel with a bearing. From there a ring of permanent magnets was added which will pass by a stationary coil and induce a current. It works, but unfortunately there’s not enough power generated to charge a battery. That means the light is only on when the hamster is running. But maybe you can figure out a way to use a super-capacitor like we saw in that exercise bike hack.

One nice finishing touch to the setup is a bicycle computer to track how much time was sent on the wheel, and the distance traveled.

[Thanks Dizzy]

Automotive current monitor

If you’ve ever had a car with an electrical system problem you know how hard it can be to pin-point the source of your woes. Here’s a hackery solution that uses a diy PCB to monitor the current being drawn off of the alternator.The sensing is provided by an Allegro ACS758 integrated circuit. This chip measures current up to 150A and outputs an analog signal that can be measured by a microcontroller. In this case an AVR ATmega8 measures the signal and spits the info back to a PC via the serial port. This data can be graphed to help locate when too much current is being drawn for the battery to remain charged.

Check out that CNC milled PCB, what a beauty!

[Thanks Joshua via Elektronika]