Retrotechtacular: Crystals Go to War

More than one of our readers suggested we highlight this beautifully-shot process documentary about the laborious and precise manufacturing of piezoelectric quartz crystals in the early 1940s. Just a few years later, Bell Labs would perfect a method of growing synthetic crystals, sending droves of brave men and daintily-handed women from the Reeves Sound Laboratories to the unemployment line.

Early radio equipment relied upon tuned or L-C circuits for clocking. These were prone to drift by a few kHz, which prompted the use of crystal oscillators for stable frequencies in the 1920s. The lives of our armed forces and those of our WWII allies depended on reliable communication equipment, so the crystal oscillators they used were top shelf, produced by hand from Brazilian crust.

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Digital “Crystal” Breathes New Life into Old Radio

[Bill Meara] of the Soldersmoke Podcast has a nice old Drake 2B radio, and wanted to use it for the 12 meter amateur band. These old radios normally make switching tuning bands easy — you just swap out one frequency crystal for another and you’re set.

Only [Bill] didn’t have the 21 MHz crystal that he needed. No problem, because he had a junk crystal, a hacksaw, and a modern direct-digital synthesis (DDS) chip sitting around. So he takes the donor crystal, cuts it open, and solders the two wires directly from the DDS to the crystal’s pins. Now he’s got a plug-in replacement digital oscillator that doesn’t require modifying the nice old Drake receiver at all. A sweet little trick.

The video’s a little bit long, but the money shot comes in around 5:00.

Now, one might worry about simply plugging a powered circuit (the DDS) in place of a passive element (the crystal), but it seems to work and the proof of the pudding is in the tasting. We wonder how far this digitally-controlled-analog-receiver idea could be extended.

The Cheapest Crystal Oven


The crystals you’ll find attached to microcontrollers or RTCs are usually accurate to 100 parts per million at most, but that still means if you’re using one of these crystals as a clock’s time base, you could lose or gain a second per day. For more accuracy without an atomic clock, a good solution is an oven controlled crystal oscillator – basically, a temperature controlled crystal. It’s not hard to build one, and as [Roman] demonstrates, can be built with a transistor and a few resistors.

The heating element for this OCXO are just a few resistors placed right on the can of a crystal. A thermistor senses the heat, and with more negative feedback than the Hackaday comments section, takes care of regulating the crystal’s temperature. A trimpot is used for calibrating the temperature, but once everything is working that can be replaced with a fixed resistor.

This deadbugged circuitry is then potted in five minute epoxy. That’s a bit unconventional as far as thermal management goes, but the results speak for themselves: [Roman] can get a clock with this circuit accurate to a few seconds per year.

Piezoelectric Crystal Speaker for Clock Radio Is Alarmingly Easy to Make

cockadoodledooLet’s face it: most of us have trouble getting out of bed. Many times it’s because the alarm isn’t loud enough to rouse us from our viking dreams. [RimstarOrg]’s homeowner’s association won’t let him keep a rooster in the backyard, so he fashioned a piezoelectric crystal speaker to pump up the volume.

[RimstarOrg]’s speaker uses a Rochelle salt crystal strapped to a bean can diaphragm. In his demonstration, he begins by connecting an old clock radio directly to the crystal. This isn’t very loud at all, so he adds a doorbell transformer in reverse. This is louder, but it still won’t get [RimstarOrg] out of bed.

Enter the microwave oven transformer. Now it’s sufficiently loud, though it’s no fire bell alarm. He also demonstrates the speaker using a piezo igniter from one of those long barbecue lighters and a crystal radio earpiece. As always, the video is after the jump. [RimstarOrg] has a lot of relevant linkage in the summary so you can learn how to grow your own Rochelle crystals.

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Repairing Dead USB Flash Drives

Over the last few years, [Tobias] has repaired a number of USB Flash drives. This strikes us as a little odd, given small capacity Flash drives are effectively free in the form of conference handouts and swag, but we’re guessing [Tobias] has had a few too many friends lose their thesis to a broken Flash drive.

In all his repairs, [Tobias] found one thing in common The crystal responsible for communicating with the USB controller is always broken. In a way, this makes a lot of sense; everything else on a Flash drive is silicon encased in an epoxy package, where the crystal is a somewhat fragile piece of quartz. Breaking even a small part of this crystal will drastically change the frequency it resonates at making the USB controller throw a fit.

[Tobias]’ solution for all his Flash drive repairs is to desolder and change out the crystal, bringing the drive back to life. Some of the USB Flash drives even have multiple pads for different crystal packages, making it easy to kludge together a solution should you need to repair a Flash drive five minutes ago.

Accurate timers with an AVR


An awful lot of microcontroller projects use timers to repeat an action every few minutes, hours, or days. While these timers can be as accurate as a cheap digital wrist watch, there are times when you need a microcontroller’s timer to measure exactly, losing no more than a few milliseconds a day. It’s not very hard to get a timer to this level as accuracy, as [Karl] shows us in a tutorial.

The problem with keeping time with a microcontroller has to do with the crystal, clock frequency, and hardware prescalers of your chip of choice. [Karl] started his project with an ATMega168 and a 20 MHz crystal and the prescaler set at 256. This made the 78.125 interrupts per second, but the lack of floating point arithmetic means one second for the microcontroller will be 0.9984 seconds to you and me.

[Karl]’s solution to this problem was to have the ATMega count out 78 interrupts per second for seven seconds, then count out 79 interrupts for one second. It’s not terribly complicated, and now [Karl]’s timers are as accurate as the crystal used for the ‘168’s clock.

27 MHz transmitter/receiver pair made with 555 timers

Get your feet wet with radio frequency transmitters and receivers by working your way through this pair of tutorials. [Chris] built the hardware around a couple of 555 timers so you don’t need to worry about any microcontroller programming. He started by building the transmitter and finished by constructing a receiver.

Apparently the 27 MHz band is okay to work with in most countries as long as your hardware stays below a certain power threshold. The carrier frequency is generated by the transmitter with the help of a 27.145 MHz crystal. The signal is picked up by the receiver which uses a hand-wrapped inductor made using an AL=25 Toroid Core. We’d say these are the parts that will be the hardest to find without putting in an order from a distributor. But the rest of the build just uses a couple 555 timer chips and passive components, all of which will be easy to find. The video after the break shows the project used to receive a Morse-code-style message entered with a push button. It would be fun to interface this with your microcontroller of choice and implement your own one-way error correction scheme.