We must admit that we’ve been guilty of using a microcontroller to make two LEDs blink alternately in the past. It’s not the worst transgression, but it stems from our discomfort with analog circuits. Luckily, [Ray] published an illustrated guide on building multivibrator circuits. This is a simple method of assembling a two-output oscillator. All it takes is a pair of NPN transistors, which are then switched by on and off based on a resistor-capacitor (RC) timer.

[Ray] does a good job of walking us through how the circuit works at each stage of one complete cycle. You’ll need to read carefully, but the supplementary schematics he uses to water down snap shots of the various electrical states really helped us understand.

Of course, blinking LEDs isn’t the sole purpose of a multivibrator. It is a method of producing a clean square wave which can be used as a clock signal for TTL logic chips. Oh, who are we kidding, see the blinky goodness for yourself in the video after the break.

Make sure your test equipment is handy, then give this video series about crystal oscillators a spin. [Shahriar] of the Signal Path Blog put together a four-part video blog post totaling about an hour. In the discussion he covers the ins and outs of crystal oscillators and ring oscillators. His focus is on how these parts are used as timekeeping devices for microcontrollers. This isn’t a lecture that skims the surface of the topic, it takes you down the rabbit hole, discussing theory, how the devices are built, how to use them, and the pitfalls of doing so.

Our favorite part is in the fourth segment when [Shahriar] measures the effect that temperature has on crystals by spraying them with an inverted compressed air canister. We always thought we were just screwing around when freezing stuff like that. It didn’t occur to us that we were conducting serious experiments.

We’ve embedded the first segment of the video after the break.

[Scott Harden] has been working through a design for a variable inductor to use as a PTO, or permeability tuned oscillator. What you see above is the most recent fruit of these efforts. The variable inductor is made up of the green coil of wire with a threaded bolt in the core. Turning that bolt moves the tip in or out of the coil, affecting its inductance.

Traditionally, tuning RF oscillator circuits has been a function of an adjustable capacitor. But capacitance is only part of the circuit, with inductance being the other important portion. Since variable capacitors that are capable of affecting a large change on the frequency of a circuit can be quite expensive he set out to find another way. This is what prompted the development of his first PTO project.

[Scott] produced a demo video of the hardware seen above which we’ve embedded after the break.

The tabulaRasa is a digital wave table oscillator, and features control of frequency, wave table selection, and interpolation. The device is split up into 2 parts. One is a pcb with a healthy amount of resistors, 3 potentiometers, ST TL074 JFET op amp, atmega328 and a SD socket.

The second part is software for your computer that allows you to edit or create your own waveforms. There are 3 different modes of control. Breakpoints, which allows you to set the waveform points and allows up to sixteen. Harmonic allows amplitude control over 16 harmonically-related sine waves, finally, the third mode lets you load in short sound clips.

Once you’re happy, save to a SD card and pop it into the board, and you’re ready to make some noise. The project page states at the end “tabulaRasa is in the last stages of development, and will be available soon.” so you cant get your hands on one just yet, but if you’re interested [Greg] has a kickstarter page setup where you can find out details on pricing.



Radio communications depend on stable oscillator frequencies and with that in mind, [Scott Harden] built a module to regulate temperature of a crystal oscillator. The process is outlined in the video after the break but it goes something like this: A small square of double-sided copper-clad board is used as a base. The body of the crystal oscillator is mounted on one side of this base. On the other side there is a mosfet and a thermister. The resistance of the thermister turns the mosfet on and off in an attempt to maintain a steady temperature.

This is the first iteration of [Scott's] crystal oven. It’s being designed for use outdoors, as his indoor setup uses a styrofoam box to insulate the oscillator from ambient temperatures. He’s already working on a second version, and mentioned the incorporation of a Wheatstone bridge but we’ll have to wait to get more details.

Behold the Bodystick, an instrument built and demonstrated by [Erich Lesovsky]. It’s a bit like a string bass but instead of strings there is a strip of VHS tape. Apparently not all VHS tape will work, but if you have the right kind you can run voltage through it and then change the resistance with a touch of your finger. It seems that the hand not touching the tape needs to be touching a conductive pad, completing the circuit. The resulting resistance changes the oscillator values on a CD40106 CMOS chip. This project is a bit out there (just like [Erich's] Mega-Tape-O-Phone), and in keeping with its peculiarity is the demo video after the break. Enjoy!

We’ll just say, [Kenneth] really likes clocks. His most recent is a pure 7400 series TTL based one, ie no microcontroller as seen in the past, here, here, and here. The signal starts out as a typical 32,768 crystal divided down to the necessary 1Hz, which is then divided again appropriately to provide hours and minutes.

As far as TTL clocks go, this is nothing too original; until it comes to his creative button interface. By using a not as sexy as it sounds multivibrator, he can produce a clean square wave instead of the figity signals produced from buttons to advance and set the time. Like always, he also provides us with a thorough breakdown of his clock, after the jump.

The DS1077 is a 5volt, 133MHz to 16kHz programmable clock source. The internal frequency divider is configured over a simple I2C interface, and the chip requires no external parts. Not bad for under $2. We used the Bus Pirate to test this chip before using it in a project. Grab the datasheet (PDF) and follow along.

DS1077, $1.69 direct from Maxim + $10 shipping.

This chip isn’t available at any major distributors yet, but Maxim has them for under $2/each with a flat $10 shipping charge. This is an 8pin SOIC surface mount chip, so we made a small breakout board for testing.

Test circuit

Pin connections

We powered the DS1077 from the Bus Pirate’s 5volt power supply. Two resistors, R1 and R2, pull-up the I2C bus to 5volts when it’s not in use. Capacitor C1 is 0.01uF and C2 is 0.1uF, as recommended by the datasheet. Control pins provide some additional functions, but we bypassed them to ground during our test. Output1 is the primary clock signal pin.

Interfacing

We put the Bus Pirate into I2C mode (M, options: I2C, 100kHz). The external pull-up resistors hold the bus at 5volts, so it’s important to leave the on-board 3.3volt pull-up resistors off (default).

I2C>{0b10110000} <– DS1077 write address
210 I2C START CONDITION
220 I2C WRITE: 0xB0 GOT ACK: YES <– got ACK
240 I2C STOP CONDITION
I2C>

First, we broadcast the DS1077′s address and see if it acknowledges. The address of the DS1077 is 1011, plus three programmable bits (000 by default), and the read (1) or write (0) bit. We got an ACK, so we know that the circuit is working and our connections are good.

The DS1077 is controlled by writing values to the locations shown in the table.

I2C>{0xb0 0x0d 0b00001000} , <–write to BUS register
210 I2C START CONDITION
220 I2C WRITE: 0xB0 GOT ACK: YES <–DS1077 write address
220 I2C WRITE: 0x0D GOT ACK: YES <– BUS register
220 I2C WRITE: 0×08 GOT ACK: YES <– BUS register setting
240 I2C STOP CONDITION
I2C>

By default, the DS1077 saves all changes to the EEPROM. We don’t need this during testing, so we disable it by setting bit 3 (0b1000) of the BUS register (0x0d). The first four bits must be left as 0, the last three bits select the address to accommodate multiple DS1077s on the same I2C bus. See datasheet page 7.

I2C>{0xb0 0×02 0b00011000 0b00000000} <–set the 16bit MUX value
210 I2C START CONDITION
220 I2C WRITE: 0xB0 GOT ACK: YES <–DS1077 write address
220 I2C WRITE: 0×02 GOT ACK: YES <–MUX register
220 I2C WRITE: 0×18 GOT ACK: YES <–data byte 1
220 I2C WRITE: 0×00 GOT ACK: YES <–data byte 2
240 I2C STOP CONDITION
I2C>

The MUX register controls the prescalers, CTRL pin functions, and frequency divider.  We disable the prescaler and CTRL pins, and enable the 10bit frequency divider.  The MUX register is explained on page 5 of the datasheet.

Specific frequencies are generated by dividing the 133MHz reference frequency through the prescalers and a 10bit (1025 level) programmable divider.  The clock is divided by the amount specified in the DIV register, plus two. When DIV=0, the output is 133MHz/2=66MHz.

This scheme gives the best frequency resolution in low ranges, and no steps between 133MHz and 66MHz.

I2C>{0xb0 1 0b11111111 0b11000000} <–DIV=1025
210 I2C START CONDITION
220 I2C WRITE: 0xB0 GOT ACK: YES <–DS1077 write address
220 I2C WRITE: 0×01 GOT ACK: YES <– DIV register
220 I2C WRITE: 0xFF GOT ACK: YES <– bits 9:2
220 I2C WRITE: 0xC0 GOT ACK: YES <– bits 1:0
240 I2C STOP CONDITION
I2C>f  <–do a frequency count
9xx FREQ COUNT ON AUX: 16128Hz (16kHz) <– DS1077 frequency
I2C>

We set all the bits in the DIV register to 1 for maximum frequency division. ‘F’ measures the frequency on the AUX pin, which is connected to the DS1077 clock output. With DIV=1025, the frequency is about 16kHz.

I2C>{0xb0 1 0 0} <– DIV=0, 133MHz divide by 2
…
9xx FREQ COUNT ON AUX: 0Hz <–66MHz, too fast to count

————-

I2C>{0xb0 1 0 0b10000000} <– DIV=2
…
9xx FREQ COUNT ON AUX: 3339696Hz (33MHz) <–133MHz/4

————-

I2C>{0xb0 1 0b00000001 0b00000000} <–DIV=4
…
9xx FREQ COUNT ON AUX: 22192384Hz (22MHz) <–133MHz/6

We can play with the divider and generate a range of frequencies. The output is always equal to the reference frequency (133MHz) divided by DIV+2. The Bus Pirate’s input pin is only capable of measuring about 50MHz, so the highest speeds don’t register. A future version of the Bus Pirate should include a gigahertz prescaler for high frequency measurement.

I2C>{0xb0 0x3f} <–write E2 register

Finally, we can write the E2 register (0x3f) to save these setting in the EEPROM. The DS1077 will now return to these settings at power-on.

Conclusion

The DS1077 simplifies complex clock sources by moving a programmable oscillator and frequency divider into a single chip. It isn’t available from distributors, but you can buy it directly from Maxim. If you need better control of high frequencies, check out the DS1085 with 10kHz steps from 133MHz to 8kHz. The DS1085L is a 3.3volt, 66MHz version available at Digikey.

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.