Homebrew Digital Clock

Fig 1. Finished Clock Prototype

Fig 1. Finished Clock Prototype

I started playing with Arduino boards a while ago, and soon discovered an interesting IC, the Maxim DS3231. It is a chip that provides extremely accurate time, backed up with a small battery. Why is this important? Well, for quite a long time, consumer watches and clocks have been timed with a quartz crystal. This provides fairly good accuracy, but production processes have become very slack due to cost cutting, and very few of these will keep very accurate time. Sure, my cell phone has the time, but it’s usually laying around somewhere, or in my pocket. I like to just turn my head and check the time sometimes. So I like to keep a digital clock around, as well.

With an Arduino, I found that I could build a digital clock that is accurate to an amazing degree, only several seconds difference per month. As a bonus, I found that I could program it to handle time zones, and the dreaded change to and from Daylight Savings Time, automatically. So I began to build a long series of prototype clocks, based on many different kinds of displays. One of the most current versions used four 8×8 red LED matrix displays, and looked pretty cool. It became part of a system that also read and displayed weather information from Environment Canada’s data server.

Fig. 2 - The JY-MCU PRO 3208 display module

Fig. 2 – The JY-MCU PRO 3208 display module

Always scouring Ebay for new display ideas, I came across the JY-MCU PRO 3208 display, very new on the market. It has some advantages over the other matrix displays, including larger LEDs, an onboard AVR processor and provision for an RTC (not installed), and was better integrated as a display board, having proper mounting holes that the others lacked. So I ordered one up. When it arrived, I powered it up and it dutifully lit up with a digital clock face (provided by the internal firmware). But my goal was to customize it.

Expecting to find a plethora of software on the internet for it, I was horrified to discover that it was all supporting the previous version, the JY-MCU 3208 (no PRO). So after some aggressive back and forth with the Ebay seller, they finally provided me with a link to download the information package (all in Chinese!). Armed with a schematic and a library that they had copied from Sparkfun, I proceeded to fire it up. The first obstacle was that the processor is a tiny AVR, pin compatible with the well known 328p, but driven with a watch crystal instead of a resonator. This would make a direct upload from the Arduino IDE impossible without modifications.

However, it slowly dawned on me that the onboard processor and the HT1632 display driver IC both share the same programming port pins on the display. Furthermore, I could disable the processor with a single jumper wire. So it would be possible to attach the display to an Arduino and run much more interesting software on it. So off I went to the table saw in the basement, and cobbled up a box that would hold the display, and two Arduinos – a Mega 2560, and an Uno clone. The Mega was equipped with a protoboard piggybacked with a socket for a NEO-6M GPS receiver, and a JY-MCU DS3231 based RTC (real time clock) module. The reason for this giant overkill, was that I was heading for China, and would not have access to all my components and tools. So I provided everything with header pins, and took along a generous supply of DuPont female to female jumper wires. This way I could easily experiment and change things without much effort.

inside the clock

Fig. 3 – Inside the clock – top view

The box consists of four thin plywood sides, held together by two pine side supports. These supports also provide a backing to mount the display with four screws. A thin slot was cut with the table saw, just inside the front edge, to form an escutcheon to hold the display filters. I used high quality Lee photographic studio filters, both a gray and a red colour, to filter out internal reflections and increase contrast. I tested a lot of different filter combinations using the Lee filter sample book, to find the most effective combination. I have also added glass in some other prototypes, but didn’t want it breaking in my suitcase, so I just used the filter material, which is a little too floppy for such a big panel. I will fix that later. The display has more than ample brightness to overcome the light loss of the filter, and still appears “too bright” to my eyes at full brightness. It can be set to any of 15 brightness levels in software. I’m sure that running it at lower levels will also increase the LED life span.

clock_connections

Fig. 4 – JY-MCU PRO 3208 External Control Connections

I’m going to tell you everything you need to know to build one of your own. The onboard AVR on the display is disabled by connecting a jumper from the pin on the display ICSP connector to an adjacent ground (on the display power connector), as mentioned above. This enables a connection to the HT1632 display driver on the same connector, which you can see in Figure 4, to the left. In this configuration, the module receives all its power and control signals from the Arduino. There are only 5 wires to connect, not including the jumper wire (blue in the diagram). There are more details about the hardware connections on the repository page that I link to further on. Before you get too excited about the GPS, we’re not using it for this project. I have developed software that can set the RTC automatically from it. Although it is currently running on the Mega, it will run perfectly well on the Uno. There is no good reason why it wouldn’t run on almost any flavour of Arduino. The circuit requires one DS3231 based RTC on the I2C lines, it doesn’t matter which brand.

Fig 5. Mounting and CPU connections

Fig 5. Mounting and CPU connections

I had a real task cut out for me when I tried to write code for the board. The library that was linked to by the seller (by the way, I have no idea who wrote it or where it came from, but it appears to include the source code for the onboard clock, so I’m merely assuming that it is the manufacturer), displayed the characters in a cockeyed fashion, which I eventually determined is because the LED matrices are rotated clockwise. Thus rendered quite useless. So I inhaled deeply and began to modify the library. It turned into a four week task, working a few hours a day.

I began by writing set and clear pixel routines which are useful in their own right, and then created character and number display methods based on those. I discovered a good font for characters, the ANSI CP437 character set, which has its origin in the original IBM PC! For the clock, I improved the number 0-9 fonts and replaced some seldom used graphics characters with those. They look a lot more legible since they are taller and thicker. I only used them for the clock, if you print something with numbers they will match the letter set more.

Here is a link to my software repository, where you can download all the code – HT1632 Arduino Library for JY-MCU PRO 3208. The repository includes three application programs – A display test program, the clock program, and an implementation of James Conway’s incredible “Game of Life” which is a famous “cellular automaton” or mathematical creature simulator, if you like. There is also a small wiki documenting the class methods so you can use it to write your own programs.

Here, at the time of writing, is the link to the onboard firmware and schematic diagrams of the JY-MCU module – module documentation – click on “??(349KB)” (where the ?? are 2 chinese characters). To unpack the .rar file, you may have to install 7-zip.

The clock program is fairly sophisticated. It polls the RTC 10 times a second, hence can be accurate to 100 milliseconds when freshly set. It has a serial configuration program that allows the user to set the time and configure 12/24 hour time, time zone, a blink mode, and display some other internal registers that are sometimes of interest. The RTC must first be set to UTC or Greenwich time. Once set to a local time zone, the time rules for that zone are selected. This means that Daylight Savings Time is handled automatically and correctly. Basically, this clock is really a “set and forget” appliance, because, although specified to lose no more than about a minute a year, reports from the field indicate that it will usually outperform that, and deviate only by a few seconds. Compare that with most digital watches and clocks, which typically gain or lose many seconds a day.

I’ve been busy adding features. It now responds to button presses on an IR handheld remote. You can select different clock information, or the Game of Life. Another feature is auto brightness. It calculates the sun’s position in the sky and use that to set brightness according to the actual sunrise and sunset times at the clock’s location. There is an optical sensor in the back, as an alternative. The location is set from GPS inputs. That inspired me to also add lunar phases, which it keeps track of, but I’m not sure yet how to display. It also has a DHT22 temperature and humidity sensor, so that information can also be displayed.

I have working prototypes that automatically synchronize my clocks to GPS or atomic clocks on the internet. But those are in development and not yet ready for public release. Another working project I have, is a time server that broadcasts the time from a 433MHz low power transmitter, to all the clocks within a radius of approximately 100 meters that are equipped to receive it.

Another aspect of the overall effort is more artistic – to make clocks that are beautiful and distinctive. I have begun to experiment with finished wood enclosures and different colours and unusual display formats that look cool. The styling on this one came off too much like a shop sign, so I’m eyeing some smaller 8×8 LED modules in different colours like blue and white. They are not available in OEM units with integrated drivers, so I’ll have to build them almost from scratch.

Electric Guitar Pickup Measurements

Electric guitar pickups convert string movements into an electrical signal that can be heard through an amplifier. They come in many different designs, and have different electronic characteristics that give each pickup a different sound. It is possible to make meaningful measurements that will reveal almost everything you need to know about a pickup, using a computer with a good digital sound interface, and a few simple electronic circuits that are easy to home brew.

There are five essential components to my system:

  1. Laptop with Scarlett Focusrite 2i2 digital sound recording unit

  2. Rightmark Audio Analyzer software (freeware)

  3. magnetic exciter probe.

  4. Resistive/capacitive load networks

  5. Input integrator circuit

graph

Fig. 1: Some features of these pickups can be seen here – series vs. parallel, the effects of different loading

The Rightmark software is an audio generator/analyzer. It sends test audio to the sound output of the computer, in this case, the output of the Focusrite box (I use the headphone jack on it). When the software is running tests, it monitors and records audio from the input, analyzes it and has the capability of generating graphs of the results.

electronic probe

Fig. 2: The probe can be positioned between strings on the guitar if necessary.

The exciter probe consists of a small coil of wire. It was salvaged from a dead flourescent lamp ballast. The coil is wound on a plastic form that was part of an inductor. The ferrite E cores that surrounded it were easily removed. Its DC resistance is about 5 ohms and it measures about 1cm cubed. It is fed through a 100 ohm resistor, in order to provide a current feed, instead of a voltage feed. The ratio of resistance to inductance is high enough that the frequency response is essentially flat from 20Hz to 20KHz. The reason for not using a voltage driven coil of large inductance, is mainly that such a coil, in practice, must have a permeable core which will interact with the device under test and skew the measurements by increasing the inductance slightly. This probe contains no magnetic materials and so should have little effect on the readings. The coil and parts are tie wrapped to a popsicle stick for convenient positioning during tests.

Different resistive/capacitive load networks are used, depending on which tests are being performed. The most basic is a 10:1 resistive divider that functions both as a

schematic

Fig. 3: Basic Measurement Circuit

normal resistive load for the pickup, and as an isolator to prevent the test cable and sound interface input from loading the pickup. For the inductive measurements, a capacitor is placed directly in parallel with the pickup. For the purpose of recording performance curves, it is better to use the integrator circuit because it shows the frequency response more faithfully.

By running a few sweep tests in Rightmark, nearly all of the important information about a pickup can be measured – the resonant frequency, inductance, capacitance, and also the frequency response. I use a spreadsheet to perform the mathematical calculations.

In operation, Rightmark first generates a 1kHz calibration tone at the audio output. At the same time, it measures and displays the input level. The output level is adjustable via the headphone volume control, and the input level can also be adjusted with the input gain control – all on the Focusrite. With some pickups it can be tricky to get adequate signal without distortion (also displayed by Rightmark). After some sweeps, I got warning messages about clipping, but I didn’t see anything crazy in the curves to suggest a major problem. When calibration is complete, you can run a sweep test.

graph

Fig. 4: Results of a raw signal measurement – no integration. The lower curves are with a test capacitor in parallel, used to measure inductance.

Using only the raw signal from the voltage divider with most pickups, the result is a peak with a 6dB per octave slopes on either side. This fits theory perfectly, but requires some explanation about why it is not flat in the pass band. In fact, other researchers data does show a nominally flat signal in the pass band. It is because the probe they are using has a built in 6dB/octave loss due to being a voltage driven inductance. The pickup coil responds according to Faraday’s law of induction, where the output voltage is proportional to the rate of change of the magnetic field. A current driven probe produces a constant amplitude varying magnetic field, since the intensity of the field is proportional to the current. Then the induced voltage in the pickup coil is the derivative of the current, which rises at a 6dB/octave rate (since higher frequency waves have steeper slopes, for the non calculus aware reader).

Parameter Measurement Procedure

Measurement of the pickups two most important parameters, inductance and resonant frequency, are made easily with the Basic Measurement Circuit. First, the pickup is connected to the test circuit, and the exciter coil is positioned next to area of the pickup that is of most interest (in practice it makes little difference). The first step is to measure the inductance. For this, S1 should be closed to insert C1 in parallel with the pickup coil.

Run a sweep with Rightmark. This should generate a peak in the lower part of the audio spectrum, such as the ones in purple and green in Fig. 4. Record the frequency of the peak. Rightmark can be adjusted to narrow down the frequency range so that it is easier to read the exact value, if desired.

The inductance can now be calculated from the equation:

L = 1 / (2*pi*fc)2 / C  , where fc is the capacitively loaded peak frequency

The value of C1 is much higher than the intrinsic capacitance of the pickup itself, and so it “swamps it out” and it can be ignored. Now open S1 to remove C1 from the circuit and run another sweep. This will generate another, much higher peak like the white and blue ones in Fig. 4. The frequency of the peak is just the unloaded resonant frequency of the pickup. Record this parameter.

Now the intrinsic capacitance can be calculated from:

C = 1 / (2*pi*fc)2 / L ,  where fc is the unloaded peak frequency

Equalized Response Curve Graphs

circuit

Fig. 5: Original prototype Integrator circuit to equalize for current driven exciter probe

Since there is an overall +6db/octave bias, the flat region in the pass band has a positive slope, and the stop region above the resonant frequency has a -6db/octave slope (since +6-12 = -6). In other words, in the case of a voltage driven probe, the rising slope and the falling slope cancel each other out. Therefore, I built an integrator to place in line between the pickup and the sound card, to eliminate the bias. This has two benefits – it makes the results read more intuitively, and it reduces the dynamic range of the sweep. The non-integrated signal is at the limit when measuring to 12kHz, because the signal gets quite big above that due to the rising slope.

Update 2016/09: I completed the design of a much better integrator circuit. cin_0321It has already been built and tested by myself and Andrew Flanders, who has been conducting similar research for a few years. It uses a Linear Technologies LT1058 or a Texas Instruments TL084 JFET op amp to achieve a very accurate integration function. It produces much better results than the plot shown in Fig. 6. The input has a built in high impedance preamplifier, which has very low intrinsic capacitance, hence does not interfere with the characteristics of the measured pickup very much. I used Eagle to design the PCB and had it manufactured by PcbWay. You can download the manual here: pickup_measurement_procedure

The electrical model of a pickup coil is well documented, and consists of a large inductance “L” with a distributed internal capacitance “C” and resistance “R”. The internal R determines the Q or “sharpness of resonance” of the tuned circuit of the L and C. However, the internal resistance is not the same as the load resistance, which in practice is the volume potentiometer in the guitar. Powered pickups like the EMG’s use an internal fixed resistor for this purpose.

graph

Fig. 6: Here the effect of resistive load is very clear. Notice that this parallel wired pickup has an extremely good high end, but requires a low resistance load to be very flat. The scale is expanded to make small dB differences clear.

The complete circuit includes the pickup and load resistance, and any capacitance that is deliberately or inherently placed in parallel with the load. This circuit has the form of a second order RLC low pass filter, which has a loss above the cutoff frequency of 12 dB/octave. Conveniently, the degree of resonance or “damping” is a well understood phenomenon for this filter, and easy to calculate. This means that the value for the pickup resistive load (usually the volume control) can be tailored to achieve a desired degree of peaking or flatness, without any trial and error. For tests, it is desirable to use a very high resistance because the peaks allow a more accurate measurement of the resonant frequency.

It is tempting, and interesting, to run the tests on pickups while they are installed in the instrument. Although it is not possible to measure inductance and other pickup parameters this way, it can directly reveal a lot of information about the frequency response of the guitar electronics as a whole, including the operation of the tone controls.

circuit layout

Fig. 7: This is the whole test circuit, looking ugly now because it is built in air! At the top is the battery powered integrator. Magnetic probe is placed on the pickup. It’s a mess because I had no electronic bench in China

The (original) integrator is a standard circuit built around an LM4558 op amp. It allows essentially flat measurements from 100 Hz to 20 kHz. Since it has a -3dB cutoff around 32Hz, the response will show a -6db/octave roll off below that. From over 20 measurements of different pickups in different configurations, I discovered that all of them are flat within a fraction of a dB, below about 200Hz. Therefore, I concluded that it is not worth measuring, and that the 100 Hz value can be taken as a very accurate indication of what it it at, say, 20Hz. It is possible to lower the cutoff frequency of the integrator to include lower frequencies, by changing some resistor values.

This is a prototype and lacks conveniences, however I believe it shows that it is not very difficult to obtain measurements that are useful for installing, evaluating, or designing electric guitar pickups. Next I will explain how to interpret the readings and perform the necessary calculations to derive electrical specifications.

The main effect of a resistive load on a pickup, is to control the peaking at the maximum frequency that the pickup can reproduce. There are two ways to describe the amount of peaking.

One is called “damping factor”, which is sometimes labelled “k”. Values of k greater than one result in a minimum peak, and if much too large, result in a gradual, excessive signal loss at the maximum frequency. Values of k less than one result in a signal peak at the maximum frequency. It produces a strong resonance if k is very small. A typical guitar volume control provides a value of k = 0.5 or less, but I prefer to aim for a more flat response from k = 0.707.

The much more common way to describe peaking, is the Q factor. Q and k are related by the formula:

Q = 1/(2 * k)

Designing a Resistive Load for a Magnetic Pickup

An actual pickup was measured for this procedure. It is a Strat type single coil with steel pole pieces and a ceramic magnet. Instead of using a stock potentiometer and calculating the Q, we can choose a Q that we like, and find the replacement potentiometer value that will produce it.

1. Measure the inductance

The measured DC resistance is 6.2k.

With a 0.1uf cap in parallel with the coil, the measured resonant frequency is 400 Hz. Calculate the inductance:

fc = 1/(2*pi*(LC)0.5)
L = (1/(2*pi* fc))2 / C
L = 1.58 H

2. Now knowing L, we can calculate the coil’s intrinsic capacitance.

The measured self resonance frequency is 8500Hz.

C = (1/(2*pi*fc))2 / L
C = 222 pF

3. Calculate the load resistance to provide a Q of 1.8:

Q = R*(C/L)0.5

R = Q/(C/L)0.5 = 1.8/(222*10-12/1.58)0.5

= 152K ohms

  1. The stock volume potentiometer that was used with this pickup had a resistance of 200K ohms, so the actual value of Q can be computed:

Q = R*(C/L)0.5 = 200,000*(222*10-12/1.58)0.5 = 2.37

Note that these calculations assume that there is no load capacitance, such as a tone control or amplifier cable adds. If the volume control is at the maximum position, the cable capacitance is in parallel with the pickup capacitance and can be added to the calculations. When the volume control is at lower settings, the capacitance is decoupled from the pickup to some extent, and has less influence. The same can be said for the standard tone control, except that we accustomed to call the maximum the “lower” as it is lower in frequency (it has the maximum effect when it is turned down all the way). Changing either the cable or tone capacitance alters both the Q and the resonant frequency.

When the capacitance increases, the Q will increase, and the resonant frequency will decrease. So it is easy to see how a tone control will influence the sound. But if we are interested in the base conditions, it is best to estimate the situation when the tone control is at a maximum (thus mainly out of the circuit), and calculate values including the cable capacitance.

A typical cable capacitance is about 80pF/meter or 25pF/foot. Thus a 3 meter or 10 foot cable will have about 250pF of capacitance. The Strat pickup that we measured above had an intrinsic capacitance of 222 pF. Parallel capacitances add, so the total capacitance is 250pF + 222pF = 472 pF. Since we have almost doubled the capacitance, it will obviously affect the output audibly. Recalculating the Q with cable capacitance:

Q = R*(C/L)0.5 = 200,000*(472*10-12/1.58)0.5 = 3.46

This Q is quite high, but produces the tonal flavour that Strat pickups are well know for. It is not the focus of this section, but worth noting that the resonant frequency has dropped from 8,500 Hz to 5,830 Hz. It is obvious that in order to achieve the desired results, we have to include the cable capacitance in all the load resistor calculations. So, for example, with the Strat pickup, we wanted to achieve a Q of 1.8. Including the cable capacitance:

R = Q/((C+Cc)/L)0.5 = 1.8/((222*10-12+250*10-12)/1.58)0.5

= 105K ohms

An interesting outcome of this analysis is that once the Q for a given resistance, or the resistance for a given Q, are known, different values of Q or resistance are extremely easy to calculate because they are linearly related. A doubling of resistance will produce an exact doubling of Q. So if you are changing your volume potentiometer from 250K ohms to 500K ohms, you can expect an exact doubling of the Q, no matter what it is. In general:

Qnew = Qold * (Rnew / Rold)

 

Differential Distortion Revisited

Differential Distortion PCBI designed a guitar distortion device in 1995. It was published in Popular Electronics magazine, August 1995 issue. I dubbed it “Differential Distortion”. With the advent of the web, the circuit was copied and added to many online compendiums of guitar circuits. I was very happy to see that, because I thought some people might benefit from some important features of the unit. Mainly, the extremely low battery drain, which might make it more practical to build into a guitar if desired. It’s also quite small, with a low component count.

Sometime around 2005, I had access to a full SMD workstation, and drawers of SMD resistors and capacitors. So the bug bit me again, and I decided to build the circuit using SMD parts. I ordered the transistors online, and had the boards made by a small run OEM manufacturer. The parts were insanely small – 0402 resistors look like grains of pepper in a jar. But I had a stereo microscope to help. I built a few of them, tested them. Then a big house move hit me, and the project languished in boxes for a few years.

In 2013, I went to an electronics mall called Chenghuangmiao in Chengdu, in the province of Sichuan in China. I have never seen so much electronics in all my life. It struck me that, not only there, but everywhere now, SMD parts have taken over from the old through-hole components. The new stuff is smaller, cheaper, and often has tighter tolerances. As I left the mall, I looked in a doorway and saw someone doing CAD layout. Looking around the store, I realized that they could build prototype boards. So it was back to the drawing board again.

I consulted my old plans, and went back to shop. But the low noise 2N5088 transistors were nowhere to be found. I realized that I would have to substitute another part. The sellers there are basically merchants, only a few have any deep engineering knowledge. So my attempts to explain my need for a “low noise” transistor were futile. One store assured me that, “all our transistors are low noise”, I think they believed I was impugning the quality of their wares. So I simply copied their lists of available parts, and went home to use the internet to look at data sheets. This led me to choose the excellent and common MMBT9014/MMBT9015 NPN/PNP types.

At the same time, I started reading opinions and experiences with the circuit that were posted on various forums. I realized that I had a few improvements of my own in mind. So a complete redesign was undertaken, with an eye to retaining all the worthy features. It has to be said, that the role for an analog device like this has been sidelined by digital audio and DSP emulation in the commercial market. Also, that as a consequence, nostalgia has led DIY builders in the direction of re-creating older designs, rather than looking in new directions. Still, some people had built it, and even reported the results of some modifications that they had dreamed up. Some implementations I didn’t agree with, but there were some good ideas to work with.

First, I made some important engineering changes. The single resistor emitter bias in the input transistor circuit of the original, was too unstable and sensitive to component and battery voltage changes. So I fixed that with an extra capacitor and resistor in the emitter circuit to lower the DC gain. The audio output level was too low, so I increased it. The input impedance was too small, so I increased it as much as I could. The 0402 parts were too small, so I used 1206’s. There is an advantage that the resistor values are readable. Next generation, I think I can step down to 0804’s and still be able to hand assemble with only soldering iron and tweezers.

Next I accomodated the mods. I added an inverting output to the differential pair. This allowed a kind of “distortion depth” control to be added. Or call it “fuzziness”, “bite”, “grunge” if you like. I also added a bias control circuit that allows the differential offset to be varied to change the harmonic tone and sustain. Some people thought the sound was too harsh, so I added an optional low pass filter on the output. The classic Fuzz Face that is a classic fuzz design, had an extremely low input impedance (stupidly low!). When guitar pickups are loaded this way, it cuts out the highs. However, the sources I was using claimed that it cut out the lows, so I added an adjustable high pass filter on the input to emulate that. It was implemented after the board design, so it is the only off board component option. That means that the area of input filtering needs a lot more experimentation.

At every step, I wrestled with the design goal of simplicity, with a lot of bang for every buck. So I didn’t allow myself to get too fancy. Most of all, I didn’t want to use any more battery current. Since I was mass producing one board, I tried to make it possible to change options just by changing the external wiring. For example, it can have the variable controls, or fixed settings, using only some jumper wires.

The board was laid out from a schematic and some sketches in about an hour, by a diffident but efficient young man, who was terribly late and had to be coaxed in by phone by the management. He did a good job, but didn’t fully understand my request to lay out thermal breaks on all the pads. With wave or oven soldering, the entire board is heated, so small components can have leads soldered to large areas of conductor. If you are hand soldering or doing repairs, this creates a nightmare as the conductor sucks up all the heat from the iron and refuses to get hot enough to make a proper joint. Or sometimes, the extra heat damages things. He did follow the instructions on ground connections, but didn’t realize that I wanted it on every connection. So a few of the components on the board were really pesky to place. I will be really demanding on this issue next time.

My order of 200 boards was completed in about a week. I found one mistake, my fault because I left the schematic at home and had to draw them the circuit from memory. I figured out that I could fix it with one 1/16 watt resistor, you can see it placed as a jumper on the top left of the board. Some boards had defects, but they had done a full inspection and marked them out. They even placed parts on two of them for me! The price was very reasonable, of course. It’s China. I remarked to my Chinese friend, that in my country a PCB company would look down their noses at me for even daring to ask to have such a job done. There are some mail order houses, but they are very pricey for a full build – mask, screen and assembly. There was no extra charge for the layout work!

I built about 80 boards. I don’t have them where I am now, but I plan to make some available as DIY kits. Originally, I thought I would supply the board and components separately, but as I mentioned, the SMD assembly is too difficult. I could do it next time with the board improvements. The circuit is simple enough that it can actually be built up on perfboard or whatever you like, as you like. I want to build a version that has the potentiometers and switches all on board, so it is a complete module that requires only battery and audio connections. The idea is to drill holes in the pickguard so it can sit in the control cavity of a guitar.

The first schematic I have posted is a shop drawing, so there are some things I need to tell you. The board is about 2.0 x 2.5 centimeters. The input is on the left, output on the right. The two potentiometers VR1 and VR2 are 10k linears. Switch S3 chooses clean or fuzz, S4 is the output low pass, S2 is the input high pass. You can substitute a 250k potentiometer for S2 if you want it variable. To eliminate the bias control, jumper T10 to T11. To eliminate the fuzz control, jumper T8 to either T6 or T7, whichever sound you prefer. To eliminate S3, just use T4 as the final output (T4-T5 jumper is not required). The dot marks on the potentiometers indicate the fully clockwise position – all the way to eleven as Spinal Tap says! 🙂

The second schematic shows the circuit design better. If you are interested in how the circuit works, or want to build your own, this is the one to study.

As a stomp box, it is possible to have the battery switched on from the jack, so it is automatically on whenever it’s in use. I didn’t show that, because it’s not how I do it. But it is possible. The people that copied my circuit included it in their version. Go have a look.

Here is the shop drawing, showing the off board wiring:
Differential_Distortion_Schematic
Here is the general schematic:
differential_distortion_general
Here is the original 1995 schematic:
diffuzz.gif