Two Channel Signal Generator for Guitar

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About: Beyond Boutique. Custom-built guitar effects pedals of my own design. Anything I post to this Instructables site is for private, non-commercial use only. No transfer of intellectual property from The Fi...

This project is an easy to build, original design for a dual channel Signal Generator for guitar and other uses. It covers the entire range of guitar notes (for you guitarists, from open Low E string - 83 Hertz, up to the 24th fret on the high E string). It has two separate channels each can be set to a separate frequency, and you can easily switch between them.

This will be very useful to help you diagnose issues with guitar pedals and with your guitar amp, without using your guitar itself. If you like to build guitar effects this will be useful to have during the building- and testing stages of you guitar effect.

It has two outputs:

  • a 1/4 inch guitar plus so you can connect the device to a guitar effects pedal or two a guitar amplifier
  • A pair of red and black binding posts, that you can directly attach to wires on a guitar effect you might be building, or can plug in a pair of test leads.


I designed this with the following constraints in mind:

  • Must be original. I am not going to buy some pre-built board from aliexpress, slap it in a case, and call it a "Maker project" for you.
  • Designed for the beginner to be able to construct it with no tears.
  • Covers the entire range of guitar notes and then some
  • Workmanlike not beautiful. No this is not how my $150 to $200 custom guitar pedals I sell look. Lol
  • Simple, intuitive operation: three control knobs and a switch. Guitarists will immediately sense how to use it.
  • Very low parts count and all parts are common and easy to obtain. A few resistors and capacitors and a single chip.
  • Foolproof design, very little that can go wrong. Robust.
  • Quiet battery operation. Runs on (2) common AA batteries; no need to plug anything in to the wall.
  • Two separate channels so you can set up two different notes or tones, and switch between them
  • No microcontrollers, no Arduino, no programming, no software. My favorite programming language is solder.
  • Draws microwatts of current. (2) AA batteries should last for many months to years, even if you forget to shut it off. The prototype only draws 33 microamps of current.
  • Covers the output voltage range of all passive single coil and humbucking pickups from guitar makers (Fender, Gibson etc.), through the most powerful aftermarket pickups.
  • Peak voltage approximately 1.5 Volts AC (RMS) which covers active pickups too. Will not harm your guitar amp nor guitar pedals.
  • Quick to built. Estimate 1-2 hours maybe less.

Let's get started building it.

Step 1: A Tiny Bit of Acoustics (optional)

The guitar is an instrument that covers five musical octaves. The open low E string (called E2 on the chart) has a fundamental frequency of 82 Hertz (cyles per second). The open high E string (E4 on chart) is about 330 Hz. The 24th fret on the high E string (E6 on chart, as high as most guitars go) us 1,319 Hertz.

On the sample box I built to show you I have marked off some of these frequencies with a Sharpie on the face of the control pannel

The project you are about to built covers frequency range of about 82 Hz to 4,500 Hertz, or one and a half octaves above the highest note on the guitar -- however the usable range of this device is up to E6.

A single coil passive guitar pickup like you would see on a Fender Telecaster or Stratocaster puts out about 100 millivolts (a millivolt is 1/1,000 of a volt). Some hot aftermarket pickups have a output of about 190 to 300 milivolts (mV). A humucking pickup like a Gibson starts at about 200 mV. Aftermarket pickups can go up to the 400-500 mV range for example the hottest DiMarzio or Seymour Duncan pickup is in that range for a humbucker. This signal generator covers that range. If you look at the picture of the sample box that I built, I have marked the volume control in 100 mV increments. Yes we all know if you whack the strings really hard you can generate instantaneous peaks of up to 1,000 mV (one Volt) but for a signal generator you want to use the typical range.

You have my permission to build one of these for your personal non-commercial use and am sure you will find it useful; yet I retain the exclusive commercial rights to sell these either assembled or in kit form, at some later time as I wish,

Step 2: Here Is the Circuit Board, Parts List and Wiring Diagram (layout) for This Project

The circuit board is the common, Radio Shack part # 276-159B

General-Purpose IC PC Board

For about $2 US you get two of these boards in a package, just snap them apart because we only use 1 of the two boards. Radio Shack has some financial issues but still sells online; this common board comes in and out of stock frequently.

The wiring diagram, also called the layout diagram, is shown. I drew this myself and drew my own template for the board. The shiny copper side is underneath that is where you solder the components; in this Instructable and in my layout diagram, we are looking at the top side of the diagram

Parts list:

1/2 of a Radio Shack 276-159B printed circuit board

(2) 1K ohm resistors

(2) 220K ohm resistors

(2) 0.47 uF capacitors

(1) 47 uF capacitor

(1) CD40106 Hex Schmidt trigger inverter -- (substitution: a 74HC14 also works fine as does a CD4584)

(3) 100K ohm potentiometers (linear pots)

(1) battery holder with wires, that holds (2) AA cells

(1) toggle switch, 3PDT on-off-on mini toggle switch, latching type*

(1) quarter-inch mono guitar jack

(2) binding posts with nuts, red and back

(1) case to mount it in I used a Radio Shack project box part # 270-1805


* Can get these on eBay as cheap as $2 for 2

https://www.ebay.com/itm/2Pcs-3PDT-Mini-Toggle-Swi...

Step 3: Here Is a Closeup of That Layout Diagram

I fixed a couple of previous errors in the first diagram; it is all set now.

Step 4: Install the Chip and the Jumpers. Next Install the Resistors and Capacitors

The first thing to do is to install the 40106 Schmidt trigger chip itself and the five (5) wire jumpers on the board. Please them one at a time on the board, turn the board over and solder them. The legs on chips when you get them are "a little wide" to fit into the board. I generally hold the chip on a table top and with a gentle but steady rocking motion, bend all 7 pins at a time. Then turn the chip over and do the same to the other side. Go easy on this we want to not bend or break off any pins

Note that the chip has a U shaped cutout that indicates the TOP of the chip. Make sure the U points to the top of the board, not the other way around.

Next install the (2) 1K resistors, the (2) 220K resistors, and the (2) 0.47 uF capacitors. Just like the 40106 chip has a top and a bottom, the caps have a polarity. You will see a white stripe on the side with minus symbols (-) on it -- that is the negative side. The negative side of a cap also has a shorter lead than the positive side. In any case, for this project the (-) stripe faces the TOP of the circuit board, and that is why you do not see it in the photo.

Next install and solder the two green Ground wires that carry the ground signal from the top of the board to the bottom.

Step 5: Now Wire Up the Two Frequency Control Pots and Solder the Wires to the PCB

All three pots in this design are 100K so thank goodness, there is no possibility of screwing them up. Turn one of the pots upside down and solder a yellow wire to the left-hand lug, and solder an orange wire to the center lug -- see photo. Now do the same for another one of the 100K ohm pots. These two pots will control the frequency of the notes for Channel A and Channel B respectively. Give each wire a tug after the solder cools, a loose control wire is not wanted. After I solder a pair if wires I braid them i.e. loosely twist the yellow and orange wires around each other in a spiral, like a braided piece of rope. Install and solder the two frequency control pots on the PCB, paying attention to the photos and looking at the layout GIF file carefully.

Also install the (2) OUT wires, one each for Channel A and Channel B, as seen in the photo and on the layout. On the layout diagram they are blue but on the project I used purple wire because I was running low on blue wire. Color coding wire is useful in practice: I like to use White to represent Inputs, Blue or Purple for Outputs, Green for Ground wires, Red & Black for the 2 terminals of a battery (see battery holder in a later step, and use yellow and orange for controls.

Step 6: Wire Up the Toggle Switch to Channel a & B OUT Wires, and Red Battery Wire

First, take a look at the battery holder. It take (2) 1.5 Volt AA size batteries, in series, so that when there are batteries in it, the voltage is 3 Volts. The holder has a red (positive) wire and a black (negative wire)

DO NOT INSTALL THE BATTERIES JUST YET.

In this step we are going to hook up the toggle switch so that when it is in the center "off" position, power is off, and when the switch is in either the up- or down "on" position, power is on.

To help us we are going to cut our own piece of wire and strip the ends of it to 2 different lengths. The 'short' end is about 1/4" long and the long end, about 1/2" long. Call this the funny wire. The long end we are going to use to bridge both "on" ends of the switch. The center of the switch, on left side we solder the red wire from the battery holder. Now only after the battery wire is soldered to the center tab of the switch, put a bend in the "long side" if the wire you cut, and solder it to both the top and bottom lugs of the switch. See photo and please observe it carefully. You will notice two things:

  • The long piece of wire does not make contact with the center terminal where you soldered the red battery wire. This is very immortant. If the two wires touch, the device will always be on: that is what we call a "short circuit". We must avoid this.
  • The long end of the funny wire is soldered to both the top- and bottom solder lugs of the switch.

Now take the other end of the red funny wire and solder it onto the PCB at the very top of the board, refer to the layout digram. The red wire is the one marked +3 VDC ie.e. 'positive three Volts DC'.

Finally, solder the black wire from the battery holder onto the PC board. It is the wire that the layout identifies as GND (for Ground).

What we are left with once we install the batteries several steps from now, is the black wire from the battery holder is always connected to the circuit board; the red battery wire is soldered to the center lug of the swtich and flipping the switch connects it to the funny wire; the funny wire is soldered to the circuit board. Now we have a battery power supply that is controlled by a toggle swtich -- exactly what we wanted.

Theory and practice:

We want a switch that is "off" in the center position so the device we are building gets no power. In the center, off position of the toggle switch it is disconnected from either the top of bottom solder lugs. However when you flip the switch up OR you flip it down, there is now an electrical connection between the middle lug (the battery wire) and the funny-length piece of wire you just soldered to the top and bottom lugs. The end effect of this is just what we want: flipping the switch either up- or down positions powers up the device. Again, this is not complicated but takes a bit of hand-eye coordination to make sure the funny wire does not contact the center terminal, and short-circuit.

That takes care of the power. Now let's hook up the output wires from Channels A and B Notice that the middle three solder lugs of the switch are UNUSED. Solder the two purple braided wires to the top- and bottom solder lugs of the swtich. Now in the lug between them solder a white write. This is a common output wire. When the swtich is in the up position, the Channel A output feeds into the white wire; when in the down position the Channel B feeds into the white wire. That is how we switch the two Signal Generator channels to a common output, that uses the toggle switch to select which channel. In my sample build I marked ends of the two purple wires with a black and a white Sharpie so I would know which one is Channel A and which is Channel B. That is because they are both the same color and I braided the wires. Either way the device will still work I just like it to be that toggle switch "up" is Channel A, and "down" is Channel B ;

Notice that the white, common OUT wire is not connected to anything yet. Be patient. We are going to hook it up the the Volume control in the next step.

Step 7: Wire Up the Volume Control Pot

Remember the white, common OUT wire from step 5, that was not connected to anything yet? Well in this step we are going to wire it up to the volume control. Refer to the photo.

Notice that the other two 100K pots only had two wires: that is because in this circuit they are wired as variable resistors. However, the third and last 100K pot is wired as its more traditional role: As a Volume control, or as we call it in electronics, a voltage divider. That requires three wires. In the photo you can see I turned the pot upside down and wrote a big V on it for Volume. Solder the white wire (that came from the toggle swith -- remember?) to the lug on the left. Solder a new yellow wire in the middle. Solder a green wire on the right. Green is for ground. Connect the other end of the ground wire from the volume control to the PCB on the bottom rung, as shown in the layout diagram.

The yellow wire my friend, is going to connect to the tip lug of the 1/4" guitar jack. The is the signal (the ground lug of the guitar jack you and solder a second green wire to the volume control, and solder that to the "ring" lug of the guitar jack. In a guitar cord, the tip is the signal, and the long tubular part we call the riing, is the ground or return signal.

Now in my build, not on the aluminum control panel but rather on the side of the case, I drilled two holes and added a Red (+) and Black (-) binding post. I wired the tip (signal) lug of the guitar jack to the red post, and the sleeve (ground) lug of the 1/4" guitar jack to the black binding post. That way you can plug in the test leads from a multimeter (the kind that have "banana" plugs on one end and point probes with a plastic handle on the other) into the two binding posts. That makes it a great way to inject a signal into an electric circuit, let's even say a guitar effects "stomp box" that you are building, testing or repairing.

Step 8: Drill the Control Panel, and Install the Switch, Control Pots, and Guitar Jack

The Radio Shack project box I used has a ABS plastic box, and aluminum lid you can drill and install controls onto (this ste), and (4) self-tapping screws to screw on the lid when you are done. First, here is the box again (see photos). It is just under 3" tall and 6" wide. I used a wood shop triangle with rule marks on it to mark the top of the case at 1" widths, along the center line of the panel. Then I made a decision esthetique and decided that since channel A is switch "up" and B is "down" I would put the Frequency A pot above the swtich and the Frequency B pot, below the switch, lined up one right over the other. I took some electronic music courses as well as electronics at university, back in the stone age: there is something called "form follows function" that applies in this case. Now, the Volume control applies to Both channels, so I decided to place that to the right of the frequency controls but between them, i.e. lined right up with the toggle switch. Finally, I left a gap of one inch to the right of the controls, and at the following inch mark, installed the guitar jack.

Next because I am an errant mortal and my eyes are no longer what they once were, I marked EACH hole I am about to drill with the size of the corresponding drill bit. Then when I do the drilling, I check and then recheck, the drill bit to make sure it matches what I wrote on the panel in white Sharpie. These are also called "Sharpie paint pens" they use actual white paint not the permanent black Sharpie ink so even after the ink dries, with a slightly damp cloth it rubs off. The black Sharpie does not come off.

Here are the drill bit sizes (though they still show up in one of the photos), and all dimensions are in fractional inches:

  1. Toggle switch -- 15/64
  2. Pots (Freq A, Freq B, Volume) -- 9/32
  3. Guitar Jack: 3/8 **

** Note 3/8" is for the famous brass Switchcraft guitar jack -- for this I used the Neutrik "Marshall amplifier style" jack made of plastic with a plastic nut, which is a somewhat bigger hole. Sometimes you want a metal (conductive) jack and other times you want a plastic (non-conductive) guitar jack. Depends on the application.

I am not going to patronize you by telling you how to insert a switch through a hole and tighten down the nuts. I like to use a nut driver attachment as the tool though. The control pots are a 10 mm nut size, the switch if I recall is 8 MM, the nut on the guitar jack, 12 mm size. I forget the two binding posts should have written it down

The AA battery holder so it is not flopping around I glued to the floor of the project box. Both the box and the battery holder are a compatible plastic. For the adhesive I use Beacon 527 and it holds great, but there are many kinds of adhesive that will work, such as E6000, Omni Stick, Tiger Bond and many others. I would not use 'super glue' though something thicker and slower driving.

Pro Tip: If you use the same radio shack case that I did you want to put a drop of good motor oil one the threads of each screw and work it in, before trying to screw it into the case. These are self-tapping screws: the holes in the case are not threaded before you screw in the screws. Trust me a good 5W-30 Mobil One oil on there makes it very easy to put in the screws and remove them later.

Finally attach the knows. I use 1/4" knobs that are Raytheon style you can find them at many electronics suppliers. They look nice have a good feel and have a white pointer line.

I labeled mine with a Brother label maker using 3/4" tape in yellow (obviously) which gives a good readable contrast and is easy on the eyes.

Step 9: Calibrating and Using It.

I will go into how to calibrate the device with both test equipment and by ear.

This is an optional step. I am not telling you to go out an buy a multimeter (I have a Fluke 87-V that retails for about $350 Lol) but it is good to have even a cheap one that can take reading in 2 things:

1. True RMS AC voltage in millivolts and volts

2. Measure frequency at least from 20-10,000 Hz

First we are going to do an audio test, ie. does this thing work? Then we are going to calibrate it with a mutlimeter.

For cheap work I use an Aneng AN8008 that I got off of eBay for about $15 and it does both of the above.

Before I start anything I turned down all the controls all the way and put a black line in the panel where the control knob pointer is pointing at. Then I turn it up all the way and mark the other position.

Start with the switch in the OFF position. Plug a guitar cord into the Signal Generator and the other end into a guitar anp.

First, turn all knobs to fully counter-clockwise position i.e. "Turn down the knobs

Flip the toggle switch up, to Chanel A position

Put the volume of the amplifier on 1. This thing gets loud.

SLOWLY turn up the control until you hear something. It should be about the same pitch as the open low E string on your guitar. Now slowly rotate the Frequency A knob clockwise": "turn it up". The pitch of the note should increase. At the end of the range the frequency picks up rapidly up to 4,500 Hz or so. You will not be able to get precise use of that last little bit of the range, but I left it in there so you can crank the thing and get a high yet audible frequency, like a very high harmonic on the guitar could make. Turn the Freq A knob to about the 10 o'clock position

Now flip the toggle switch to Chan B setting. The pitch should immediately drop back down. Each channel makes a note that corresponds to that channel's frequency knob. It is very handy to have two independent channels that you can set differently and switch between them. It gives you a preset' capability.

Another thing to keep in mind, even though I used 1% tolerance resistors for Channel A and B, the capacitors I used are 20% tolerance. So the lowest frequency is not exactly the same between channels A and B. You can do better with 5% capacitors or if want to splurge, 1%, but also be aware that the resistance across lugs 1 and 3 of a 100K pot varies more than the resistors. One of mine reads 96.7K not 100K. So my channel A goes down to Low E, but channel B is low F almost F#.

OK IT WORKS -- NOW CALIBRATE IT

turn it off. Disconnect the guitar cord from the amp. For calibration I use the binding posts as very easy to connect to a multimeter.

CALIBRATE OUTPUT LIKE A GUITAR PICKUP

Turn Volume control all the way down. Turn on Chan A. Both channels output at exactly the same level at all times. Set frequency knob anywhere you like. Set the multimeter to AC RMS voltage setting on the millivolts scale. Slowly turn up the volume, slowly and gradually, until the meter reads about 100 millivolts. Look at the pointer dial of the volume control. With a black sharpie put a dot there. I then extend the dot to a line. My lines are a bit rude looking I admit (see photo) but are good enough for calibration. Now slowly, slowly turn up the volume until it reads 200 millivolts AC voltage. Again mark it. Repate the process for 300, 400, 500 etc. going up by 100 millivolt increments. When you top out the scale it will read 0L. or something like that. At that point you can switch to the Volts scale because we are now at 1,000 millivolts = 1.000 volts. Keep going up by 0.1 volts, i.e. 1.1, 1.2, 1.3 and put a mark there as you see fit.

Frankly you would be find to just make the five 100 mV to 500 mV marks as you are now covering the complete output range of passive electric guitar pickups made anywhere in the world.

Again here are ballpark numbers

100-200 millivolts: Single coil pickups. Stock are more like 100 mV, hot after market are 200.

200-300 millivolts range: Humbucking (double coil) pickups normal range (think: Gibson) and possibly the hottest aftermarket single coil made (Dimarzio, Seymour Duncan, all our friends who we like)

400-500 millivolts range: The hottest, aftermarket humbucking pickups one can buy (Dimarzio, Duncan, etc.)

500-1,500 millivolts range: Active pickups for example a Yamaha bass with a battery preamp in it. Some amplified piezo transducers on "acoustic-electric" guitars like the Godin.

CALIBRATING FREQUENCY -- and a note about precision

We are going to measure frequency but since we are marking by turning a knob and we are human beings, and next time trying to achieve the same reading let me cut to the chase: You will never get the exact frequency On the lower end of the band like A2 = 110 Hz though you can get remarkably close, I will say withing less than 1 Hz difference. But we are frail human beings and this is not a guitar tuner nor is that the purpose. The purpose of this calibration is to get you close in the ballpark to notes on the guitar neck so you can test that Fuzz Whackmaster you are building or what-have-you. I will say this signal generator has remarkable little drift: If I leave the knobs in the same position and take two readings say, 30 minutes apart, they are the same, spot on.

DO THE ACTUAL CALIBRATION

Now set the volume control to the 700 millivolt setting. The frequency on some multimeters will read true and lock on at 100 mV but we want reliability and quick readings. Use 700 mV.

Set Freq A turn it all the way down. Turn the multimeter to the frequency measurement often marked as "Hz" but read the manual. If your box gets to 82 Hz mark that with a line an label it E2 if you wish (remember the frequency table in the Acoustics step?). Turn it up and I bet you are going to reach about the 10 o'clock spot on the dial or a bit higher until it reads 110 Hz. Mark that as A2. Using the chart as your reference and the meter reading as "truth" mark off E3, A3, E4 settings, and A4 which I marked as A440 on mine, because the international musicians "tuning note" is A440 or 440

Step 10: Will Add a YouYube Demo When I Get a Change.

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    seamster

    26 days ago

    Nice work! Thanks for sharing the details : )