evaton technologies SDIY MDRONE Assembly Manual

200 School St

http://www.evatontechnologies.com/

Pittsburgh, PA 15220-2718

SDIY MDRONE

Assembly Manual

Revision A - Parts & PCB Kit

Introduction

Thanks for purchasing the SDIY1MDRONE kit from Evaton Technologies! The
SDIY MDRONE kit is a battery-powered standalone tabletop drone synth that
can be built in just a few evenings. The SDIY MDRONE kit is available as just
parts and PCB (build your own case), or as a complete parts + PCB + pre-drilled
enclosure, for those who wish to get up and running right away2.

The SDIY MDRONE voice architecture consists of three independent square­wave oscillators, which are mixed in a fixed summing mixer. The mixer output
is then fed to a semi-resonant bandpass filter, with adjustable center frequency
and bandwidth. A “Grit” setting allows overdriving the filter for dirtying-up the
sound and making a more hard-hitting timbre.

Output is a 1/4” mono (TS) line-level audio jack.

Skill Level

The SDIY MDRONE kit, while not necessarily aimed at the complete novice,
can be built by anyone with some soldering experience. All components are
standard, readily-available thru-hole components. The spacing of the
components on the circuit board is generous, so that no special tools are
required to install. These instructions assume you already know how to solder,
and cut and strip wires.

Precautions

Soldering irons are HOT! Be careful not to touch the business end of your
soldering iron at any time. Also note that components that have been soldered
will remain hot for a few moments. If you need to hold something to solder it, it
is recommended NOT to use your fingers. Wear long pants and long sleeves to
avoid solder splash from burning exposed skin.

1

Synthesizer Do-It-Yourself

2

If you purchased this kit as a complete parts + PCB + enclosure kit, additional instructions for the

enclosure will be included in your shipment.

200 School St

Pittsburgh, PA 15220-2718

http://www.evatontechnologies.com/

Tools Required

A few basic electronic assembly tools are needed to complete the assembly
of the circuit board. If you are building your own case, some additional tools
are required to drill holes.

Tools Required - PCB assembly:

Soldering iron. Minimum 25 watt pencil iron; preferred temperature-

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controlled, but not necessary.
Small wire cutter

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Wire stripper

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Electronic solder. 60/40 lead/tin or 63/37 lead/tin, or lead-free. Use rosin-

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core solder. Do NOT use acid-core3 (plumbing) solder!
Pair of needle nose pliers (optional) for bending leads and holding

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components
Small bench vise or “helping hands” (optional) for holding the PCB.

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Small adjustable wrench or 10mm deep-well socket, for tightening

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potentiometer nuts
9/32” open-end wrench for tightening toggle switch nuts

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De-soldering braid (optional) for cleaning up mistakes

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Multimeter with ohms and volts setting. Not a necessity, but handy for testing.

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Tools Required - Case assembly:

Phillips screwdriver

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If building your own case, a 9/32”, 13/64”, and 3/8” drills are required, to

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drill holes for the potentiometers, toggle switches, and output jack.

Also Required:

9V battery. Alkaline preferred, but heavy-duty is fine, too. A properly

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constructed SDIY MDRONE will run many hours on a single battery.
1/4” mono patch cord (TS)

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3

Acidcore music is great. Acid core solder is not.

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Signal Flow

The SDIY MDRONE consists of three square wave generators that feed a
summing mixer. Each oscillator can be turned on and o! with a switch. Each
oscillator’s frequency can be controlled by a front panel knob. The output of
the summing mixer feeds a bandpass filter. The filter bandwidth and center
frequency can be controlled with front panel knobs.

Circuit Theory

While it is not necessary to completely understand exactly how the circuits in
the SDIY MDRONE work, a top-level description of the theory is presented here,
for those who’d like to get a little more acquainted with electronics. Please see
the included schematic diagram to follow along4. The schematic is divided up
into a few logical sections: The oscillators, the summing mixer, the virtual
ground reference, and the bandpass filter.

Oscillators

The three oscillators are the type known as a “relaxation oscillator.” In a
relaxation oscillator, a capacitor is slowly charged up from zero volts until the
voltage across the capacitor passes a certain threshold, at which time, the
circuit begins to discharge the capacitor. Once the capacitor voltage falls below

4

Or skip ahead to the next section; there won’t be an exam on this material!

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a certain voltage, the circuit again begins charging the capacitor. The charge
and discharge rate is determined by the resistance of the control knob on the
front panel.

If we look at the schematic, OSC 1 is formed by the capacitor C6, resistor
R17, and variable resistor (potentiometer) R16, and is controlled by the 40106
Schmitt Trigger chip. A Schmitt trigger circuit outputs zero volts when the input
is above a certain voltage. It outputs 9 volts when the input is below a certain
voltage. So, looking at the voltage on C6, it starts at zero volts, so the output of
IC1A is 9 volts. That 9 volts gets fed back onto C6, and it starts to charge.

Once C6’s voltage gets high enough, IC1A switches its output from 9 volts
to zero volts. C6 now begins to discharge. This repeats over and over again,
making an oscillation!

IC1B takes this rising and falling voltage and turns it into an on/o! signal (a
square wave!). Adjusting R16 (the front panel knob) adjusts how quickly C6
charges and discharges, thus setting the frequency.

OSC 2 and OSC 3 work the same way.

Summing Mixer

U1A is an operational amplifier (op-amp). It adds together the three signals
coming through R2, R4, and R18. An op-amp connected in this configuration is
called a Summing Amplifier.

Virtual Ground Source

R5, R6, and U1D form a bu!ered voltage divider. 9 volts goes through R5
and R6 to ground. Because R5 and R5 are equal in value, the voltage at pin 12
on U1D is exactly half that voltage, or 4.5 volts. U1D is configured as a “unity
gain follower,” meaning that the output at pin 14 is exactly the same voltage as
the input on pin 12, but it can drive a bigger load without the voltage being
a!ected.

The “virtual ground” of 4.5 volts is required because the SDIY MDRONE
operates from a single 9V supply. Most op-amp circuits require a positive
voltage supply and a negative voltage supply to operate. We can “fake out” the
op-amps by creating this “virtual ground” that is exactly half the battery
voltage, and then pretend that the 0 volts terminal of the battery is actually
“negative 4.5 volts with respect to virtual ground”. Then the +9V terminal of the
battery is “positive 4.5 volts with respect to virtual ground.”

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Bandpass Filter

The bandpass filter is an active filter that uses two op-amps and capacitors
and resistors to filter out signals that are outside of its “pass band”.
Frequencies of sound that are below the filter’s pass band are removed, and
frequencies that are higher than the pass band are also removed, but
frequencies that fall inside the pass band are passed through to the output. The
width of the pass band (the di!erence between the highest frequency that can
pass and the lowest frequency that can pass) is called the filter bandwidth.

The frequency that falls exactly halfway between the lowest frequency that
can pass and the highest frequency that can pass is called the center frequency
of the filter, or FC. R23 adjusts the bandwidth of the filter, and R13 adjusts the
F

C.

While this is a very basic description of the filter operation, a more thorough
description of how the filter does this is beyond the scope of this manual.
Further reading is suggested if you want to gain a better understanding of op­amp filters5.

Kit Contents

A bare printed circuit board (PCB) and 18-foot (6m) spool of wire

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2x 0.01 µF 16V capacitors

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3x 0.1 µF 16V capacitors

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3x 1.0 µF 16V capacitors

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1x 470Ω resistor

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2x 1.1kΩ resistor

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4x 4.7kΩ resistors

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9x 10.0kΩ resistors

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1x 47kΩ resistor

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4x 100kΩ linear potentiometers

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1x 100kΩ audio potentiomer (Frequency Pot)

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1x 5kΩ audio potentiometer (Volume Pot)

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2x 14-pin DIP IC sockets

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1x LM324 op-amp IC

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1x CD40106 Hex Schmitt Trigger IC

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5x DPDT switches

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9V battery snap and battery holder

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1/4” mono audio output jack

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5

While there are many good books on the subject, an internet search of “active filter design” will also
yield some very useful datasheets and application notes from various op-amp vendors that are quite
thorough. The filter in the MDRONE is a dual-amplifier passband design.

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Assembly Instructions

Note On Assembly Of Front Panel Components

While most of the components on the SDIY MDRONE get soldered directly to
the PCB, the front panel components are first soldered to wires, and then the
wires are soldered to the board. Leave long enough wires to allow easy
assembly at the end. While assembling the PCB, it’s best to just solder the front
panel components to long leads (6 to 8 inches), and leave them laying beside
the board. Don’t attempt to install the front panel components into the front
panel until the board is completely assembled.

Preparation

To begin assembly, clear a space on your workbench6. It helps to have good
lighting, and your tools located where they can be easily reached. Plug in your
soldering iron and let it come up to temperature. It helps to have a dampened
sponge nearby, to wipe the solder tip on to keep it clean. Place the bare circuit
board in the middle of your work area, with the white silkscreen legend facing
you.

The next few paragraphs will go step-by-step through the process of
assembling the circuit board. You may wish to check these steps o! as you go,
to keep track of your place.

____ Sockets

First, install the two IC sockets in the locations U1 and U2 on the PCB. There
is a notch at one end of the socket. Line up the notch in the socket with the
notch in the white component outline on the board.

To install the socket, place the socket in the holes on the board for the
socket, and make sure the socket is flat against the board all the way around.
You can use a piece of clear tape to hold the socket in place while you solder
the pins. Remove the tape after soldering.

____ Virtual Ground Resistor Divider

Install two of the 10kΩ resistors (brown/black/orange) into locations R5 and
R6 on the board. Carefully bend the leads of the resistors so that they fit
through the two holes in the board at the ends of the component outline. Bend
the leads slightly on the back side of the PCB to hold the resistors in place, and

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Hey, that’s my first step. My bench is always a complete mess!

Page 6

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solder the resistors in place. When you are sure you’ve made a good solder
joint on all four pads, cut the excess leads o!.

____ 9V Battery Snap

The 9V battery snap comes with pre-stripped leads. The red lead is the
positive lead, and the black lead is the negative. Solder the black lead to the
hole marked P8 on the PCB. Solder the red lead to the hole marked P7.

____ Power Switch

Get one of the toggle switches out, and solder a 6­inch wire to one of the center pins. It doesn’t matter
which center pin. Solder another 6-inch wire to
another pin on the same side of the switch. Although
the kit ships with DPDT7 switches, we will be using
them as if they were SPST8 switches. The switch is
“ON” when the toggle lever is pointing away from the
pair of pins you’ve soldered to.

Now solder the other end of one of those wires to
the hole marked P6 on the PCB. Solder the other wire
to the hole marked P5 on the circuit board.

____ Test Power-Up

At this point, we can perform a simple test of the circuit board, if you have a
multimeter with OHMS and VOLTS settings. First, we’ll check that there is not a
power-to-ground short circuit. Set your meter to the OHMS (Ω) setting.

Put one meter probe on
one contact of the 9V
battery snap, and put the
other probe on the other
contact of the 9V battery
snap. The meter should
read an infinite number of
ohms (open-circuit). If not,
go back and check your
solder joints.

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Double-Pole-Double-Throw

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Single-Pole-Single-Throw

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