I’ve been designing a 4-layer PCB for a product for a few months now and I finally got to the stage of assembling the prototype after the PCB and components arrived. Most of the passive components I used were 0603 surface mount parts so reflow soldering was the only viable option (if I wanted to maintain my sanity).

Since I don’t have a proper reflow oven, I decided to use the next best thing I own: No not my hot air rework station, a toaster oven!

There are, of course, plenty of guides and tutorials online for modifying a toaster oven for reflow soldering. They all involve a temperature PID loop to precisely control the heat profile for various solder types, components, etc,.

I really just wanted to see if I could just turn it on and see what happens. Spoiler Alert: It works.

I just preheated the toaster oven for a few minutes, stuck the PCB in there, and watched for the solder to start flowing. This took about 30 to 40 seconds. I wish there was more to this so I could do a more in-depth write-up but honestly that was it — nothing fancy and no precise timing or temperature profiles.

I gotta say, the final board is quite a beauty!

The Fine Print

Do this at your own risk! I did it because I’m just building a prototype and the final board will be professionally assembled. In addition, I know what I’m doing and I read the datasheets of all my critical components. You’ll notice that I did not put the electrolytic capacitor in the oven — I could not guarantee not to violate the very precise timings and temperatures specified in the datasheet. Also, please mind the fumes.

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Picking up from Part 2, the third and final part of the wood click gears with motor drive build covers the stepper motor drive. The gears ended up driving this really beautiful clock. Note that I did not build the clock, just the gears driving the hands.

This part of the build was fairly simple since it was just a matter of driving a stepper motor at a constant speed. I also included a switch to be able to reverse direction.

Block Diagram

The drive system is extremely simple as shown in the block diagram. I did not create a schematic for the build and just used the block diagram for reference instead. The circuit was powered by a standard 5V regulated wall charger.

Stepper Motor and Driver

Since the project didn’t require a large motor, I used the 28BYJ-48 stepper stepper motor with the ULN2003 driver. This is an extremely popular stepper/driver combination with a great deal of documentation available online.

ATtiny85 Microcontroller

I opted to use the ATtiny85 microcontroller since I only needed a few pins to control the stepper motor driver. The ATtiny85 has 5 usable I/O pins — I used 4 for the stepper driver and 1 for the forward/reverse switch. Lastly, it is largely Arduino compatible so many library work without modification.

Code

The code (included below) was fairly straightforward — there is nothing fancy happening and I used the AccelStepper library. It is able to run the motor more efficiently than the default Arduino stepper library and includes a half-stepping mode, which is recommended for this motor.

Here is a great tutorial on using the Arduino Uno to program the ATtiny85. As a side note on the tutorial, you can get away with programming it without using the capacitor. 

// Simple program to drive a stepper at a constant speed
// using an Attiny85
// Paul Bupe Jr

#include <AccelStepper.h>;

// Defining some useful constants
#define HALFSTEP 8
#define MINUTE_STEPS 1.13777
#define SECOND_STEPS 68.266
#define SECOND_STEPS 115

// Motor pin definitions
#define motorPin1  0     // IN1 on the ULN2003 driver 1
#define motorPin2  1     // IN2 on the ULN2003 driver 1
#define motorPin3  2     // IN3 on the ULN2003 driver 1
#define motorPin4  3     // IN4 on the ULN2003 driver 1

#define dirPin     4     // Input for Clock Direction

bool last_dir = HIGH;
float step_speed = SECOND_STEPS;

// Initialize the stepper library in half-stepping mode
AccelStepper stepper(HALFSTEP, motorPin1, motorPin3, motorPin2, motorPin4);

void setup()
{  
   pinMode(dirPin, INPUT);
   stepper.setMaxSpeed(1000); // Arbitrary max speed 
   stepper.setSpeed(step_speed);	
}

void loop()
{  
  int dir = digitalRead(dirPin); // Get direction from switch
    // Only executes if the direction changed
    if (last_dir != dir) {
      if (dir) {
        stepper.setSpeed(step_speed); // Clockwise
        last_dir = HIGH;
      } else {
        stepper.setSpeed(-(step_speed)); // Counter Clockwise
        last_dir = LOW;
      }
  }
   stepper.runSpeed();
}

Putting Everything Together

With all the parts in and program written, I did some quick prototyping just to make sure everything worked as expected.

I then designed a quick mount in SolidWorks and printed it out using my 3D printer.

Then it was just a matter of crimping a few wires and soldering everything together.

After that I tested everything out then mounted it on to the back of the gear assembly.

Finally a quick test with everything assembled and it was good to go!

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Amplifier Specifications

The full specs for this amplifier are available on the seller page but the general specs are:

• Power output (w/ 36 VDC power supply): 100W x 2 (6 ohms, THD 10%), 99W x 2 (6 ohms, THD 1%)
• Frequency response: 20 to 20,000 Hz
• Minimum impedance: 6 ohms
• Fixed gain settings: 24 dB, 25 dB, 26 dB

Chassis Design

I designed the chassis as a typical rectangular design using SolidWorks. The main design feature is that I added some slots for ventilation on two sides. Since this amp was purposed for desktop use, I opted to remove the fan and use passive cooling. Rodney Bryant did an awesome job of bringing my design to life! Here are a few pictures he took while building the chassis:

Electrical Design

The wiring for this build was fairly straightforward, mostly switches and such. First, I opted to use a single 3.5mm input instead of an RCA inputs since. I used an external power supply and placed a resettable fuse on the positive power rail. I then wired three switches to the amp: a main power switch in the rear, a standby switch, and a mute switch. Since I opted for passive cooling, I removed the fan and wired an LED to the fan connector.

Noise Floor

The main issue I encountered was the high noise floor (loud humming when nothing is connected and even when something is connected and is at low volume). I lost the shielding benefits presented by a metal chassis since this chassis was made out of wood, .

Because I was using a 3.5mm input, I had to solder some connectors onto the board instead of using the already present RCA connectors. The noise floor was drastically reduced when I connected an input to the RCA jacks as opposed to my 3.5mm input. This at least narrowed down the issue to my wiring. I tried the usual recommendations (shorter wire runs, shielded cables, twisted pairs) but nothing seemed to work.

Eventually I realized that in all my testing I had the ground from the 3.5mm input connected to the volume potentiometer and then from there to the amplifier. Instead, I needed to connect the ground directly from the input to the amplifier, not the potentiometer. This design is closer to the “star ground” technique where all the grounds are tied to a central point. Making this simple wiring change immediately eliminated the high noise floor and made the amplifier usable.

Volume Control

The volume control knob used in the initial design was a dual 50K “audio taper” potentiometer. Volume control potentiometers must have a logarithmic response because the human ear doesn’t perceive changes in loudness in a linear fashion. Here is a great article about the nonlinearity of human hearing.

I discovered that all the potentiometers I ordered (first from Amazon then from Mouser) were very flimsy and also introduced some scratching noises (they were all graphite pots) sometimes when switching. This was the opposite of the smooth and heavy feel I wanted and why I bought a metal knob.

A stereo stepped attenuator is typically the “professional” solution to my problem but I was already over budget for the build and I didn’t have room to fit a large component. On a hunch I went to Goodwill and happened to find an old record player with a volume potentiometer having the exact characteristics I needed. I switched it out and it turned out to be a perfect fit — I didn’t even need to drill out the hole. Unfortunately I didn’t take any pictures when making the change.

Notes

Overall this was a fairly straightforward and fun build. My search for the perfect volume control potentiometer and my improperly grounded input caused the biggest delays in the build. Otherwise, this amp is a perfect addition to my setup!

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Project Summary

This was a quick build of a 50W desktop audio amplifier based around the SainSmart 12V 50Wx2+100W TPA3116D2 board. The amplifier performed well into a 6 Ohm load and was able to hit “uncomfortable” levels (at a 4 foot listening distance) without distortion.

Parts List

• SainSmart 12V 50W x 2+100W TPA3116D2 2.1 Amplifier Board
• Self-Stick 1/2″ Noise-Dampening Bumpers
• Kmise Z2807H3 14 x 17 mm Mini Aluminum Knob
• Black and Red Plastic Shell Speaker Terminal Binding Posts
• BOX3-1455N-BK Black Aluminum Box (6.30 x 4.06 x 2.10 in)
• 2.1mm Metal Panel Mount DC Power Jack
• 3.5mm Stereo Panel Mount Input Jack

The Build

This project did not require any significant electrical design and was more an exercise of fitting components inside a relatively small enclosure. The initial plan was to have both RCA and 3.5mm inputs but the final product contained only the 3.5mm input due to the very limited space inside the enclosure.

SainSmart Amplifier Board

The build quality of the amplifier board was disappointing and was the cause of significant frustration during the build of this project. The biggest issue was the fact that the potentiometers were not evenly spaced and were also crooked.

Secondly, the power switch was slightly recessed which meant that the mounting holes for the potentiometers had to be drilled out to a larger diameter in order to allow for the switch to come out for enough to be reachable. The obvious solution would have been to desolder all the pots and resolder them properly. I did not do that.

Chassis

About the hardest part of this build was fitting the amplifier module in the chassis and cutting out the holes in the faceplate at the correct height. Due to the poor assembly of the board, no assumptions could be made about any features being equidistant so there was a lot of measuring and even more informed guessing.

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Project Summary

• Successfully developed a system and protocol to autonomously deploy multiple UAVs for emergency (or planned) communication relief in an outdoor setting.
• Designed and developed a fully functional web-based flight control graphical user interface for system.
• Created an algorithm for UAV swarming in a hexagonal cell pattern using GPS localization.
• Developed a fly-by-wire system to autonomously control multiple UAVs by modifying open source flight control software.
• Designed parts and equipment in SolidWorks to facilitate project.
• Properly utilized the 915 MHz ISM band for telemetry.

Download a copy of the complete thesis here.

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