Mobile Electronics Workbench

Back in the old days I used to drag all of my electronics project parts out of the garage in boxes, set them up on the dining room table, and get to work.  This worked out great until dinner time, at which point I would shove everything back into boxes and scatter it around the room until after dinner was over.  There were extension cords, power strips, soldering irons, power supplies, and components.  Components everywhere.  It got to the point that it was more of a hassle to clean up than the projects were worth, and I sort of went on a hiatus from electronics projects.  I looked around the web at solutions other people had come up with, and I decided I needed a mobile electronics workbench.  A portable lab.

I wanted something with a built-in power supply, a spot for a breadboard, some soldering space that nobody would get bothered about if it got burnt spots on it, component and tool storage, a built-in extension cord, and good lighting.  Here is my solution:

The final product

Light and tool hanger folded in

The back side, with power cord cleats

 I built it out of 3/8" plywood mostly, with 1/4" plywood on the back for light weight.  The wood is glued together and reinforced with small nails.  The tool door holds various strippers and tweezers, and folds in neatly.  My eyes are not what they once were, and so I bought a magnified light on Craigslist for $30, cut it down to fit, and mounted it so that it can move up and down and still fold into the box.  The dimensions of this light dictated the overall dimensions of my box, and it just barely fits, which means it fits perfectly.  The front opening folds down to make a nice soldering and prototyping area.  There is a little shelf for holding craft boxes of components.  My light and soldering station plug into a 4-plug outlet on the inside, so I've got 2 spare plugs for other things that need AC at the table.  All told it weighs about 35 pounds with everything in it.  Not too bad.  It has helped me complete many more projects, both big and small, than I would have without it.  I can just bust out a project, fold it up for dinner, and pull it back out after the kids have gone to bed.  Easy.

Awesome as it is, I have some future upgrades planned for it.  I would love a little vise to hold my circuit boards while I work on them.  Currently I am using an old Dell ATX power supply, and it makes a high pitch hum that bothers everybody in my house.  A good cheap lab supply would be ideal, especially if it was smaller too.  I have had this in service for a year now, and space on the inside is valuable.  I use the Sparkfun soldering station, which I absolutely love, but the iron holder is horrible.  I would like to build my own iron holder into my mobile bench, especially since I have to take the iron out of the stock holder to get everything to fit inside after the doors are folded up.

Overall it has been great.  Now I just need to find a way to store my etching supplies, my laminator, my electric skillet, and my oscilloscope in there.  I guess they'll stay in the garage on an as-needed basis for now.

LEGO 2.4 GHz Power Functions Radio Control DIY Circuit

Update: The H Bridge chip this circuit uses has a voltage drop that makes devices that use it operate more slowly than they should.  An updated version 2 is in the works that uses the DRV8833.

When LEGO came out with the infrared controlled Power Functions system I was super excited.  I have dreamed of being able to build remote controlled LEGO cars and racing them with my friends since I was a little kid.  The actual system left a lot to be desired though.  If you want a nimble car, you have to use the LEGO 8885 infrared transmitter, but it only offers full speed forward and reverse, and if you are using it for steering it gives full left or right.  There is no proportional control.  You can’t go at half speed or steer just a little to the left.  You can use the other LEGO transmitter, the 8879, which gives you seven incremental speeds forward and reverse, or seven positions on the servo for steering.  That sounds perfect, but in reality it will only send a few commands per second, and if you try to give it more commands per second than that it will ignore them.  This means that it can take 3-5 seconds to steer from a full left to a full right, and your transmitter dial will be out of whack with where you expect it to be when you try to go straight again.  Suboptimal for sure.

I love the options the variety of Power Functions motors provide, but the control system needs improvement, so that is what I decided to do.  I am documenting it here so you can do it too.

Overview:

My circuit uses a cheap 2.4 GHz radio transmitter to quickly control two Power Functions devices.  You can control two motors, or two servos, or one motor and one servo.  The 2.4 GHz receiver sends out servo control signals that are typically used to position a hobby servo or control an electronic speed controller.  Our LEGO motors and servos cannot use this signal though, so we are going to use a programmable microcontroller to translate the hobby servo signals into a pulsing signal that can be used to power our LEGO motors and servos.  The microcontroller we are going to use is the Picaxe 14M2.  I chose this microcontroller because it is small, cheap ($4), is easy to program in the BASIC programming language, and the only thing you need to buy to program it is a $6 cable if you have a serial port on your computer.  If you don’t have a serial port, you will need a USB programming cable, which is more expensive, at $26.  The Picaxe chip can’t output enough power to power our motors and servos, so we are going to feed the Picaxe output signals into an H-bridge motor driver chip, the SN754410.  I chose it because it is easy to use, tough, and cheap, at less than $3.  It can supply 1 amp of power to two different motors, which is enough for most Power Function situations, but I have not tested it with a L or XL motor yet.  I think it will handle the L motor ok, but I know the stall current of the XL is closer to 2 amps, and that’s way too much for the SN754410.  The SN754410 does have overcurrent protection though, so it will shut down before it does any damage to itself.  You can stack two SN754410s on top of each other, and solder their legs together to double the power it can handle, so if you are planning on running an XL hard, you should consider that.  The other part of the circuit is the 7805 5 volt voltage regulator, which takes the 9 volts from the battery pack and converts it to 5 volts, which is required to power the 2.4 GHz receiver, the Picaxe, and the logic functions of the SN754410.  The SN754410 also needs the full 9 volts to power the motors.  For the record, I am not using any external protection diodes on the SN754410, with no ill effects.  There is some debate about this on the internet.

Here's a video of the results:

 

And of the first road test:

 

Functional Concept:

The concept here is that the radio controlled receiver sends out a pulse of somewhere between 1 and 2 milliseconds every 20 milliseconds on each of the two channels we are using.  1 millisecond tells a servo to go full left, 2 milliseconds tells the servo to go full right, and 1.5 milliseconds tells the servo to go to its center position.  We are using the Picaxe PULSIN command to measure the length of that signal on each of the two receiver pins.  We then have the Picaxe perform some math on that data to end up with an output signal that we send to the SN754410.  Here is a link to a great page on what the pulse width modulation signal looks like coming out of the Picaxe and into the SN754410.

If we have a servo signal less than 1.5ms, then we need to send out a PWM signal to the SN754410 on its 1A pin, while sending out a constant 0v signal to its 2A pin.  As the servo signal becomes closer to 1ms, we need to increase the “on” time of the PWM signal to 1A.  If the servo signal becomes larger than 1.5ms, then we need to send 0v to the 1A pin, and start sending a PWM signal to 2A.  The math I used to do this is in my Picaxe code.  I made sure that there was a dead zone around 1.5ms so that the motors are sure to be stopped, and I also made sure with my code that the output signal could not be such that my PWM signal has a larger than 100% duty cycle. 

Here is what you will need to buy:

HobbyKing HK-GT2B 3CH 2.4GHz Transmitter and Receiver        $23       Hobbyking.com

For $23 you can’t beat that deal.  If you already have a transmitter, you can use any receiver that works with your transmitter, but the pins may not match what I have on the etched board.

14M2 Picaxe chip                                 $4                    Sparkfun.com

Picaxe Serial Programming Cable          $6                    Sparkfun.com

SN754410 H-Bridge Motor Driver            $2.35                Sparkfun.com

7805 5v Voltage Regulator                     $1.25                Sparkfun.com

0.1 uF Capacitors (qty. 2)                      $0.25 ea.          Sparkfun.com

Female Header                                     $1.50                Sparkfun.com

3.5mm audio jack                                 $1.50                Sparkfun.com

This is to connect the programming cable to a solderless breadboard to program your Picaxe.

Two 10k ohm and one 22k ohm resistors

You can buy these at Radio Shack in 5 packs, or in a big assortment.

Copper Clad and etchant

I love Electronic Goldmine’s scissor-cut copper clad.  Easy to drill and cut, and nearly clear, but you can use Radio Shack's too.  We only need single sided, so grind off the unused side if your copper clad is stiff.  I use Radio Shack PCB Etchant to etch my printed circuit boards.

LEGO Power Functions extension wire (qty. 2) short 8886           $3         Lego.com

Harbor Freight has an assortment of tiny drill bits that I use to drill my PCB holes.  I use the 0.8mm bit for most of my components, and just a bit bigger for the voltage regulator.  I file down the legs of the voltage regulator so they are nearly as skinny as the other components, so it sits flush against the PCB.

I’m going to assume you have some electronics, PCB etching, and soldering experience.  If not, here are some links to get you the background you need to get started on this project:

Toner Transfer PCB building:

http://mad-science.wonderhowto.com/how-to/diy-lab-equipment-etch-your-own-circuit-boards-using-laser-printer-0134931/

I don’t use tape, but rather fold the paper over and push the copper clad into the crease.  Also, I have found that ironing for 3 or 4 minutes works well.  Don’t forget to rough up the surface and wash the board before you iron on the toner.  Sharpies work well for touching up before you etch.

Through Hole Soldering:

https://learn.sparkfun.com/tutorials/how-to-solder---through-hole-soldering

I absolutely LOVE my Sparkfun 937b soldering iron (part number TOL-10707).  My joints have improved drastically since I moved up from cheap irons.

Everything you could want to know about Picaxe:

http://www.picaxe.com/

http://www.picaxeforum.co.uk/forum.php

Here are some pictures of the process:

Prototyping.  Note the HobbyKing receiver not yet de-cased.  Is that the top of a free-with-any-purchase Harbor Freight multimeter?

This is the best toner transfer etch I've ever done.  This is the first board I used AutoCAD to do my art.  For my previous PCBs I used Microsoft Paint, which works, but is not awesome. 

 

 

The HobbyKing receiver just plugs right into that socket I built out of 3 four pin headers side by side.

Here is the BASIC code that you need to program the Picaxe chip with. 

symbol bforward = B.2              ;the pin that outputs not 0 when motor B goes forward

symbol breverse = B.4              ;the pin that outputs not 0 when motor B goes reverse

symbol aforward = C.2              ;the pin that outputs not 0 when motor A goes forward

symbol areverse = C.0               ;the pin that outputs not 0 when motor A goes reverse

symbol cha = C.4                      ;the pin that receives the channel A pulsin

symbol chb = C.3                      ;the pin that receives the channel B pulsin

symbol chainput = w0                ;variable that channel A pulsin uses

symbol arevout = w2                 ;variable that gets output to motor A when in reverse (areverse pin)

symbol aforout = w3                 ;variable that gets output to motor A when in forward (aforward pin)

symbol chbinput = w4               ;variable that channel B pulsin uses

symbol brevout = w6                 ;variable that gets output to motor B when in reverse (breverse pin)

symbol bforout = w7                 ;variable that gets output to motor B when in forward (bforward pin)

output bforward                                    ;make pin bforward an output pin

output breverse                         ;make pin breverse an output pin

output aforward                         ;make pin aforward an output pin

output areverse                         ;make pin areverse an output pin

Main:

let aforout = 0                           ;set these variables to 0

let arevout = 0

let bforout = 0

let brevout = 0

pwmout aforward,249,aforout    ;all 4 pins start at 0v all the time

pwmout areverse,249,arevout

pwmout bforward,249,bforout

pwmout breverse,249,brevout

Amain:

pulsin cha,1,chainput                 ;check the length of the pulse coming from channel A

if chainput < 102 then let chainput = 102 endif    ;you don’t want this less than 102 ever

w1 = chainput-102*22                                         ;intermediate math

if w1 > 1000 then                                               ;this all checks to see if it should be forward or reverse

arevout = 0

else 

arevout = 1000-w1

endif

if chainput < 152 then

aforout = 0

else

aforout = chainput-152*22

endif

if arevout > 1000 then let arevout = 1000 endif    ;this makes sure that it doesn’t get more than 100%

if aforout > 1000 then let aforout = 1000 endif    ;duty cycle which locks things up at full throttle

pwmduty areverse,arevout                     ;output the reverse pwm signal for motor A

pwmduty aforward,aforout                     ;output the forward pwm signal for motor A

Bmain:

pulsin chb,1,chbinput                            ;this is pretty much the same thing as Amain but for B motor

if chbinput < 102 then let chbinput = 102 endif

w5 = chbinput-102*22

if w5 > 1000 then

brevout = 0

else 

brevout = 1000-w5

endif

if chbinput < 152 then

bforout = 0

else

bforout = chbinput-152*22

endif

if brevout > 1000 then let brevout = 1000 endif

if bforout > 1000 then let bforout = 1000 endif

pwmduty breverse,brevout

pwmduty bforward,bforout

sertxd ("chA",#chainput," AF",#aforout," AR",#arevout,"  chB",#chbinput," BF",#bforout," BR",#brevout,13,10)  ;for tuning on the computer screen

goto Amain

You can copy and past all of the above into the free Picaxe programming editor, then upload it to the chip.  Read the Picaxe manual #1 to get an idea of how to do it.

Below is a link to the pdf of the etch artwork.  You will use this to print with a laser printer onto thinish glossy paper, then iron it onto your blank copper clad.  I also have several component placement guides on that page.  I put 4 copies of the art on one page so you can have 4 tries with a single print.  Sometimes it takes that many.  Also, save this file and print it with your own pdf viewer.  The whiteish lines you see on the black areas are ok on your screen, but not on your paper.  Google Doc's print puts the lines on the print, but Acrobat reader does not seem to.

LEGO Radio Control etch art

The Future:

What does the future hold?  I have a few upgrades planned, and here they are:

1) I'm going to find a smaller voltage regulator, as that is currently the tallest part.  Replacing it with a shorter one will enable this whole thing to be under two bricks tall.  Currently I have to use 3.  Also, as long as I'm replacing the voltage regulator, I'm going to find a low-drop-out one to replace it with so my batteries can go even lower before replacing them.  EDIT: I cut the heat sink of the regulator off through the middle of the hole, and now it fits in a 2 brick high space, with no ill effects.  Also, my batteries will get so low that they will only barely drive the motors before the dropout voltage comes into play.

2)  I want to make a 6 channel board, since a 6 channel HobbyKing transmitter and receiver is hardly any more money than the 3 channel I'm using now.

3) I would love to make an 11.1v battery pack out of hobby lithium batteries.  Smaller, lighter, cheaper, more powerful than alkaline.  

4) Maybe someday I will gut a servo motor and replace the 7 increments with a true resistor strip circuit like every other hobby servo on the planet, and have full proportional steering.  Why didn't LEGO do this in the first place?  EDIT:  In practice the 7 segments feel very much like full proportional steering on a LEGO car.  I'm not going to worry about modifying their servos for this anymore.
5)  I want to build this circuit again with surface mount components.  I can buy a SMT voltage regulator, capacitors, and resistors easily.  The Picaxe 14M2 is offered in a hand-solderable SMT package (but shipped from the UK).  Now all I need to find is a hand-solderable SMT version of the SN754410 motor driver chip.  The ship LEGO uses on the new V2 of the infrared receiver is the TI  DRV8833 but it is difficult to solder (and etch a board for) and the maximum voltage is 10.8v, and I eventually want to use a 3S LiPo at 11.1v.

It was back in the day on the old TI-99/4A that I programmed my last video game.  I learned skills that are still useful today by doing that, mostly the BASIC programming language, which I use to program the Picaxe programmable microcontrollers.  It also taught me how to think about programming in general, as well as helping me think through problems.  Good stuff. 

Recently I started using MIT's Scratch program to make video games, and let me tell you, it's awesome.  I have introduced my eight year old son to it, and through it he's learning about the coordinate system, and/or decisions, velocity, variables, nesting mathematical functions, and so much more, all on top of learning to program a computer.  He loves it, and that's good, because they made the software just for kids.

Imagine writing a computer program by dragging commands into your script area, and the commands are shaped like puzzle pieces so that they only fit where they belong, and you've got scratch.  It's awesome, and I suggest you check it out right now.  I was able to write my first program in 15 minutes.

http://scratch.mit.edu/

Science!

I teach 3^rd through 5^th grade science lessons and one of my favorite new lessons has been teaching force, motion, energy, and the scientific method with 2 liter water bottle rockets.  It is a somewhat involved lesson in which we try to determine how much water in the bottle will make it fly the highest.  The rockets are a 2 liter bottle with a ring fin, a Nerf football nosecone, and a quick-release nozzle.  I built a Gardena style launcher that attaches to a garden hose and a bicycle pump in which you can fill the bottle with a predetermined amount of water (measuring with graduations marked on the launcher guide posts).  We pressurize the bottles with 60 psi of water each launch, but vary the amount of water in 0.2 liter increments. 

After we have launched and timed our rockets with 0.2 through 1.8 liter volumes of water, we plot the time aloft on the Y axis and the volume of water on the X axis of a graph for easy and visual data analysis.

Here is a link to the water rocket and launcher instructions:  

http://www.aircommandrockets.com/construction.htm

The forces acting on the rocket are the acceleration of the water through the nozzle towards the ground, gravity, and wind resistance/friction.  The potential energy is the compressed air in the volume of the bottle not taken up by the water.  The less water you have the more energy in compressed air you can store, and your rocket will be lighter at launch, but air does not have as much force to propel the rocket, as its mass is so much less.  Force = mass X acceleration, so if what you are accelerating out the nozzle does not have much mass, the launch thrust force will be reduced.  In other words, it’s a balancing act, and the fun part of this experiment is to find the perfect balance of water and compressed air.

Here is where the story gets interesting.  I taught this lesson for the first time about a month ago in my first summer session, and I expected this nice dome shaped graph, where 0.2 and 1.8 liters had the shortest time in the air, and at some point in the middle there would be a peak.  We launched 2 rockets at each volume, and indeed my expectations were correct, with the exception of the 0.6 liter launches.  There seemed to be a dip there on both launches.  I disregarded it and assumed that I had not put enough air pressure in the rockets for those launches.  One student had measured a time that was greater than the other students, so I made that dot really big on the graph, and put my X on the graph where my intuition said it should be.  In reality you can see the data points for the two launches clustered in two groups of three dots well below the point at which I placed my X.  With that data analysis, we concluded that 0.6 liters is the amount to put into your rocket to make it fly the highest.

Fast forward to my second summer session last week, in which we did the same rocket experiment.  In that class we came up with very similar data, and as I was plotting it on the graph for the class I was thinking in my head that I must have not put enough air pressure in the 0.6 liter launch when I remembered the data from the first session.  This time I respected the data, and plotted it as it was recorded, and you can see very clearly the 0.6 liter dip.  We even plotted the data points for both launches for 0.6 liters on this graph.  This time we concluded that 0.8 liters is the amount to use to make the rocket go the highest.

Now the question is WHY?  That just doesn’t make sense to me.  I am going to run this experiment on my own with 0.1 liter graduations and very accurate measurements and see if more accurate measurements give me more insight.  My initial thought is that there is a longer coast after thrust due to a higher initial velocity involved at around 0.4 liters or so, but I am going to have to research this.

The takeaway lesson here is what Paul Simon told us years ago, “A man hears what he wants to hear and disregards the rest.”  There is no place for this in science, although for 3^rd through 5^th graders it really did make the lesson a lot easier to understand.  This type of thing happens occasionally even in the higher levels of science, but my job as a science teacher is to teach kids to only trust the data, and to disregard our preconceptions of what the outcome should be.  I am glad I caught this in the second session, and I will work this concept into my rocket lesson in the future.  Science is awesome!