A more powerful battery

Earlier we tried to make a copper-aluminum battery out of pennies, foil, and old film canisters.  Long story short it didn't work.  So we quit fooling around and made a more robust setup. 

First we cut 7" sections of 3/4" PVC pipe and cemented endcaps on.

The we cut 7 1/2 " sections of 1/2" copper pipe and drilled a few small holes along the length.

Then 1/4" aluminum shafts were cut to 8" lengths.

The idea is the copper tube will extend out of the PVC pipe and the aluminum will extend slightly beyond that.

It's critical that the copper and aluminum do not come into direct contact in the cell.  So we used rubber bands to provide a spacing along the length of the tubes. 

Here is an assembled unit.

The cells need to be upright so when we add vinegar it doesn't spill out.  So we put together a wood frame to hold the battery.  (M did a lot of sanding on this.) 

We used cider vinegar poured in with a funnel. 

One cell gave almost half a volt, 0.473 V.

With the first one working we quickly set up the rest of the cells. 

There we go.  2.85 volts across a battery of six cells.  Now, can we light up an LED?

Success!  The LED on the left is bright pink from the battery voltage.  The dark red one on the right is identical but not hooked up for comparison. 

How does it work?  (I am a bit rusty, no pun intended, with my chemistry so lets see if I can get this right and not mix up the directions.)   The copper is oxidized ("rusted") as it enters the solution.

Cu -> Cu[+2] + 2e[-] 

Each atom of copper that enters the solution frees up two electrons that can flow through the circuit. 

An excess of hydrogen ions (H[+]) are present in the acidic vinegar solution.  At the aluminum end electrons flow in and react with the hydrogen ions to produce hydrogen gas (hydrogen is reduced). 

2H[+] + 2e[-] -> H2

The cells are well vented with open tops so the hydrogen does not accumulate to dangerous levels that could ignite. 

The resulting redox reaction (reduction-oxidation) drives electrons through the circuit connection.

The expected voltage can be estimated from the metal's anodic index, this is a measure of the voltage between various metals and gold as a reference.  Copper has an index of -0.35 V, aluminum has an index of -0.9 V.  The difference between them is 0.55 V, more or less the half volt we got with cider vinegar as the electrolyte (ion solution).

Nano Quadrotors

I ran across this video at The Conversation site.  Amazing and scary!  The kids and I need to build some of these!

Scribble-bot

Things never turn out as planned.  I picked up a small $1 vibrating motor (the kind that makes cell phones vibrate) and a paper thin silicon wafer solar cell.  I was going to hook them up according to something like the bristle bot over on the "get your mess on" blog, but so that the creature vibrated and moved in sunlight.  (The first robot I made, when I was in 5th grade, was with an electric motor, a solar cell I ordered out of a magazine, a cardboard base, double sharpened pencils (for axles), glue, tape, rubber bands and plastic wheels; it ran in light and stopped in shadows; like a cockroach.)   However, this time ~30 years later, the silicon promptly shattered when I tried to solder wires to the + and - sides of it.  I picked the largest remaining piece and tried to heat it as little as possible to solder but it shattered again.  So, for now, that's all for the solar component.  I still had the vibrating motor, so I hooked it up to a 1.5 V AA battery, and rubber banded them together.  It vibrated and slid around on the table top.  I needed something to add to it.  I thought about the toothbrushes we had ... but then remembered something about a drawbot (on Teacher Tom's blog that referenced "get your mess on", and at "Filth Wizardry" that commented there).  So I used more rubber bands to make a little pencil tripod... 

Above you can just see the vibration motor under the AA battery which is banded to the pencil tripod.

Another view.

T pointing out the rabbit ear configuration. 

I cut out a rectangle of cardboard and taped a sheet of paper to the back.  The cardboard is to keep the vibrodrawbot from going off the edge of the sheet.  It ran for a while a scribbled out a design.

At the end we had a masterpiece, already framed to hang up on the wall!  T was unimpressed, he said "so basically you made a robot that can't do anything but scribbles; even [F] can do that."

Undeterred I am thinking of future modifications.  An on/off switch, pens in different colors, ...

Brushless Motor

I redesigned the motor we made earlier.  I figured the brushes were creating a lot of friction and keeping its top speed down.  I wound four new coils onto spent wire spools, 1,000 wraps each of 32 AWG enamel wire, and mounted the new coils around the bottle with the magnets taped to it. 

The magnets are mounted 90 degrees apart (or as close as I could get them to 90 degrees using the pattern printed on the bottle).  If the electromagnets were small enough I could put them 22.5 degrees apart within a 90 segment and pulse them in a row.

 
However, they wont fit this close together, but I can place them around the full 360 degrees in multiples of 22.5 degrees +/- 180 or 90 degrees--in other words 67.5 degrees--and if I get the right placing it will work the same way (with, in this case, lots of tweaking and taping).

(Originally I tried to do this with three coils in multiples of 30 degrees, which in theory should work, but it was too sensitive to the placement.)  In this second case the magnets are pulsed in the opposite direction from the way the rotor turns.  I hooked up a controller and wrote a short program to drive the motor.  The LEDs on the breadbord indicate which coil is powered and to what degree.  It worked better to move adjacent coils up and down in voltage in a triangle wave pattern to trade off the rotor.

Here is a code snippet to give an idea.

All I had at the time were some PNP transistors to hook it up so everything is "drawkcab".  A "HIGH" signal means "LOW" voltage, etc.  I need to clean up the code by putting the pins in an array and having a single loop.  I am also thinking of using a smoother sine wave function to trade the rotor between coils and increase the voltage to the coils (currently it is running on 5 volts off the controller board, but the control of the rotor is very loose).  One surprising side effect, as the power is adjusted along the triangular wave the board can not do a true analog transition, but approximates this by varying the pulse of an on/off signal very rapidly, this causes the magnets on the bottle to vibrate and produce sound.  The motor "sings" as it turns.  Also, contrary to my original goal, I can not get to high speeds with this setup, but I can turn the motor as slow as I like, pause for as long as necessary, and easily reverse direction, kind of like a step motor, which gives me some ideas for applicaiton if I can build a more rigid structure, run it with more power, and gear down for a mechanical advantage without too much friction.

Joule Thief

The kids and I are doing some more battery experiments (briefly mentioned in an earlier post).  We have been using copper pennies, aluminum foil, felt, wire and diluted vinegar in old film canisters.  We can generate a few volts but so far have not had any luck powering anything off of them.  To try to convert the output to a higher voltage I made a joule thief pictured below.  The name is a play on Jewel Thief and Joule as a unit of energy.  It "steals" extra energy from the battery. 

It has two wires wrapped around an iron (ferrite) ring (although a bar would also work) and two ends are connected to a transistor (one via a resistor).  It sets up an oscillation which boosts the voltage to a higher level for part of the time.  The key to understand it is that as the current flows from the transistor to the coil, a magnetic field increases in the iron, which causes a current to flow in the second coil, which gives positive feedback to the flow of current from the transistor.  So the flow of electricity builds in the circuit.  However, voltage is only produced in the second coil when there is a change in the magnetic field.  So when the current begins to max out the secondary voltage drops, which again due to positive feedback lead to a very rapid decline of current in the system and, because the magnetic field is changing again, a sharp spike of voltage is the opposite direction from the second coil (from the collapse in voltage in the first coil).  This quick pulse of voltage is higher than what is in the original battery and can light the LED.  The circuit is trading continuous lower voltage for brief moments of higher voltage.  (This is similar to the mechanical action of a Shishi odoshi, a Japanese garden fountain that slowly builds in potential then snaps back to the beginning state making a loud "deer scaring" noise.)



In the picture above it is powering the LED to the upper right from a 1.5 V battery.  Normally 1.5 V is not enough to light up an LED (without the voltage oscillation), the LED to the lower left is connected directly to the battery for comparison and is not lit up. 

So this simple joule thief works; however, four of our homemade copper-aluminum batteries are not enough to light the LED even with the oscillating voltage boost.  Back to the drawing board.  (I suspect the zinc in some of the pennies is causing a problem.  Perhaps I will experiment with some other metal combinations.) 

Foxhole Radio Modifications

I played around with the foxhole radio we made earlier to try to increase the signal.  In a traditional foxhole radio a single diode is used, which cuts off half of the AC from the signal and converts it to a useable audio wave.  This is a half wave rectifier, which does not make use of all the power potentially available in the signal.

I used four LEDs hooked together in a diode bridge to make a full wave rectifier where current flowing in both directions in the signal are converted to an audio wave. 

Here is the actual setup.

And a closeup of the LED diode bridge.

Before with the single diode I was able to tune in stations from 10 to 15 miles away.  I was able to find the same stations again, so it is not worse, and I also found three more stations, the tower of one of these is just over 20 miles away!  Below is a map of the station locations I tuned in using this setup (click for a larger image).

So there seems to be some improvement.  I searched for mention of this online and in forums I read that using a full wave rectifier in an unamplified radio wouldn't help because of the voltage loss from the signal going through two diodes instead of just one (these types of radios work with very low power so any loss is bad).  However, my results suggest is does help.  For much beyond 20 miles I am going to run out of island.  I need to bring in signals from over 50 miles away to reach the next island, Moloka'i.  Perhaps next I will try to increase the size of the antenna.