Intro: Pocket Protest (a LM386 Amp in a 9v Battery Casing)
Whether your working on electronics, want to test that speaker, checking out a cool looking radio at a swap meet, want to soapbox the evils of soapboxing, or sit on the street corner singing your blues ...
Well, darnit, sometimes you just need a simple easy to use amp!
I bring to the world Pocket Protest (a LM386 amp in a 9v battery casing)
Pocket Protest runs on a standard rectangle type 9 volt battery for a couple hours and can power a 4 - 8 ohm speaker fairly easily, while all along being enclosed in a 9 volt battery casing
The wattage is low, but since theres no filtering or enhancement the sound that comes out is quite brash...
Its a can running off of a 9V battery, and it sounds like it too! Even if the wattage is low the "bullhorn" quality sound gets loud enough that normal people have to talk to almost shouting levels in order to speak over the device
Here is a video of the breadboard test mentioned in step 9, with a really flat tiny 25 watt 8 ohm speaker, it was around 30% volume, which was enough to overdrive the mic on the camera and my wife on the other side of the apartment
Step 1: Tools and Materials
Wire Cutters / Strippers
Loose Clip leads (alligator clips to use as mini spring clamps)
Solder (and some way to remove it in case of mistake)
Needle Nose pliers
Small Flathead Screwdriver (jewelers style or similar)
Electric Drill and 3/16th inch drill bit
Pencil / Paper
Xacto / hobby knife
320+ grit sandpaper
330 ohm resistor
led, I choose blue, go with a full sized bright led
10k audio pot pcb mount
1/8th inch stereo pcb mount headphone jack (female)
10uf electrolytic cap (16 + volts)
220uf electrolytic cap (16 + volts)
0.1 uf ceramic cap (with long full length leads, buy new)
dead 9 volt battery
good 9 volt battery
Gel Super Glue
Low viscosity clear epoxy
heat shrink tubing (1mm and 5mm)
4* 5 inch pieces of wire, multicolored 20-24 gauge recommended
~ 1 foot of 20-14 gauge zip cord or 2 conductor speaker wire
2x alligator clips (with rubber jackets optional)
Clearcoat of some type (spray)
except for the ceramic cap, the power providing battery, and alligator clips everything was scavanged from other electronics
Step 2: Amp Circuit
The pocket protest is a basic LM386 reference design with some components forgotten or replaced, it is fairly easy to breadboard out in a few moments
The audio quality is harsh, and in the configuration below you can only get about 2/3's of the volume before the sound is complete garbage, but its surprisingly loud even at low knob levels
If you notice we have to tie 6 points of the circuit to ground, fun right?!
Step 3: Biulding the Amp Part 1
First Grab your pc mount 10K pot and your LM386 chip
Hold the base of the pot ontop of the LM386 and orient it where pin 1 of the pot is lined up with pin 2 of the amp chip
Likewise pin 2 of the pot should line up with pin 3 of the chip, and pin 3 of the pot should line up with pin 4 of the chip
once you know every thing lines up use the super glue to glue the amp chip to the bottom of the pot keeping the alignment best you can
Step 4: Biulding the Amp Part 2
Go ahead and get your soldering iron heated up while we are doing this, we will need it soon.
Bend pins 1 and two of the pot down towards the chip, I found it easy to bend the pins down flat against the casing of the pot using the blunt end of my tweezers.
Next use your tweezers to bend the leads of the chip into an L shape, and then up and over the pins of the pot. you will need to do this to both pins 2 and 3 of the chip, be careful these things are brittle as it is and im using a scavenged chip so dont fool around too much.
Finally bend the 3rd pin up and away from the chip, again flat as possible against the pot, this pin is our audio input.
Step 5: Biulding the Amp Part 3
Grab your now hot soldering iron and solder chip pin 3 and pot pin 2 together
Next get one of your 5 inch lengths of wire, strip about a quarter inch off the end, twist it tight and tin the end. also tin the 3rd pin of the pot
Finally using your tweezers to help guide / hold the wire reflow the wire and pot pin together making a secure connection
Step 6: Biulding the Amp Part 4
Now we need to grab a new component, the 0.1 uf ceramic cap
This is one of the few items (aside from the power source) that needs to to have fairly long leads, and most scavenged items have really short leads most often the thickness of the pcb and not much else.
We need the long leads because the component itself will mount on the side of the pot / chip combo and it needs to make contact with pins 7 , 4, and 2 which are on opposite sides of the chip.
First take your needle nose pliers and hold the cap's leads in them as close to the base as possible without cracking the outside coating, then bend the cap upwards into a 90 degree angle.
Next lay the cap down on the underside of the LM386 chip, where the disk part of the cap is resting on the end pointing twards the same end as pins 1 & 8 of the amp and upwards to the knob of the pot .
Then using your needle nose pliers or tweezers as a break, bend one lead of the cap to mate up with pin 2 of the chip.
Now loop the cap's lead back around in a U shape and solder the U loop to pin 2. Once soldered bend the remaining lead so it mates up with pin 4 of the chip
Finally bend pin 4 of the chip over the remaining lead, onto the underbelly of the chip, then solder and clip the excess
I promise you that was the most complicated part of the entire project!
For the other lead of the cap simply bend pin 7 over in the same manner as pin 4, solder and clip the lead
Step 7: Biulding the Amp Part 5
Next, on the other end of the amp (pins 4 & 5) you need to position your 220uf electrolytic capacitor where the positive (+) side lines up with pin 5 and super glue it to the side of the amp and the pot
Bend pin 5 over the cap's lead and onto the belly of the amp chip, then solder
Then solder a 5 inch piece of wire to the clearly marked negative (-) side of the cap, this is our audio output
FInally bend pin 6 of the amp chip up against the pot, this is our power + (pin 2 being ground), and get a 5 inch length of wire (I suggest red since this is power in) and, tin the pin, tin the wire and reflow together into a secure connection
Step 8: Biulding the Amp Part 6
The last capacitor, the 10uf electrolytic needs to connect to pins 1 and 8, and this is purely optional
With this cap in place the chip has a gain of 200, and lots of distortion, but even at half volume its quite darn loud with not so bad distortion
Without the cap, the chip has a gain of 20, which is pretty clear all the way up to full volume, but its about as loud as a really cheap small radio, or telephone answering machine (which is where this chip came out of)
If you decide to forgo the cap, just clip off pins 1 and 8
If not ...
Lay the cap down on the belly of the amp chip where the positive (+) side meets up with pin 1 of the amp chip and the negative (-) side mates up with pin 8 of the amp chip and solder the 2 connections
Finally solder a 5 inch piece of wire (i used black) to pin 2 of the amp chip using the same tin, tin, reflow process. This is our common / ground wire for the circuit
Step 9: Finally Done With the Amp
At this point you are technically done, you should hook up the amp (I used a breadboard) and make sure it works
The circuit will run on ~4 volts to ~12 volts with no problems
Hook up power and ground, connect a speaker to the output and ground the other side to the power supply
Hookup an input to the, um well input, and ground the audio's negative(-) to the power supply
If all went well you should have a working amp, with quite a bit of motor boating.
While fooling around with this I found if I put a LED tween the positive (+) and negative (-) side of the power supply it would squelch the motor boating
This makes a good breadboard / hobbiest amplifier, and quite honestly it was far as I was prepared to go.... Then Instructables announced the "Pocket sized" contest
Now while this is well beyond pocket sized, its not very pocket friendly. It could easly get snagged inside ones pocket and rip apart something important, and frankly we cant have that
So moving forward I decided that I could fit the amp, the LED, and the audio jack inside of a 9 volt battery casing, making it pocket friendly
Step 10: Casing It Up Part 1
First thing to do is size up the amp against the battery casing, making sure it will actually fit
Next thing to do is wrap the amp circuit up tightly in electrical tape, this prevents anything from shorting on the battery case
Once we know its going to fit, its just a simple matter of doing the good ole 9v battery hack on your dead battery
Use a pair of wire cutters to help pull up the non terminal end of the metal casing, then use your flathead screw driver to bend the rest of the end up so you can remove the cardboard end and the battery contents
Save the Cardboard with the battery terminals, and the shell of the battery, the rest can be properly disposed of
Step 11: Casing It Up Part 2
First grab your headphone jack, and probe it out, there should be 3 pins, one is audio left +, the other is audio right +, the final pin is ground, using a meter probe out which pins go to which parts of the jack, plugging in a headphone wire makes this quick and easy
Back to the amp, we have to trim some wire, add a audio input jack and audio output leads
Start by taking your common / ground wire (black in my case) and trim it where there is about 2 inches poking out of the now taped up amp assembly, strip about 1/4 inch off the end and twist it up
Next with the piece you just cut off, strip about 1/4 inch off of the end and pigtail it back to the common wire, basically splicing the wire you just cut back together, then solder the pigtail to the ground on your headphone jack. This jack (like most) has the ground wrapped around the outside of the jack, I opted to solder the ground connection on top of the jack and clip off the ground pin.
Theres only 1 more connection to be made to the headphone jack, audio input to the amp (in my case the orange wire). Start off by bending the L and R connections inwards and over eachother.
Then Trim your audio in wire to about 2 inches, strip about a quarter inch off and solder the L and R pins to this wire
Finally Wrap up the whole thing in electrical tape, this does not have to be water tight, but enough where when we flood the case with epoxy it does not fill in the headphone jack.
Step 12: Casing It Up Part 3
Take your zip cord / speaker wire and pull apart about 3 inches of it. Clip one side of the zip cord (if yours is marked use the + side) where there is about 2 inches left.
Also clip your audio out wire (yellow in my case) to about 2 inches long, strip a quarter inch off both the audio out wire, and the short wire you just made above.
Slip about an inch of 5mm heat shrink tube over one wire, and get it out of soldering heat's way, twist the 2 wires together using an in line splice, and solder.
Slip the heat shrink tube back over the joint and heat until tight. Then finally solder on some alligator clips to the other end of the wire so you can connect your speaker.
Step 13: Casing It Up Part 4
Were down to the final electronics connections!
Grab the cardboard battery terminal up to your good 9 volt battery where it will snap on properly, and mark the top of the cardboard battery terminal with the appropriate polarity (remove from battery before proceeding)
Next grab your 330 ohm resistor, and hold the resistor where it sits in between but off to the side of the backsides of the battery terminals. bend 1 lead to mate up with the negative (-) terminal
Now get your LED, make sure you know which side is the anode (+) lead, position it where the anode (+) of the LEDis over the positive (+) battery terminal
Put a sleeve of 1mm heatshrink tubing over this lead, I did not and it became a headache
Then bend a L shape so it has a "foot" sitting on the backside of the positive (+) battery terminal
Next take the cathode of the LED and bend a U shape around the non bent end of the resistor, clip and solder
Step 14: Casing It Up Part 5
Now we must solder everything to the battery terminals
The 2nd wire of the zip / speaker wire, one end of the resistor and the wire comming from the headphone jack all need to be soldered to the negative (-) side of the battery termnial
The anode of the led , and the amp's power in pin (red wire in my case) need to be soldered to the positive (+) side of the battery terminal
Also use the solder to plug up the holes in the battery terminals, we do not want epoxy running out of the bottom
Step 15: Casing It Up Part 6
Now that you have it all wired up, you should go ahead and test it, if everything is still working its time to compact it so it will fit inside of the 9 volt battery shell
Carefully bend and fold up the wires into a rectangular shape, the knob, the speaker outputs, and the led need to point towards the top. The headphone jack needs to point out sideways on the negative terminal side of things
Use the empty battery shell as a guide
Step 16: Casing It Up Part 7
Hold the circuit against the case on its side so you can eyeball where to drill a hole for the headphone jack, and mark it with a sharpie, there is a seam to the battery shell on one side, use the side with no seam or the shell might not hold together while drilling.
Get out your drill and 3/16 inch drill bit and drill the hole where marked.
Next stuff the circuit into the 9 volt battery shell, the terminals should be on the end that we did not fold upwards when taking the battery apart.
Use your small flathead screwdriver to carefully poke the headphone jack through the hole in the side of the case. Add a drop of super glue around the outside edge of the hole and the jack to hold it in place for now.
Also apply a bead of super glue around the lip of the terminal end of the battery shell, use your needle nose pliers to hold the terminals tightly against the shell, also to hold them in proper place for now.
Finally I tested it all again to make sure that I did not accidentally break or short something while pushing on the headphone jack with the screwdriver.
Be mindful that the only thing holding the power terminals and the headphone jack in place is a thin bead of super glue, but its good to do a final test because the next step is encasing the thing into a brick of plastic
Step 17: Casing It Up Part 8
Now that its inside of its case we need to make it permanent, the easiest way to do that is pour some expoy inside of the shell and let it harden up
At my work they have some epoxy that dries crystal clear in about 12-24 hours, and has a viscosity of maple syrup, and that is perfect.
But I could not wait till Monday and used some 1+ year old "5 minuet" epoxy, which was totally wrong.
For starters it is yellow which taints the blue led
Next its much thicker (like thick honey) so you have to pour really slowly to avoid large air pockets, or poke them out with a toothpick
And since its 5 min set time you have to work fast while going slow, it ended up being a huge mess, and I am not happy with the results, so get good clear setting thin epoxy.
The idea here is to fill all of the void with epoxy so it form 1 solid brick of plastic inside the shell, and keeps everything from moving around
Step 18: Casing It Up Final
Once your epoxy is set up fully you can use a knife to clean up by scraping drips and whatnot off of the knob or the shell
When I finally got done with it, the shell was such a mess I used 330 grit emery cloth to sand off the glue residue, and the original paint.
The shell is steel and would rust if left in this state, I opted to hit it with a buffing wheel to take out some of the sanding marks and spray a clear coat over it, course you can paint it any way you want.
And there you have it, this took quite a bit of time and patience but overall its not all that hard.
I hope you enjoyed this instructable on a small useful amp and if you have any questions or comments please let me know