Introduction: How to Make a Shopping-cart Sound-system for Street Parties

About: whats an engineer to do?

This Instructable will show you the steps to create a self-contained mobile soundsystem in a shopping cart. This setup can be used for all kinds of public gatherings, including Protests, Street Dance Parties, Parkling Lot Rap Battles, and even outdoor movie showings (with a second cart for the Projector).

It uses a Shopping cart as a rugged, mobile platform. A Car-audio amplifier works great with a Car-battery, and some decent ordinary Speakers seal the deal. A built-in battery charger ensures that you never forget to respect your fragile battery, and a Mixing board (that can handle 12V DC) allows you to add a microphone or two, taking the concept to the next level.

To do movies, get a second shopping cart, a second GOOD battery (and charger), an inverter (keep AC power wires away from Sound wires) and a place to set your laptop, DVD player, projector, and portable screen. If that battery doesn't have enough juice to run the projector through the whole movie, at least the soundsystem will keep the party going. (maybe jumper cables from the soundsystem's battery...)

Step 1: Diagram of Parts (not Including Cart)

In this diagram you can see two Speakers, an Amplifier, and a Battery, as well as other parts. You can get away with only those three parts - although a switch is definitely a good idea. The optional parts include a Mixing Board, which allows you to plug in more than just an Ipod or other music player. With a Mixing board, you can plug in a Microphone or two, which obviously makes the soundsystem ten times better.

If you don't have a Mixing board, you only get the one input of the amplifier - and you can't use it for a microphone (because a microphone needs to be pre-amplified by a mixing board). If you decide to add a Mixing board, you will need one which can be powered by 12 volts DC. Vestax and Gemini make mixing boards which say 15 or 18 volts DC but they will work fine on 12 volts. You will have to get the plus and minus wires correct on the first try though, or you will blow it up. More on that later.

Step 2: Get Speakers and a Shopping Cart.

You can't skip this step. You might choose your speakers first, or your cart first. If you have an awesome special cart, you might even make speakers just to fit in it, but if you're using an ordinary shopping cart, you can usually fit whatever speakers you find.

You will notice that these speakers fit PERFECTLY into this cart. Thats because speakers are Consumer Products, and a shopping cart is a fetishistic symbol of consumption. They are made for each other. Probably though, you did not buy the cart or the speakers.

Make sure your shopping cart doesn't belong to a nearby store, which could prosecute you in case Cops want to make a big deal out of what you're doing. You may not be doing anything illegal with your street party, but if The Police want to stop you, accusing you of stealing a shopping cart is a quick way to put you in jail and confiscate your soundsystem! If your shopping cart is obviously the property of a store, that store can be easily convinced by the Cops to press charges on you. Same goes for Milk Crates.

Choosing speakers is easy if you're trying to save money. A lot of people's houses contain unused speakers (check the attic or basement) which still work fine. Often the only problem is that the foam ring around the paper cone of the largest speaker has disintegrated. This can be fixed with tape if you do it right, or you can replace the speakers if the wooden box they are in is good. Let's not digress from the main topic though: find speakers that you like, and that work well.

A final piece of advice, in case this is not obvious: TEST the speakers before falling in love with them. TEST the shopping cart (put a friend in it and pretend you're running from the cops) before choosing it. Then, on to the next step.

Step 3: Get an Amplifier and Battery (and Charger)

You will need an Amplifier. The amp discussed here is a standard ordinary car audio amplifier (except the switch we added). Amps like this have one input - a "Line-Level" stereo input. This signal can come from an IPod or other headphones-ready device, OR a mixing board with several inputs (including Microphone jacks). Amplifier wattage numbers are very subjective, so don't worry about whether it says it's 200 watts or 1000 watts, just see if it WORKS and sounds good. It will probably supply more wattage than your speakers can handle either way though, so be careful.
(When testing a suspicious used amplifier, don't connect it to a speaker you don't want to fry!)

Another thing about a Car Amplifier like this: it runs on 12 volts, like a Car Battery supplies. Using your home stereo amplifier won't work unless you have a very long extension cord. You may be thinking "but I can just use an inverter, and power my favorite plug-in amplifier!". DON'T DO THAT. You will be wasting a third of your battery life, carting around extra equipment, AND inverters make a horrible 120Hz "buzz" that gets into all your sound. Plus people who know better will scoff at your lack of efficiency.

You will need a Car Battery to power your soundsystem. It can be a used one, a small one, a largish one, or a Boat battery, or other 12 volt battery of a decent size. Deep Cycle batteries are better, but more expensive - basically, they can be drained further without lasting damage. Ordinary car batteries are not supposed to be drained more than halfway during normal use. But if a Car battery is available to you, use it. You probably won't drain it more than halfway anyway before the Cops take it away. (Try to get them to let you disconnect the battery so it's still good when you get it back!)

And the Charger: batteries like this must be charged back up after being used - allowing them to sit discharged will ruin them, besides not being ready next time you want to party. I recommend strapping the charger into the shopping cart and wiring it in - so there is no reason not to plug it in when you get the chance, even if the cart doesn't go "home" to the same place every time. You can't skip this step; charging is essential to the preservation of your battery and your whole project.

Read up on car battery chargers to understand how long to leave it plugged in, and what Charge Rate to set your charger to. If you have an automatic charger with a two-amp setting, overnight will charge your battery back to full, and you can leave it plugged in up to a day or two without danger.

Step 4: Put Power Cables on the Amplifier

The Amplifier needs to connect to the Battery, for power. A good way to do this is with jumper clips like the ones in the picture. Another way is with proper battery terminals, which you can get at the Auto Parts or Hardware store. Some batteries don't have top-posts, and clips like those shown won't even work. The choice is yours; I like jumper clips, as you see, because if someone else decides to put in a different battery because the old one got ruined, I don't have to show them how to connect to it. Lots of red and black tape are used to make sure Plus and Minus are unmistakable.

As long as the connection is secure, you can use whatever connections to the battery makes sense to you. For batteries with Posts (like Normal car batteries) you can use Hose Clamps to squeeze bare wires onto the posts of the batteries - nothing wrong with this. You can even put multiple wires in there, like the ones from the Charger that is strapped in next to the battery. I'm just saying.

The jumper clips in the picture are soldered, and use heavy-gauge wire. The connections at the ends of those black wires are the most important, but you can't see them here. Use whatever methods you or your solder-happy friend are comfortable with; just make it a good rugged connection and don't mix-up Plus and Minus even for a second (picture a big spark and the end of your amplifier's useful life).

A Car Amplifier has three power connections, in this case B+, REM, and GND. Sometimes there are wires coming out, instead of screw terminals as shown. In that case, Yellow=B+, Blue=REM, Black=GND. B+ goes to the Plus terminal of the battery - all of this should be RED RED RED. GND stands for Ground and goes to the Minus terminal of the battery and should be BLACK BLACK BLACK. REM stands for Remote, and goes to B+ when you want the amplifier to be turned ON.

The Plus and Minus wires of the amplifier can be left connected to the battery all the time if you have an ON/OFF switch. The ON/OFF switch is an ordinary two-wire switch which goes between the B+ terminal and the REM terminal of the amp. When this switch is OFF, the Amplifier is OFF even though it is still connected to the battery. If you use a U-shaped wire as shown in the second and third pictures, you HAVE to use jumper clips because that will be the only way to turn off the amplifier!!!

In the fourth picture, you can see the nice switch that almost looks like the factory did it. It connects to the B+ and REM terminals of the amp. You could instead put a House Lightswitch in a box, with wires going to the B+ and REM terminals; then you can mount the switch up top where you can reach it. Make sure all your connections are secure, and taped so they don't touch the metal shopping cart.

Don't use a big red button for an on/off switch, or Cops will say they thought it was a Bomb and that's why they shot you in the head. (Jean Charles de Menezes R.I.P.) This is something you should keep in mind when designing your Carts' appearance, so that some idiot out there doesn't come to the wrong conclusion about what your shopping cart is and why you're acting sketchy and dressed weird. If you want to be extra idiot-proof, put a sign on both sides that say "mobile sound system pride".

Step 5: Get and Prepare Speaker Wires

You will need Speaker Wire to connect the Amplifier to the speakers. This should be two-conductor stranded wire no thinner than 14 gauge. It does not have to look like what is in the picture; it can be "lamp cord" which can be bought from the hardware store. It can be white, brown, or black and red, it doesn't matter. There should be a way to see a difference between one wire and the other though; sometimes there are ridges on the rubber of one of the wires and not the other. If you can't tell the difference, you might end up hooking one or both speakers up backwards, which affects the sound.

Step one: Get the speaker wire - enough for each speaker to reach the amplifier.

Step two: Cut off the mangled ends of the wires and start over.

Step three: Strip the insulation off the wires WITHOUT nicking or cutting any strands!
(If you cut into some of the strands, chop it off and try again)

Step four: twist the strands of the wire like rope, so that individual strands won't come loose and reach out and touch their neighbors, causing a short and ruining everything.

Step five: If you have access to a soldering iron and fresh solder, "tin" the wire ends with solder so that they never come untwisted. This is optional, but it makes things so much better. If you don't know how to solder, find someone who has done it before. Use solder with flux in it which is no thicker than the wire itself, unless you really know what you're doing.

Step six: cut the exposed part of the wire, if necessary, to less than a half-inch of exposed metal.

Step 6: Connect the Speakers to the Amplifier

I'll tell you a secret. The speakers pictured in this article are "8 Ohm" like most in-home speaker cabinets. All Car Amplifiers including the one shown are made for "4 Ohm" speakers, which means that you can connect FOUR speakers instead of Two like you've been planning. It's okay if you don't, but this little piece of information is your reward for getting this far. If you wanted to connect four speakers instead of just two, you would just double them up on the screw terminals of the amplifier; that is, two wires under each of the screw terminals instead of just one, as shown.

First picture: Speaker wires connected to the speaker (on this speaker, you have to push the plastic thing, stick in the wire, and then let go). Red is Plus, the other one is Minus. This wire has a white stripe on one side, so I put Red Tape on that side, on both ends of the wire, to avoid confusion.
If you hook up the speakers' Plus and Minus backwards, the Bass might sound a little wrong.

In this First picture you can also see that I have split the wires apart, a few inches from the speaker ends. Do this carefully so that you don't cut through the insulation into either of the wires.

Second picture: Now we will put a Wood screw with a Washer through the hole. This step is important because it keeps the wires from getting yanked out of the speaker. If that happened, the ends of the wires would probably touch each other or the shopping cart, and DESTROY the amplifier. Make sure you use a washer, which is a flat metal thing with a screwhole through it, so that the wire doesn't get cut into by the screw head. Check the edge of the washer for sharp. You might could also use a piece of wood.

Third picture: The screw is put into the wood of the speaker cabinet. Now the wire is safe.

Fourth picture: Connect the speakers to the amplifier. Usually, the screws have a square washer under them, and the wire sandwiches between those squares. They are labeled + and -. One pair of screws is L or Left, the other is Right. It doesn't matter which is which, you can always swap the Red and White Audio connectors at the input if you want to switch them. That is also how you troubleshoot things to make sure both speakers are working, and to test your audio cable.

Step 7: Mount the Amplifier to a Speaker and Tie Things Down

The amplifier has wire connections coming out of both sides, and it must not be kicking around the shopping cart. This one is screwed to the back of one of the speakers, so that the power switch and the input connectors are facing up. It is close enough to the bottom for its Power wires to reach the battery. You may also notice that the plastic knobs have been removed from the amplifier so they don't get turned accidentally - you could also tape over them, and switches, like on the mixing board.

In the second picture, the Mixing board has been placed on top of a speaker which is upside-down. The plastic "bottom" of the speaker cabinet works great to hold the mixing board safe, but it will be tied down anyway. There must be a place to put the microphone and ipod during rough travel with the shopping cart.

The mixing board has been "taped up" so that any controls which do not matter to the usual use, or which are broken, won't be accidentally moved. All of the other controls are labeled very clearly with white tape and black marker. People who have something to say or music to play don't need to be arsed with trying to figure out which Line 6 or Phono 2 Aux Mic Send the Ipod cord is, and the Main Volume knob needs to be clearly labeled - think about it. The microphone volume knob should be clearly indicated. This microphone has an on/off switch which has been taped ON - people should only have to worry about one control, which is the knob on the mixer.

Third picture: tie everything up securely. If you are lucky, you can find a someone experienced with ropes who will do a good job. The battery is the MOST IMPORTANT part of this, because if it moves even a little bit, its posts might touch the metal of the shopping cart and that can be really really bad.
It should be tied in with its own rope, something which doesn't stretch and won't degrade from contact with battery acid, which is all over the outside of batteries. You might want to use a "battery hold-down kit" available at Auto-parts store, and has metal hooks which can hook on the metal of the cart, as seen in the last picture.

Step 8: Add the Charger

I recommend putting the battery charger on the cart, and wired it in. This one is tied into the bottom of the shopping cart with bicycle inner-tube. This charger is a Ten-amp "automatic" charger, which means that it will turn off automatically when the battery is full, IF it is working right. A two-amp charger would be more gentle, but definitely require overnight charging. Use whatever you have, and learn more about it if you are concerned.

WARNING: Charging a battery emits Hydrogen gas. This is the most flammable gas there is, and it burns so rapidly that a hydrogen explosion is faster than the speed of sound. Charging at a higher charge rate (like Ten amps instead of Two) will result in more hydrogen. Charge in a ventilated area, and be careful of sparks around a charging battery, including the sparks made by disconnecting the charger. Unplugging the charger from the wall first (instead of the battery) is appropriate.

This charger still has its jumper clips on it, and that's how it is connected. You can chop off those connectors and attach them to the battery with another method if you want. The most important thing is that nothing allows a short from the battery terminal to the shopping cart, or the other teminal. Large jumper clips should have enough tape (red for + and black for -) to cover any exposed metal parts which are not intended to touch the battery posts.

Finally, tie the AC power cord of the charger to a high point on the shopping cart, so that it doesn't drag on the ground in case people forget to tuck it in after charging. It should not hang low enough to drag. Put an extension cord in the cart in case you need to plug into an outlet where the cart can't reach.

Step 9: Add a Mixing Board (optional)

You can add a mixing board to the system, but it is not necessary. Without a mixing board, you will simply have a cable from the inputs of the Amplifier to a mini-jack connector intended for an IPod headphone jack (or laptop headphone jack or anything you want to play from.

If you do have a mixing board, you will connect its output to the amplifiers' input, and then connect a mini-jack connector cable to one of the mixing board's inputs. You can have two such cables, and then you would be able to cross-fade or mix multiple music players. This is fun, because you can have two DJ's at the same time, taking turns choosing the song, without having to yank the cord and switch it in between songs, which is horrible.

If you have a mixing board, you will be able to have a microphone as well. This makes a lot of things possible which weren't before, and is a good reason to add a mixing board. Think poetry slams, rap battles, political exhortations, and vocalizations over whatever music is chosen at the time.

To get a mixing board into the mix, you will need a mixing board which is made for a DC power supply. In the picture, you can see the power jack on the left, labeled 800mA DC 18V. The thing that matters here is DC. It doesn't matter if it wants 12V, 15V, 18V, it will work with the 12V we will give it. The fact that it says 800mA which is 0.8 Amps is irrelavent to what we are doing.

The connector probably is labeled showing which part of the socket is Plus and which is Minus. In this instance, the mixing board was opened, and wires were soldered to the circuitboard, bypassing this connector. That way, the loose connector won't fall out in the middle of a rap battle, but for most people it will be much easier to simply cut off the wire from the original power supply and use that.

If you use the cable that came with it, you will cut off the AC transformer (where it plugged into the wall) and ditch that box - its job is to make DC power from what the Wall outlet supplies. You will have to figure out which of the two wires coming out of it is Plus and which is Minus. If you hook up the mixing board to the battery with Plus and Minus mixed up, it will be instantly destroyed.

If you have the cord coming from the Mixing boards' power jack, and have cut off the Adaptor, now strip and expose the two wires. One of them is Minus, which is also Ground, which is the same as the outer metal ring on all of the audio connectors on the mixing board, and the GND terminal seen on the right.

NOTE: Nothing is connected to the AC Wall outlet during this test. Nothing is connected to the car battery for this test. If you have any doubts about this process, find someone to help you who has experience with this kind of thing.

Use a VOM (Volt Ohm Meter) aka Multimeter, set on the OHMS setting (200 ohms setting if it has multiple ranges). Touch one of the multimeter leads to the GND terminal of the mixing board (or even tighten the GND screw onto it so you don't have to hold it there). Touch the other multimeter lead to one of the wires going to the power connector and wait for the reading to stabilize. If it is approximately zero, that means you have found the Ground/Minus wire. Measure the other wire now, and wait for the reading to stabilize. It will likely measure as though the meter is not even connected to anything at all. That is what the Plus wire to the mixing board will act like. (This is assuming the power switch of the mixing board is ON).

When you are sure which wire is Plus and which is Minus, label them clearly with Red and Black tape. Connect the Minus to the Amplifier's GND screw (in addition to the other wire which is already there). Connect the Plus of the mixing boards wire to the B+ or the REM screw of the Amplifier. If you connect it to the REM terminal, the Mixing board will be turned off when the Amplifier is turned off by its power switch, which is a good way to go.

Step 10: Wireless Microphone From a Cordless Phone (bonus Points)

This could be an instructable all itself, but that's what search is for. If you have gotten the mixing board to work, you have what it takes to add a Cordless Phone to your shopping cart. This is the coolest thing ever, and i'll tell you why.

A microphone plugged into your mixing board has a wire. This means it never runs out of batteries, and is always going to work, and people always know which end to talk into. It's great because it is a normal way to use a PA system, if the person knows to stay out of the Feedback Zone.

But a cordless phone hooked up as a Wireless Microphone lets you hand the handset around in the crowd, and people who would never dare step up to the shopping cart and pick up the microphone might be tempted to say a few words into it before passing it off. It really involves people. Also, using the hangup/pickup button you can turn it off and on without going up to the mixing board.

Here's how to make it work: Find a cordless phone - the handset and base must match. Plug it (the base) into the wall and let it charge the handset. Take the telephone cord that comes out of the base, and cut the Wall end off of it. There will be two or four wires; if there are four, ignore the outer two.

The two wires should be stripped and soldered to a round connector or two, and plugged into the mixing board (two connectors so that it comes out of both speakers). If the wire is hard to solder, find another telephone wire (the ones that come with Apple computers are easy to solder to,) When the phone is charged, and plugged into the mixing boards' other input, pick up the phone, tell it to "pick up", and talk through it - your voice should come out of the system. (At this point the base is still plugged into the wall, while you're trying it out).

If the phone works (many phones won't do this, so try another one) you are ready for the next step; cut off the power adaptor (the one that was plugged into the wall) and figure out which wire is plus and minus. Even though the phone says 9VDC it will be fine with 12V from your battery. If the phone is made for 9VAC it might not work.

NOTE: Nothing is connected to the AC Wall outlet during this test. Nothing is connected to the car battery for this test. If you have any doubts about this process, find someone to help you who has experience with this kind of thing.

If the phone is made for DC and you have cut its power wires from the wall adaptor, there are now two wires going to the phone (they are side by side, you need to pull them apart like you did with the mixing boards' wires) and two wires coming from the power adaptor. One of the wires will have a stripe or printing along it. Set your VOM or Multimeter to Volts DC (20V or 200V scale) and measure the two wires coming from the AC adaptor while it is plugged into the wall. You should read a voltage, 18V is normal even for a "9VDC" adaptor, but what matters is whether there is a Minus sign before the number. Switch the red and black leads from the meter, and see how the Minus sign shows one way and not the other. When the Minus sign is NOT showing, the Red and Black of the multimeter correctly indicate which wire is Plus and which is Minus. Label them with Red and Black tape. Which one is the striped one? It is the same for the wires coming out of the Phone base.

Now you can join the plus and minus wires of the phone base to the B+ and GND posts on the amplifier. You should not connect the Plus of the phone base to the REM post, because you want the phone to be "on" even if the amplifier is turned off, because you want to Charge the cordless phone BEFORE you go out using it.

Note: if the phone was made for "9V" and you are powering it on 12V from your battery, it MIGHT be charging the handset more aggressively than it was designed to. If the handset is warm, it is probably fully charged. If your rechargable pack gets ruined or won't hold a charge, you can exchange it for a new one at Radio Shark or another used cordless phone - and watch that Plus and Minus!

Good luck and don't give up until you get it working!