Ben's Solid State A/V Switcher. Comes in any color you'd like as long as it's black.
While "solid state" may bring bring to mind electronics from the 60s (or, in some cases, yet-unreleased flash hard drives), but in this case it simply means the actual switching is done with circuitry, not mechanically. Ironically, the circuits are activated with a mechanical switch, but they could be controlled electronically if you were so inclined.
In this How-To we'll show you how to wire up your own A/V switcher which you can expand and use any way you'd like, for composite, S-Sideo, component, even VGA signals. Sure, you can buy a newer, really expensive TV's with multiple inputs (even component), but for those of us whose credit cards aren't quite up to that challenge, more inputs for less cash would be rather desirable. Let's get started!
For this project's example we'll be building a 3 input, single output composite video, left / right audio switcher. This will allow us to, say, switch between SNES, NES, and the God of War II adapter (aka PS2) using the single input on the back of the TV. Quality. We'll also explain how to wire up a VGA or S-Video port as well.
Here's the electronics we'll need to build this project. Quantities are noted as they will vary depending on how many inputs we want.
Bus switch - Digi-Key part # 568-3619-5-ND. We'll need one of these for every input you wish to have. They're pretty cheap, about 56 cents each.
10k resistors - Digi-Key part # 10KQBK-ND. We'll need one of these for every input.
1k resistor - Digi-Key part # 1.0KQBK-ND. Just need one of these.
Pushbutton on/off switch - Digi-Key part # 519PB-ND, Radio Shack catalog # 275-1565. We'll need one per input. Any switch that can stay on or off will work, these are a couple good choices. The Digi-Key model is much cheaper of course.
7805 voltage regulator - Digi-Key part # 497-1443-5-ND, Radio Shack catalog # 276-1770. Converts the voltage from the power supply down to 5 volts for the circuitry. We just need one.
Heat Sink - Digi-Key part # 294-1108-ND, Radio Shack catalog # 276-1368. Sucks excess heat off the 7805 regulator. You can use a chunk of scrap aluminum if you wish, even a PC's expansion slot cover with a hole drilled in it, as this project isn't going to generate much heat. Again, just one is needed.
Project box - Radio Shack catalog # 270-1805. We grabbed a large one so we could add more ports in later on if we wish. Any plastic box will work, go nuts and be creative. I'll let you guess how many are needed.
Here's the Digi-Key part numbers of any jacks you might need. Get as many as you plan to use for inputs and the output.
Yellow RCA jacks - CP-1415-ND
Red RCA phono jacks - CP-1413-ND
White RCA phono jacks - CP-1414-ND
Blue RCA phono jacks - CP-1416-ND
Green RCA phono jacks - CP-1417-ND
S-Video jacks - CP-2440-ND (also called a mini-din)
VGA jacks - 215FE-ND (also called a d-sub 15)
Other things you'll need:
Thin wire - Such as ATA-33 hard drive wire. To connect the bus switches.
Low wattage soldering iron
Solder - As usual we suggest the thinnest stuff you can find.
An old wall-wart DC power supply - These usually range from 5-15 volts, 9 is very common. Any old supply should work, such as from a Zip drive, game console, cell phone adapters. Look for "DC output" on the label, as well as a mA rating of at least 200. This relates to how much current it can supply. You can also power this project with batteries if you wish, for whatever reason.
Power switch - For the entire switch box, "main power" you could say. Any small switch will work, or you can skip it and have the switcher be always-on.
Don't miss the bus
This project works using bus switch IC's (integrated circuits or "chips"), in this case, 8-bit switches. Each switch has 8 inputs and 8 outputs, actually arranged as a pair of 4's that can be switched on or off separately. By connecting multiple bus switches together and putting all of the outputs on the same line (or bus) we can build a multi-input, single-output switcher.
Wiring the bus switches
Here's the trickiest part of the project. Like most ICs these days, the bus switches are surface mount and thus are harder to solder manually than the DIP sockets from the olden days. We covered soldering like this back in How-To: consolize an arcade PCB but here's a recap:
The 20 pin bus switch integrated circuits
- Clamp the IC in something to hold it steady, such as a "Helping Hands" or vice.
- Put a very small amount of solder on each pin to prep it for the wires.
- Cut 2 sections of 8 wires each from the ribbon cable and strip a small bit of plastic off the ends.
- Tin a small bit of solder onto each exposed wire.
- Hold each wire to each pin and heat it to connect the pre-soldered parts together. For best results, start on the center wires and work your way out.
How to wire the bus switch in many different ways
Thus, one of these IC's could act as (2) 4 signal switches, such as composite and left and right audio, or S-Video (luma and chroma) and left and right audio. To use the switch with all 8 lines at once, simply wire Enable A and B together so they are both activated at the same time.
When a switch is turned off (or open, not engaged) the output pins are at high impedance, which basically means they're neither on nor off and won't affect whatever else is on the bus. This is what allows us to use multiple bus switches on the same bus and is similar to how a computer memory map works.
The following examples will show how the bus switch can be wired for a variety of video input types. We'll talk about hooking multiple bus switches together and the rest of the wiring (such as power, ground and the enable switches) later on. In every example "Bus Out" refers to the signals / pins that will connect to the main output jacks on your video switcher. So no matter how many inputs you have, all the Bus Out signals go to the same place. (That's why it's called a bus, and you're probably already sick of the word.)
Note how the bus outputs are at opposite sides of the chip, thus they can be easily interconnected using a thin piece of wire between them under the IC.
Building a switch box
Alright now that we've described how the bus switches are wired let's put together a switch box.
- Wire all of the input jacks grounds together. (Same for S-Video or VGA if you're using those.)
- Wire one terminal of each pushbutton switch to ground.
- On the left we see a spot called "+5". This is where the input voltage will be connected and we'll branch several things off it.
- Connect a 10k resistor between +5 and the unused terminal of each switch (this will become the "enable terminal"). The enable terminal connection is what we'll hook up to the enable pin of the bus switch to activate each input.
- Install your 3 LEDs (we used random junk ones we had laying around) and connect all of their positive (long) leads together. Connect a 1K resistor between this and +5 volts.
- Connect the ground lead of each LED to its corresponding Enable Terminal.
When you click down a switch, the enable terminal gets connected to ground or is "pulled low" as mentioned during the wiring section. This puts ground into the negative terminal of the LED, thus it lights. The ground also goes into the enable pin of the IC (which, as mentioned above, will be connected to the enable terminal) and activates that switch.
The 10k resistor is used as a pull-up resistor, which keeps the terminal high (+5 volts) until it's pulled low by the switch, and the resistor itself keeps the ground from having a direct path back to the +5 volts, which of course, in the words of Egon Spengler, "would be bad".
Here's what the switcher should look like thus far.
Next let's put in a power supply, using as cheap of parts as possible. As mentioned in the parts list you can use pretty much any old wall-wart DC power supply. Some, such as those for cell phones, output between 3.3 and 5 volts and thus you won't need a 7805 regulator as the power is already within the range of what the bus switch IC needs. However most adapters output at least 9 volts (even those labeled as outputting less) so you should always test the wires with a multimeter first to find the voltage.
- Screw the 7805 regulator to your heat sink and put it in the box however you see fit. The bus switches don't take that much power so there's not going to be much of a load (heat), so don't bother overdoing it.
- Connect the other terminal of the switch to the left pin of the 7805. This is the input.
- Twist another long wire (colored black if you have it) to the ground wire of the power supply. Solder both of these together to the center pin of the 7805.
- Solder another wire (red is nice) to the right pin of the 7805. This is the output, and it'll connect to the +5 volt spot we talked about when we wired the input jacks together.
Wiring the bus switches to the input jacks
The wiring of the bus switches themselves was covered earlier, so here we'll just show you how they are physically connected.
Earlier we talked about an "output bus" and here it is -- a very elaborate piece of copper perfboard. Laid across the pads are some bits of wire, these are the output busses, video, left audio and right audio. Ribbon cable connects these to the output jacks. Doing it this way makes wiring a little easier as you have a long strip of a connection to connect all of the bus switch outputs too and won't have excess wires between the two halves of the case.
Enable A on the second bus switch goes to the leftmost pushbutton as seen above. The inputs of this bus switch connect to the leftmost set of jacks, and the output bus connections go to the same place as the first bus switch did. Remember, a bus is connected to everything.
Here's a pinout of a VGA port, in case your Google is broken and you have trouble finding one because there's no internet connection to your cave.
If the switcher doesn't work check the following...
LED indicators don't light up:
- Check that the LED polarity is correct - long lead is positive, short is negative.
- Check that the +5 volt and ground connections are connected to the 7805 regulator.
- Ensure that the selection switches have one terminal connected to ground.
- Make sure the power supply and regulator is supplying +5 volts to the bus switches.
- Check for short-circuited connections on the inputs and output bus.
- Make sure all input and output jacks have their shells/shields connected to the main ground.
- Don't enable multiple inputs at once, and check that only the input device you want to use is on to avoid interference.
- Using a multimeter set to voltage, see if +5 or ground (reads as 0) is getting to the enable pins.
- Check the wiring of the inputs and outputs on the bus switches.
- Make sure the bus switches have their Vcc (+5 volts) and ground connections made.
- Check that your test unit (game system, disc player) is on. As a last resort you know.
You now have the information required to build any type of custom A/V switcher you'd like. It's a great way to hook up multiple gaming system or to put more than one device into a TV's single component video jack. Have fun with it and see what you can come up with. Remember, since the switching is done with a TTL level input signal you can control the switches with other electronics such as timers, counters, PIC's and Basic Stamps. Now go forth and multiply!