Introduction: Salvage Mini-PCB-boards for Arduino Shields
Save time and money by salvaging mini-PCB-boards and re-using them as an Arduino shield. These small PCBs are often packed with buttons and switches, and they often have convenient connections built onto the board.
I'm embarrassed to admit that it took me three years to think of this idea, but my progress seems typical for new electronics hobbyists - and each step was necessary for me to take/ My progress went something like this:
YEAR_1: This electronics stuff is scary - I dare to use only pre-soldered Arduino Uno shields. Plug and play is safe and easy. By the end of the first year, maybe begin to solder pre-designed kits like the Danger Shield from Sparkfun or the RGB LCD kit from Adafruit. Begin to see the limits of using other people's designs...
YEAR_2: Read and follow the dozens of how-to-scavenge electronic components Instructables. Spend hours scavenging parts and even more hours sorting them into meaningful piles. Promise yourself that you will reuse these parts
YEAR_4:Wait a minute - why not just re-use the boards. I now know enough to figure out what they do, how they work and how I can use them.
All those steps were critical to my learning about electronics. I needed to understand something from each phase, from the simplest ones to the all-hand-built projects. It was also important to my ego that I learned how to make stuff from scratch. Now that I don't have as much to prove, I'm free to take the easiest route I can - and that's where scavenging pre-made PCBs comes in.
Step 1: Where to Find and How to Salvage
Where to Find Mini-Boards
Mini-boards are found in a wide range of electronic products. You can spot a likely candidate by looking for a highly sculptural case that has several distinct areas filled with buttons and other controls.
- Small boom-box radios are a likely source of mini-boards. If the radio has buttons and dials sticking out at different angles, one group on each side and a center console - then it's a good bet there are several small boards inside.
- Home printer/copiers are also a good source. If your printer/copier has multiple keypads and controls on separate parts of the front face, there is a good chance it also has multiple boards inside.
- Kids' toys are another great source, especially cheap ones. The toy-makers have to create several small boards to fit into the oddly shaped body of the toy.
How to Extract and Salvage
Harvesting these mini-boards is really very simple, but it can be annoying at times. There can be lots of screws and connectors to unhook, and the board you want might be buried several layers deep. Stay patient and keep working (put down the hammer) and you will get a useful board for free.
Once you start taking the case apart you can begin removing the mini-boards. They are usually attached to the case with a few, very small screws. They will occasionally be held by clips, glue, tape or caulking. Be patient and remove as many of the screws and clips as possible.
If the board still refuses to move, get a magnifying glass and look for any remaining hard-to-spot fasteners. If the board bends when you pull on it, look for the hidden screw at the bottom of the bend. Sometimes a screw is hiding in the center of the board, right in the middle of a cluster of components that make it difficult to see.
Also check the back of the board. Feel around to see if there is a wiring connector still plugged into the back of the board. It will be tempting, but try not to cut any wiring or cables just to remove the board.
Sometimes you will need to remove other components before you can get to the board you want. Enjoy the process and marvel at the design skills required to create such a nine-level, triple-nested interlocking mechanism.
Step 2: Best Types of Boards to Save
The best boards are the simplest boards - a few buttons and switches that are easy to re-purpose for your projects. It is tempting to salvage the large boards with 20 buttons, 10 potentiometers and lots of LEDs. I have done that, but I've never really used them after I reverse engineered them. I just don't need all those inputs for my projects. They make great training exercises, so go ahead and try to decode a huge board - just don't be surprised if you don't actually use them very often.
But I do use the simple 4 button boards all the time. They are convenient and simple. And if I need more inputs, I simply hook up a second, equally simple board.
What Else to Save
You might just want the mini PCB boards, but it's a good idea to save other items:
- button covers
- knob covers
- electrical connectors
- wiring harnesses
- mounting screws.
face-plates from surrounding case
The covers for buttons and knobs help keep the components clean, add visual clarity, and cut down on design and construction time when you design a case for the board. Saving the covers for "soft" buttons (PCB with elastomer caps) is especially important. It's a lot of trouble to make these types of buttons work without the original cover caps.
The wiring harness and connectors make hooking into an Arduino much easier. Sure, you can solder directly to the board or use jumper wires on the pins - and you probably will if you use this is a permanent project. But for prototyping, a nice ore-made wiring bundle makes it much easier to set the board or boards safely off to the side.
Unless you have a collection of tiny screws, it's best to save the mounting screws. They fit that board and held it securely before, so you can also use them to mount in a new case.
Faceplates and frames also make re-enclosing the mini-boards much simpler. Not every electronic appliance has a useful faceplate or frame around the mini-boards. But sometimes, mostly as a visual cue for functions, a mini-board will have its very own cover.
Plastic sandwich baggies are a great way to keep all the parts together for storage.
Step 3: Complexity and Cautions
Chips, ICs and Black Blobs - Some boards are too complex to re-use easily. If you see an IC chip or a dreaded "black blob" on the board, you may want to scavenge the individual parts, then throw the rest of the board into the recycle bin. The IC chip will perform some obscure function that changes how the input/output operates. It might even turn them into a serial data signal. If the board has 18 buttons but only four wires coming off it, you can be sure something complex and mysterious is going on inside the chip. If you know enough to track down a datasheet for the IC and decode the serial messages then go for it - it's good training. But most new hobbyists will get frustrated. (Simple 7400 series chips can be a good first research project - but only if you know what that means)
Resisters and Capacitors - Other boards will have diodes and capacitors and resisters scattered around the board. These are usually just "conditioning" circuits and are easier to handle than ICs, but you will need to test to see if the input/output voltages work in the voltage range that is safe and meaningful for your microcontroller. If you send 5v into the board, but only get 1.8v back, then your controller might not even recognize the current as a button press. On the other hand, if you send in 5v and get 24v back then you will completely fry your Arduino. So be careful and test the voltages before you hook it up to your controller.
LEDs - When you buy an LED from hobby-stores like Adafruit or Sparkfun, you are told if the LED can be safely used with a 3-5v microcontroller. But some of these scavenged boards have tiny LEDs that only need 1v. If you hit them with 5v they will go up in smoke. The very small indicator LEDs are a good example. You can often switch in a sturdier LED with a little solder work, or add in a resister to bring down the voltage to a safe level. Just be aware that scavenged boards are not as predictable as the pre-made boards designed for hobbyists.
Step 4: Reverse Engineering and Organizing
Hopefully, you are somewhat familiar with basic electronics. While still simple, reverse engineering commercial electronics is actually a bit more complicated than following the step-by-step instructions in hobby kits. You might run into some odd items and techniques not found in kits designed for beginners. At the very least you will need to know how to use a multi-meter to trace down circuits and how to tie your circuits into existing circuits.
A good first step is identifying where the power comes into the board. From there you can begin tracking where the electricity is distributed and how it's controlled. Attach one of your multi-meter's probes to the power line, then pick another line and press buttons until the reading changes. You now know what button controls that line - simple right...
Often the power and all the return lines are grouped into one bundle. Other times the power and returns are in two or more groups. Simple, button-only boards may not have a dedicated ground line - all the grounds will got through the buttons. More complex boards, with potentiometers and encoders especially, will have a dedicated ground wire that always carries current. Some boards even have two power lines on two separate circuits, say a 5v and 12v circuit - all on the same board.
Yes, this can get complicated, but it is a great way to learn real-world electronics and circuit design. If you stick to simple boards at first, then slowly work your way up to more complex boards, you will learn a lot without getting too frustrated.
Where to Hook In for Testing
The connector pins and wiring harness are the best place to test for signals. I like to use jumper-wires to tie into the pins or harness, then use alligator clips with my multi-meter probes. This keeps the connections secure and both hands free for pressing buttons and adjusting the meter.
For buttons and switches, I like to use the "continuity" setting of my meter. When a circuit is closed (current is flowing) the meter beeps to tell me it's getting a signal (current is flowing).
If your meter does not have a continuity setting, then use resistance (ohms) settings. If the circuit is open the meter will read one way ( 0, or 1 or a "-") But if the circuit is closed, the meter will begin to display a number because there will be a small amount of resistance inside the circuit. Be sure to dial in a reasonable scale, usually low, not 100k, in order to get a reliable reading.
For potentiometers (rotary or slide), I also use the resistance setting. For encoders, you can use continuity or resistance to track the wiring scheme.
Colors, Labels and Single-Line versus Branching
Most of these mini-boards will have a single power line going into them. Sometimes the power wires will be a different color, like red or black. But other times all the wires are colored. In that case, try one of the wires/pins on the outside edge of the connection.
Sometimes there will be a printed label next to the pins on the PCB itself. Sometimes, they will even tell you what voltage is expected. The labels might match a number also printed next to the button (pin 1 = button 1), sometimes it might say R1 (right 1) or Vol for volume.
The layout of the PCB and circuit also gives you hints to help find the power line. The power trace often has several tie-in points, one for each component. This can sometimes result in a branching structure for the power trace. By contrast, the return lines from the buttons usually have just one tie-in for each component, and the line goes directly (unbranched) back to the wiring connector. You can often visually track the trace back to the correct pin on the wiring connector without using the multi-meter.
Types of Buttons
There are two styles of buttons you will find in most modern electronics - the traditional "click" style and the new style of gapped-PCB with flexible caps.
You also need to be aware of the different types of clicky buttons. There are numerous variations, but the most common are:
- Normally Open - OFF until pressed
- Normally Closed - ON until pressed
- Momentary - only changes when pressed
- Latching - toggles between ON/OFF, keeps state until next press
The PCB etched button pads with elastomer caps (like Adafruit's Trellis pad) are easy to work with. But make sure you save the squishy pads that come with them. These silicone pads have a conductive surface that bridges the gaps in the PCB and completes the surface. If you can, also save the frame that goes with the buttons - this will help keep the buttons in place during use.
Potentiometers and Encoders
You probably already know how to work with basic potentiometers and encoders. But radios sometimes have more sophisticated versions of these parts.
A basic potentiometer has three leads, but radios can have dual potentiometers (faders) that control power between two sides at the same time. They sometimes have an on/off switch or LED built into the same component. So you might see potentiometers that have six, seven or more leads. But don't worry, it's just multiple copies of familiar components - plus the wiring is well documented on-line so you can figure out these new configurations.
Encoders are similar - the number of pins and the firing-sequence can vary, or they might have on/off switches and LEDs built in. These are all standard configurations however, and are well documented on line.
So it might take you some probing to completely track down all the pins on your pots and encoders. But it's certainly achievable, even for relative noobs.
Add Your Own Labels and Connectors
Whew, if you have successfully traced all the connections on the board - congratulations. Now document and label them before you forget and have to do it all over again.
- Use a permanent marker, paint or nail polish to number or color code the buttons and associated wire (button #1 goes to wire #1, or green button to green wire).
- Tape a diagram to the bottom of the board
- Rearrange the wires so the connections are "in order" and match the buttons
- Make a connector board with label and diagram
- Use Fritzing or circuits.io to create a diagram that you can use to plan your next project
Basically, do anything that helps you remember what wire goes to what component. You might think you'll remember, but six months from now it will all be a mystery again. So a little scribbling now can save you a lot of time later.
Hook-up and Use
Now that you know what the scavenged PCB does and how it works, you can hook it up to your Arduino and use it like any other shield. You can install the board permanently into a project, or you can use it only for prototyping. Or you could split the difference and enclose the mini-board in a case and use it as a standard part of your development process. Salvaging mini-PCB-boards might not be quite as complex as making a shield from scratch, but it's still a rewarding and educational experience.