The Raspberry Pi Punnet

 

However, using this involves using some sort of connector. In fact one option is my Buffer Board described here. However, many people might want to access the signals and not want to resort to soldering.


So I have designed this solder-less screw terminal access break out board. This connects to the Pi through a ribbon cable and transfers the signals to labelled screw terminals. There are three versions presented here. One that just transfers the data signals, and two that offer some form of protection for the processor’s GPIO lines.

Screw Connection Breakout Board

At the top left hand corner of the Pi is a 26 pin header labelled P1. It is here you can get access to logic signals that provide inputs and outputs to the computer.

The P1 header is designed to connect to a 26 way two row 1.2xx mm pitch IDC connector. Now IDC stands for Insulation Displacement Cable and is a way of making lots of connections with minimum effort. It works by having ribbon cable, named after the first world war flaying ace Barron von Ribbon Cable, being cut into by a special connector. This is quite a precise connector because it cuts through the cable’s insulation without cutting through the conductor. Hence the name insulation displacement. It practice it is easy enough to construct a cable. Simply slide the connector on the ribbon cable and apply pressure. This can be from a purpose made press or if you are only doing a few you can squeeze it up in a vice. The trick is to get the cable carefully aligned, apply a bit of pressure, check the alignment again and finally tighten the vice. Being careful not to crush the whole assembly.

Basic Breakout

This cable then connects to a 26 way header on the breakout board. The board is built onto strip board and the header has 0.1” spacing. This means you have to cut away a small slice of copper between the rows of holes to prevent each pair of connections shorting out. You can do this by using a very sharp knife, I used a scalpel with replaceable blades. The technique is simple, get a steel rule and score a line along the copper strips. Then move the rule about 1mm and score again. Then you remove the copper in between the two score lines with a scooping motion of the knife. You get a nice clean break. Examine it carefully to make sure there are no whiskers of copper still joining the two sides. Then you can solder the header onto the board and also the screw connectors.


If you are using the basic, no protection circuit then that is about it. All you need to do is to wire each one of the active pins through to a screw terminal connector. You can do this basically in two ways. First of you can arrange the screw connectors in any grouping you like and use flexible wire to connect up each terminal to each pin. Or you can use tinned copper wire to make links. By using horizontal links only and a few extra track cuts on the underside you can make quite a neat job. If you want to take this approach then the layout for top and bottom of the board are like this:-

The added extra you will spot here is an LED and resistor, this is useful to have to give you assurance that the board is connected correctly and is getting power. If you want a larger version of this layout, all the diagrams are downloadable here:- Breakout Diagrams.zip.

If you want to incorporate this sort of protection into the break out board then the neatest way to do it is by using surface mounting components. The inexperienced are often wary of using these but it is not too difficult to cope with. You need a good pair of tweezers and a fine tipped soldering iron. The ides is that the resistors are soldered across a hole in the board where the copper on both sides of the hole has been removed by a scalpel. This is done after each screw connector apart from the two power and the ground pins. So for example look at the G9 input and you will see the resistor as a small box and each side you will se the shading that represents a cut. As I mentioned before larger images of this are available as a download. The zener diodes are soldered between tracks with the side with two connectors on being the anode and the side with a single connector the cathode. The layout also includes two ground link lines. There are where the tinned copper wire is pushed through the same hole to form a link on both sides. You can see from the grey hole on the layout where these are. You can just about get away with 24 gauge wire but 25 to 30 AWG will be a touch easer to use. There is a video at the end of this page showing how this was constructed.

For those of you for whom surface mount still fills you with dread I have done a through hole version. It turns out to be a bit bigger and the links are a bit more complex but it is probably easer to build for a novice.

Back to the Punnet

The snag with just having the raw output available to inexperienced people is that accidents or errors in wiring often prove fatal for the GPIO pins, and while a Raspberry Pi is not very expensive to replace for a few pence you can save that money to spend on other things. This protection circuit does not prevent the GPIO pin being used as an input or an output but it will protect them from a moderate amount of abuse. It uses a zener diode across the input. Basically this will prevent the input voltage from going any higher than 3V3 because at that voltage it

breaks down and starts to conduct thus clamping the maximum voltage on the GPIO pin at 3V3. Also is a negative voltage is applied then it acts like a normal diode and conducts preventing more than -0.7V being applied to the pin. Voltages outside this range will damage the GPIO pins. The 330R resistor will limit the current flow through the zener, dropping the excess voltage between that applied and 3V3 across it. Thus the GPIO is protected when it is used as an input. However, it also has a role in protection when the GPIO pin is used as an output. By limiting the output current to a maximum of 10mA it will stop the GPIO pin from being overloaded. Also if the pin is being used as an input and a switch is connected between the input and ground, there is protection if the pin is accidentally made an output and the switch is grounded. This would normally be a fatal condition but this simple resistor prevents it from happening. This circuit also allows an LED to be connected directly to the board, but always remember to teach people that a resistor is needed in the circuit it just happens to be built in here. There is a down side to protection, there always is, and it is that you can’t get more than 10mA from the GPIO pin and any logic you drive will only be pulled down to zero through 330R. Fortunately most logic families nowadays can cope with this.

The resistors are yellow and are all mounted upright, That is one lead is bent through 180 degrees and the component is fitted radially. The same goes for the zeners marked in red all except one. Where as it does not matter which way round the resistors go it does for the zeners. The cathode should be the top connector, this is normally marked with a band. The long leg of the LED should go to the 5V line.

Then what ever version you use you want to label the connectors, it is best to leave an extra two or three rows of holes in front of the screw connectors to do this. The labels can be self adhesive labels cut to size and written by hand or can be printed from a computer or label printing machine for a neater look. Remember that the top row labels will need to be rotated 180 degrees so it reads correctly.


Finally before using the board it is a good idea to test it to make sure the protection is working. First measure the resistance between each header pin and the corresponding screw output. It should be 330R, a reading of zero or very low indicates the copper has not been cut completely where as a high resistance implies a bad joint or resistor. Then you need to test the zeners, wire up the board to an external 5V power supply through the 5V and ground connectors. The green LED should light. Next remove the 5V line and connect it to each screw input in turn. Then measure the voltage on the corresponding header pin and make sure it is no more than 3V9, any more suggests the zener is not soldered correctly or the ground is not correct. A voltage lower than 1V suggests that the zener has been placed the wrong way round.


If all is good it is time to connect it up to the Pi. The key on the cable should be pointing into the Pi and the cable away from it. I put a dab of white correcting fluid on the connectors to mark the correct way it should fit. Now power up the Pi and the green LED should light. If not check the cable again and the way it is constructed. In the download package there is a file giving the links to some of the components used in this project.


So now you have easy solder less access to the Pi’s GPIO ports along with a modicum of protection, ready for your exploration of the GPIO pins and their possibilities.