Project: EOG – Making PCB Snap Connectors

Now that I have an EOG system working to some extent, it is inconvenient to wire myself up each time I want to take a measurement. Not to mention that the silver chloride electrode pads are single use which is wasteful and expensive (I reuse them multiple times, but the connection and adhesion definitely degrade). On top of this, I don’t like being connected to the system while it is connected to any mains powered device due to risk of an electrical fault being directed straight to my head – I can unplug my laptop, but my oscilloscope isn’t battery powered. So I decided to make an adapter from my signal generator to the snap connectors so I can inject false EOG signals.

IMG_5696

Standard silver chloride electrode

The EOG leads connect to one of these electrodes via a kind of snap connector, like those used on clothes but in a different size, and I have been unable to find them anywhere online. So I needed to find someway to make my own snap connector so I can connect the EOG leads to a PCB.

My first attempt was using my 3D printer to create a something the right size and it turned out it was pretty easy to make a piece of plastic that would connect to the EOG leads in the same way as a the electrode and then screw onto a PCB from behind. But of course the 3D printed objects aren’t conductive. I considered using copper tape to coat the plastic but I figured it would wear very quickly. My solution was to use a piece of wire wrapped around the connector which is folded through a hole and makes contact to the screw used to mount the connector on the PCB. So the electrical connection would go from the PCB -> screw -> wire -> EOG lead.

This went through a number of iterations to optimise the dimensions.

IMG_5606

From left to right shows the process of refinement

IMG_5678

First adapter attempt

I made three of these and assembled it into my first adapter board to adapt 0.1″ headers to snap connectors. The trouble I found was that screwing into the back of the plastic was not strong enough to create enough pressure to make a good electrical contact from the PCB to the screw.

I then realised that the head of an M2 bolt is actually just about the right diameter to connect to the EOG leads. With an extra nut used to distance the head of the screw from the PCB, it made a very good connection made entirely of metal.

IMG_5699

M2 bolts work well as snap connectors

The final revision is quite configurable but by default the negative and ground leads are shorted to the ground output of the signal generator and the positive lead is connected to the signal output of the signal generator via a 10kohm potentiometer to simulate the output impedance of a human.

The bolts I used are digikey part number is H738-ND. Design files are on my GitHub.

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