Digital Input causes RAIL on all cyton channels
Hi All,
I am attempting to set up a two-button Digital Input on my Cyton. The schematic for the simple circuit I built can be seen here (replace Arduino with Cyton) 
Here is also a corresponding Playground although this link will expire in 12 days or so
My issue is that when I plug it in to the Cyton (VDD to +, Ground to GND, Pins in 13 and 18 ), and start to Stream Data in the GUI, I get all >98% RAILED readings on the channels (I've been using only 8 channels so far). I created a video describing the problem with more details, but I'm happy to provide more info.
My biggest concern is that my circuit is set up in such a way that's not good for the Cyton. Definitely don't want to fry it! lol. I noticed that after removing my breadboard and restarting streaming, all the channels were RAILED. I had to do Impedance checks on each channel (I'm not really sure what it does tbh, I just clicked around until they no longer showed Railed). I would greatly appreciate any guidance on how to
1. Set up my dual button circuit so that it works cleanly every time
2. Generally ensure my channels don't rail while I collect data (My experiment will involve collecting continuous data for over an hour every day so a stable signal will be very helpful)
My secondary question is: Why are some pins showing as 1 when there is no input to them. In the video channel 12 is 1 for no reason. And when I turn on Digital read without any input I get 2 or 3 channels that are 1 as well. I guess it's not a huge problem but curious nontheless :-)
Thanks!
Comments
Hi, Omar,
It' unclear to me how you have wired your buttons, but your mistake is likely there. Be sure to use the correct Cyton pin names (Dxx), see final link below.
See this related post, for a better schematic.
https://irenevigueguix.wordpress.com/2016/05/03/two-external-buttons-on-openbci-32bit-board/
Note, that it is no longer necessary to change the Cyton firmware. Just switch the Cyton into 'Digital Read' mode, and use appropriate pins. Digital Read replaces the Accel 3 axes Aux values with the logic levels of those pins.
https://docs.openbci.com/docs/02Cyton/CytonExternal#external-triggering-the-easy-way-firmware-3xx
Regards, William
Nearly a year later,
, but I did want to thank you William for your help. I have two follow-up questions:
1. My external button setup utilizes two pins (say D11 and D12) and it seems to be working correctly in the Digital Read Widget. However I noticed that some of the other pins are noisy (for example D17 may be lighting up sporadically)
2. I have seen instances where I press my external button, and the 8 channel timeseries data will jump or shift for a second. Could it be possible that the digital input is causing an artifact in the electrode reading?
I can try to provide a CSV recording if that would be helpful.
Best, Omar
Hi Omar,
Digital logic inputs MUST be connected to a source of 0V or 3.3V, they cannot have floating, unconnected inputs, otherwise values can be random.
Thanks...
If the input is floating, then that will happen.
Because the voltage in the external 'buttons' wires is 3.3V, that is millions of times stronger than the microvolts seen in the EEG. If your EEG electrodes are anywhere near the button logic, it could 'couple' into the electrode wires via inductive or capacitive means.
William