'External' Triggers for Ganglion
I need precise time triggers embedded in the EEG data to mark the onset of stimuli. It would be nice to send the trigger directly from the same Arduino which I am using to generate the stimuli.
From my searches I've found this guide: https://docs.openbci.com/Cyton/CytonExternal/
However, this is for Cyton. Should I use this guide anyway or should I follow a different procedure for Ganglion?
Comments
Matt, hi.
Cyton has three additional 'Aux' (low-resolution, 0V to 3.3V, 10 bit ADC) channels that are read from ADC pins on the PIC32 processor. Ganglion does not have additional channels.
You can use one of your 4 normal channels to receive the incoming external trigger information. But I would suggest using a voltage divider network to get the trigger into say the zero to 50 microvolt range. That way it will look more like a normal biosignal instead of a huge 3.3V or 5V logic level. The Ganglion has extensive compression used in the radio stream; so keeping the trigger signal in the biosignal range helps that. There are numerous free calculator pages online.
https://www.google.com/search?q=voltage+divider+calculator
Additionally it's a good idea to use the opto isolator approach shown on the Cyton page. You can wire both the isolator and the voltage divider on the same small breadboard.
https://docs.openbci.com/Cyton/CytonExternal/#optoisolation
Regards, William
Hi William,
Could you explain briefly some of the reasons for your suggestions?
Thanks,
Matt
Would this work as an opto isolator?
https://www.amazon.com/Gikfun-OPTOCOUPLER-PC817C-Arduino-AE1143/dp/B01JG8EJVW/ref=sr_1_7?crid=3NTFRVVIH30VV&keywords=opto+isolator+arduino&qid=1670701703&sprefix=opto+isolator+arduino,aps,84&sr=8-7
If not, is there another you could suggest?
You can use another optoisolator. Be sure the specifications match the one used in the External Trigger link. Also possible that the pin numbers differ.
The voltage divider is only two resistors. So minuscule cost. If you check the compression link I gave, you can see that large voltage swings, such as 0V to 3.3V, are not handled well by the compression algorithm.
William
You should not connect Cyton pins to any external voltage source. Which the optoisolator solves.