how to control current in stimulating electrode

Thread Starter

surina

Joined Nov 11, 2015
10
Hi, I am going to design some kind of stimulating electrode circuit with some limitation parameters. I am using arduino pro mini as my microcontroller and using potentiometer (MCP4131) to control the voltage. I am having a problem on controlling the current produce since what its safe condition is 2mA. I have a rough ideas on this circuit which I will use op-amp (LM358) and transistor (BD135) connecting to the stimulating electrodes but I dont know if it will produce current as low as 2mA. So, my question are :

1) how can i control the current to be as low as 2mA?
2) do i need to place more resistors on the circuit and how much it is to be supposed to?

Any help would be very appreciated !! Thanks.
 

DickCappels

Joined Aug 21, 2008
6,532
Then what is its purpose?

A lot of tDCS experimenters use the LM334 or similar programmable current sources. Would that be suitable for your application?
 

Thread Starter

surina

Joined Nov 11, 2015
10
The future works for this circuit is to put it on some parts of nose. But in meantime, I want to develop the circuit for stimulating electrode and trying to learn and practice on controlling its current and voltage.
 

MikeML

Joined Oct 2, 2009
5,444
If you limit the electrode current to 2mA, what is the open circuit voltage allowed between the electrodes (i.e. compliance of the current source)?

Doesn't the Arduino need to control the magnitude of the electrode current, i.e. 0 to 2mA?

Isn't a stimulus that has a DC-component a really, really bad idea (Electrolysis of the electrode)?
 

Alec_t

Joined Sep 17, 2013
11,517
Your opamp is configured incorrectly. As shown, its ouput will be permanently low and the BD135 will be off.
I suspect you will need quite a high voltage to get 2mA to pass through nose tissue, if the electrodes simply contact the skin rather than penetrate it.
 

DickCappels

Joined Aug 21, 2008
6,532
To make @Alec_t 's circuit produce a constant 2 milliamps, regardless of the voltage at + supply 2, when the input voltage is 5 volts (plus some tolerance) As long as the input voltage from your digipot doesn't exceed 5 volts, the current from the collector of Q1 will not exceed 2 ma.
 

Alec_t

Joined Sep 17, 2013
11,517
Dick is right. I assumed the digipot maximum output is 5V. R1 and R2 divide this down to 200mV max. If the electrode current tries to rise above 2mA the voltage developed across R2 will try to rise above 200mV and the comparator (LM358) output will switch low to turn off Q1 and so hold the current at 2mA max.
 
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