how to control current in stimulating electrode

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
10,067
Is this for tDCS?

surina

Joined Nov 11, 2015
10
Nope. The stimulating electrode will not be apply for neuro stimulation.

DickCappels

Joined Aug 21, 2008
10,067
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?

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.

Dodgydave

Joined Jun 22, 2012
10,945
Post your circuit, then we can assist you more,.

Alec_t

Joined Sep 17, 2013
13,776
A BD135 is overkill if you want to draw only 2mA. The opamp alone could provide that.

surina

Joined Nov 11, 2015
10
here is my circuit and it is just a rough idea. but it seems like i have to get rid of BD135?

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
13,776
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
10,067
@Alec_t what do you think the best arrangement for the opamp would be for surina's purpose?

Alec_t

Joined Sep 17, 2013
13,776
If it were me I'd probably try something like this ...

I don't say it's the best way, since I have doubts on the efficacy of a DC stimulus .

surina

Joined Nov 11, 2015
10
If it were me I'd probably try something like this ...
View attachment 95445
I don't say it's the best way, since I have doubts on the efficacy of a DC stimulus .
Oh great, thanks. but the circuit still can draw high current if the voltage is increase. I just want to make the circuit can have a maximum current as high as 2mA.

DickCappels

Joined Aug 21, 2008
10,067
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
13,776
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.