To allow this code to work, I copy over your data.
% Power source
source_V = 5; % voltage of the power source
source_I_max = 0.04; % max current draw from the power source
% Resistors
resistor_R = [100 220 330 1000 10000 100000 1000000]; % available resistors
% LED
led_V_forward = 1.85; % forward volage of the LED
led_I_max = 0.02; % max current for the LED
I think that you need to be more careful in defining your optimization variable:
resistor_idx = optimvar('resistor_idx',"Type","integer","LowerBound",1,"UpperBound",7);
Also, I don't think that led_I is an optimization variable. So remove the line
% led_I = optimvar('led_I') % Remove this line
You need to create an optimization problem for maximization.
prob = optimproblem("ObjectiveSense","max");
You need to use fcn2optimexpr to create your objective, because you cannot use an optimization variable as an index.
fun = fcn2optimexpr(@(x)(source_V - led_V_forward) /resistor_R(x),resistor_idx);
prob.Objective = fun;
Put the constraints in the problem.
prob.Constraints.led = fun <= source_I_max; % LED should draw current below limitation of the power source
prob.Constraints.led2 = fun <= led_I_max;
Solve the problem like this:
[sol,fval] = solve(prob)
You see that integer-constrained problems are not so simple to set up. But you can do it if you are careful to follow the limitations.
Alan Weiss
MATLAB mathematical toolbox documentation
