tau = 10.0;                      % membrane time constant in ms
dt = 0.05;                       % sampling interval in ms
noisesd = 0.05;                  % measurement noise in mV

time = 0.0:dt:5*tau;             % time vector
voltage = expdecay(time, tau);   % exponential decay
voltage = voltage + noisesd*randn(1, length(voltage));  % plus noise

plot(time, voltage);