# Sign issue deriving SHM of Electric dipole in external uniform electric field

If we have an electric dipole as shown:

Net torque on system = $$Fdsin\theta$$= Rate of change of angular momentum = $$I \ddot \theta$$

For small displacements along line of E field, $$sin\theta \approx \theta$$

So $$Fd\theta=I\ddot \theta$$.

I want to show this is SHM, where have I dropped my negative sign?

As drawn you have taken $$\theta$$ to be positive in the anticlockwise direction?
The torque is in the clockwise (negative) direction so should be written as $$-Fd \,\theta$$.