Key dropped, put it back together, but do not know where the immobilizer gadget goes

The key works and the car starts, but only when I hold that little tube close to the immobilizer censor by the ignition. When I open up the key, I do not see any obvious place where that must fit.
Is there maybe like a diagram somewhere to be downloaded, please? I cannot afford getting it replaced of fixed by a professional, they are way too expensive!
Will really appreciate some practical but simple advise. Please!