Charlie

First of all, the form needs to be based on the tblreadings table, i.e.
tblreadings will be the Record Source for the form.  Is this what you've
got?  Ok, which field in tblreadings is this combobox supposed to enter
data into?  whatever this field is, you need your combobox bound to this
field, i.e. the Control Source property of the combobox will be set to
the name of this field.  And then the Row Source property of the
combobox will be set to the name of "a table I have made".  So, that's
the basic concept... hope it helps.  If you need more explicit help,
please post back with more details.