Table Update With Dynamic Data

OK, I'm trying to add some audit type information to a set of tables in my database. This involves adding 2 columns and sticking some data into those two columns. The columns themselves are added successfully, but I'm having troubles sticking any data into them - I'm getting a lot of errors. What I want to do is put a single character code in one column, and a datetime in the second. Here's the SQL I'm trying...




Code Snippet
SELECT @SQL = N'UPDATE ' + QUOTENAME(@TableName) + ' ' +
N' SET TRANSACTION_CODE = ''' + @TxCode +
N''', EFFECTIVE_DATE = ''' + @TxDateTime + ''';'
SELECT @Params = N'@TxCode varchar(1), @TxDateTime datetime'
EXEC sp_executesql @SQL, @Params, @TxCode, @TxDateTime





But this errors out with:

Msg 241, Level 16, State 1, Line 64

Conversion failed when converting datetime from character string.

I tried changing the datetime column to varchar, and CONVERT(varchar(30),@TxDateTime). That gets rid of the above error, but replaces it with the following (twice, I might add - one for each column?):

Msg 213, Level 16, State 1, Line 1

Insert Error: Column name or number of supplied values does not match table definition.

Any help is appreciated - let me know if you need more info.