Need a vba to save anytime sheet changes at all

I need a vba for the below sheet to save anytime the sheet is updated or changes in anyway. I thought I had one but its not working

Picking log-Rev6.xlsm

I tried and modified a cell at random. it called the sub but D1 in picking log is blank so it fails how or when is cell d1 fill?

D1 is filled at start of work it is the batch number. need to save as cell D1 anytime sheet is changed.

Try modifying the top part of the code to

Private Sub Worksheet_Change(ByVal Target As Range)

Dim txtLocation As String
Dim txtExtension As String
Dim txtFileName As String
Dim response As Integer
Dim batchnum As String

batchnum = Range("D1").Value
'Set Target = Range("D1")
txtLocation = "D:\picking logs\"         'Change this to the location for the files always with a \ at end
txtExtension = ".xls"      'The extention for the file
txtFileName = txtLocation & "Batch " & batchnum & txtExtension

still not saving as D1 every time batch is updated.