Display old and new values for a cell - vba

Question

I have a worksheet that logs changes that uses have made to cells. It goes as follows
Public OldVal As String
Public NewVal As String
Private Sub Worksheet_SelectionChange(ByVal Target As Range)
If Target.Cells.Count > 1 Or IsEmpty(Target) Then Exit Sub
OldVal = Target.Value
End Sub
Private Sub Worksheet_Change(ByVal Target As Range)
Dim LDate As String
If Target.Cells.Count > 1 Then Exit Sub
NewVal = Target.Value
Sheets("corrections").Cells(Rows.Count, "A").End(xlUp)(2).Value = Now & "_Sheet " & ActiveSheet.Name & " Cell " & Target.Address(0, 0) & " was changed from '" & OldVal & "' to '" & NewVal & "'"
OldVal = ""
NewVal = ""
End Sub
The problem im having is that for some reason it will never display the previous value. it will output it only as Sheet FA Cell B5 was changed from '' to '12' even if say for example 10 was in the cell prviously.
I also was curious to know is there a way that you can have it so that this code is not running at all times. Id prefer to have a button you click and at that point it will initiate and start logging changes.
Thanks

Answer 1

I got your posted code to work with a very small change:
Public OldVal As String
Public NewVal As String
Private Sub Worksheet_SelectionChange(ByVal Target As Range)
If Target.Cells.Count > 1 Or IsEmpty(Target) Then Exit Sub
OldVal = Target.Value
End Sub
Private Sub Worksheet_Change(ByVal Target As Range)
Dim LDate As String
If Target.Cells.Count > 1 Then Exit Sub
NewVal = Target.Value
Application.EnableEvents = False
Sheets("corrections").Cells(Rows.Count, "A").End(xlUp)(2).Value = Now & "_Sheet " & ActiveSheet.Name & " Cell " & Target.Address(0, 0) & " was changed from '" & OldVal & "' to '" & NewVal & "'"
Application.EnableEvents = True
OldVal = ""
NewVal = ""
End Sub
For your second question, start with:
Application.EnableEvents = False
Hook your button onto a macro like this:
Sub StartLogging()
Application.EnableEvents = True
End Sub

Answer 2

Your code is working fine for me. As for the enable/disable macro, you just need to insert this line before (/above) each IF (in both of your macros). Optional check for a more appropriate cell to store the Yes/No option (rather than "X1"):
If Sheets("corrections").Range("X1") <> "Yes" Then Exit Sub
' where you can change X1 for a more appropriate cell
To create the buttons just add the shapes/objects and assign the macros below:
Sub Enable_Logs()
Sheets("corrections").Range("X1").Value = "Yes"
End Sub
Sub Disable_Logs()
Sheets("corrections").Range("X1").Value = "No"
End Sub
Note! To add buttons with macro assigned to them: press Alt + N, +SH and select a shape. Then, right click on the shape > Assign Macro (and select the corresponding macros). Note! for the 1st time you run the macro, you should manually set the "Yes" value in cell X1.

Answer 3

Thr problem why my OldVal was not showing up was that it was being held in array. So when I told it to look at OldVal(1, 1) it works just as it should. Thanks for the help. The final working code is:
Public OldVal As String
Public NewVal As String
Private Sub Worksheet_SelectionChange(ByVal Target As Range)
If Sheets("corrections").Range("G1") <> "Yes" Then Exit Sub
OldVal = Target.Value2
End Sub
Private Sub Worksheet_Change(ByVal Target As Range)
If Sheets("corrections").Range("G1") <> "Yes" Then Exit Sub
If Target.Cells.CountLarge > 1 Then Exit Sub
NewVal = Target.Value
Sheets("corrections").Cells(Rows.Count, "A").End(xlUp)(2).Value = Now & "_Sheet " & ActiveSheet.Name & " Cell " & Target.Address(0, 0) & " was changed from '" & OldVal(1, 1) & "' to '" & NewVal & "'"
OldVal = ""
NewVal = ""
End Sub