Private Const WM_SYSCOMMAND   As Long = &H112
Private Const SC_MONITORPOWER As Long = &HF170&
Private Const HWND_BROADCAST  As Long = &HFFFF
Private Declare Function SendMessage Lib "user32" Alias "SendMessageA" (ByVal hwnd As Long, ByVal wMsg As Long, ByVal wParam As Long, ByVal lParam As Long) As  Long
'
Public Sub SetScreenPowerOff()
    Call SendMessage(HWND_BROADCAST, WM_SYSCOMMAND, SC_MONITORPOWER,  1&)
End Sub
Public Sub SetScreenPowerOn()
    Call SendMessage(HWND_BROADCAST, WM_SYSCOMMAND, SC_MONITORPOWER,  0&)
End Sub

Private Const WM_SYSCOMMAND As Integer = &H112UI
Private Const SC_MONITORPOWER As Integer = &HF170I
Private ReadOnly HWND_BROADCAST As IntPtr = New IntPtr(&HFFFFI)

<DllImport("user32")> _
Private Function SendMessage(ByVal hwnd As IntPtr, ByVal wMsg As UInteger, ByVal wParam As Integer, ByVal lParam As Integer) As Integer
End Function

Public Sub SetScreenPowerOff()
  Call SendMessage(HWND_BROADCAST, WM_SYSCOMMAND, SC_MONITORPOWER, 1)
End Sub
Public Sub SetScreenPowerOn()
  Call SendMessage(HWND_BROADCAST, WM_SYSCOMMAND, SC_MONITORPOWER, 0)
End Sub

private const uint WM_SYSCOMMAND = 0x112U;
private const int SC_MONITORPOWER = 0xF170;
private static readonly IntPtr HWND_BROADCAST = new IntPtr(0xFFFF);

[DllImport("user32")]
private static extern int SendMessage(IntPtr Handle, uint wMsg, int wParam, int lParam ) ;

public static void SetScreenPowerOff() {
  SendMessage(HWND_BROADCAST, WM_SYSCOMMAND, SC_MONITORPOWER, 1);
}
public static void SetScreenPowerOn() {
  SendMessage(HWND_BROADCAST, WM_SYSCOMMAND, SC_MONITORPOWER, 0);
}

procedure Syst_MonitorSleep(Valeur: boolean);
var
hWnd: THandle;
begin
//handle de broadcast, la procedure fonctionne aussi avec application.handle
hWnd := 65535; 
if Valeur then //si Valeur est a TRUE on met le moniteur en veille
SendMessage(hWnd, WM_SYSCOMMAND, SC_MONITORPOWER, 2)
else //sinon on le force à sortir de la veille
SendMessage(hWnd, WM_SYSCOMMAND, SC_MONITORPOWER, -1);
end;