A Simple Virtual Voltmeter Using GDI+ and the GP-3 Board

private void DrawNeedle(Graphics g)
{
// determine hinge point of the needle
Point centerPoint = new Point( (ClientRectangle.Width)/2, ClientRectangle.Height - 20);
// determine the topPoint of the needle
Point topPoint = new Point( (ClientRectangle.Width)/2, 60);
// Create a new graphics path
GraphicsPath gp = new GraphicsPath();
// draw the primitives inside the graphics path
//draw the left side of the head of the needle
gp.AddCurve(new Point[]{topPoint, new Point(topPoint.X-3, topPoint.Y + 5), new Point(topPoint.X - 5, topPoint.Y + 5)}, 0.3f);
gp.AddCurve(new Point[]{new Point(topPoint.X-5, topPoint.Y + 5), new Point(topPoint.X - 3 , topPoint.Y + 5), new Point(topPoint.X, topPoint.Y+10)}, 0.3f);
Point sidePoint1 = topPoint;
Point sidePoint2 = centerPoint;
Point sidePoint4 = topPoint;
sidePoint1.Offset(-1, 10);
sidePoint2.Offset(-1, 0);
Point sidePoint3 = sidePoint2;
sidePoint3.Offset(2, 0);
sidePoint4.Offset(1, 10);
// draw the stem of the needle
gp.AddLine(sidePoint1, sidePoint2);
gp.AddLine(sidePoint2, sidePoint3);
gp.AddLine(sidePoint3, sidePoint4);
// draw the right side of the head of the needle
gp.AddCurve(new Point[]{new Point(topPoint.X, topPoint.Y + 10), new Point(topPoint.X + 3 , topPoint.Y + 5), new Point(topPoint.X+5, topPoint.Y+5)}, 0.3f);
gp.AddCurve(new Point[]{new Point(topPoint.X + 5, topPoint.Y + 5), new Point(topPoint.X + 3, topPoint.Y + 5), topPoint}, 0.3f);
// close the path
gp.CloseFigure();
// initially create the matrix as an identity matrix
Matrix m = new Matrix();
// make sure the value does not exceed the bounds of the meter
// (in physical terms, if the value exceeds the bounds of the meter,
// then pin the needle.)
float calcValue = m_Value;
if (calcValue > m_MaxValue) // pin to maximum value
{
calcValue = m_MaxValue;
}
if (calcValue < m_MinValue) // pin to minimum value
{
calcValue = m_MinValue;
}
// convert value to an angle spanning meter
float angle = (calcValue/(m_MaxValue - m_MinValue)) * MaxAngle - (MaxAngle/2);
// rotate the identity matrix around the hinge point of the meter according
// to the angle-value
m.RotateAt(angle, centerPoint);
// rotate the graphics path using the rotated identity matrix
gp.Transform(m);
// make the needle a solid red color
g.FillPath(Brushes.Red, gp);
}

Listing 3 - Drawing the Needle to point to the Value

Simulating the Meter

The meter uses the built in user control timer to simulate the movement of the needle from the old value to the new value. This way, the meter really looks like it has a voltage dragging the needle with a magnetic field. (We also use double-buffering in our constructor, to make the motion of the needle appear smoother.) Listing 4 is the timer event handler that moves the needle a small step each time a tick event is received by the timer (every 150 milliseconds). The needle moves an incremental step in the direction of the new value and stops when it reaches the value set by the user. The event handler uses the mathematical sign function to determine the direction of the needle and multiplies the result of the sign function by the step used to increment the needles motion.

private void timer1_Tick(object sender, System.EventArgs e)
{
// determine the step interval for each tick of the needle
float step = (this.m_MaxValue - this.m_MinValue)/200.0f;
// determine direction of step
step = step * Math.Sign(m_newValue - m_Value);
// step through and simulate needle movement
// first increase the value of the needle towards the new value
m_Value = m_Value + step;
Invalidate();
// if the needle is headed up the scale,
// check to see if we exceeded the new value
if ((step > 0) && (m_Value >= m_newValue))
{
// needle reached its destination
// stop the timer, and set the final value
timer1.Stop();
m_Value = m_newValue;
return;
}
// if the needle is headed down the scale,
// check to see if we dipped below the new value
if ((step < 0) && (m_Value <= m_newValue))
{
// needle reached its destination
// stop the timer, and set the final value
timer1.Stop();
m_Value = m_newValue;
return;
}
}

Listing 4 - The Timer Event handler simulating the motion of the needle

Improvements

In order to make a decent voltmeter, you need to be able to scale to several voltages. One possibility is to feed the voltage being measured into an amplifier that either amplifies or divides the voltage coming into the A/D converter. Figure 3 shows a possible divider circuit I've scratched out (but haven't tested) that could help measure voltages in ranges from (0 - 500 mv), (0 - 5V), and (0-50V) respectively. Vin would be connected to the measured voltage point and Vout would be connected to the A/D converter channel on the GP-3(along with some protection circuitry). By using a multiplexer circuit (an electronic selection switch) in combination with the GP-3 logic output to select the Mux(multiplexer), and a virtual selection knob on the Windows Form, you could add range selection to the virtual voltmeter in Figure 1. Each click of the virtual selection knob would redraw the meter to reflect the new range and electronically select a new voltage divider resistor in the circuit in figure 3.

Figure 3 - Range selection for the voltage being measured

Conclusion

Once more we explored the power of going beyond our computer screens and sensing the outside world with the help of .NET. In our next related project, I'm hoping to demonstrate a more practical example of using .NET and the GP-3 together. In the meantime, if you want to experiment with the GP-3, you can download the GP-3 kit from AWC. Have fun controlling hardware and getting charged with .NET!