Fluctuate float value in an Unity shader - unity3d

Question

I'm trying to fluctuate between two values inside a shader to achieve a glowing effect.
I need it to be done inside the shader itself and not via C# scripting.
I've tried using the _Time value that Unity gives us for shader animation but it isn't working:
Shader "Test shader" {
Properties {
_ColorTint ("Color", Color) = (1,1,1,1)
_MainTex ("Base (RGB)", 2D) = "white" {}
_GlowColor("Glow Color", Color) = (1,0,0,1)
_GlowPower("Glow Power", Float) = 3.0
_UpDown("Shine Emitter Don't Change", Float) = 0
}
SubShader {
Tags {
"RenderType"="Opaque"
}
CGPROGRAM
#pragma surface surf Lambert
struct Input {
float4 color : Color;
float2 uv_MainTex;
float3 viewDir;
float4 _Time;
};
float4 _ColorTint;
sampler2D _MainTex;
float4 _GlowColor;
float _GlowPower;
float _UpDown;
void surf(Input IN, inout SurfaceOutput o) {
if (_UpDown == 0) {
_GlowPower += _Time.y;
}
if (_UpDown == 1) {
_GlowPower -= _Time.y;
}
if (_GlowPower <= 1) {
_UpDown = 0;
}
if (_GlowPower >= 3) {
_UpDown = 1;
}
IN.color = _ColorTint;
o.Albedo = tex2D(_MainTex, IN.uv_MainTex).rgb * IN.color;
half rim = 1.0 - saturate(dot(normalize(IN.viewDir), o.Normal));
o.Emission = _GlowColor.rgb * pow(rim, _GlowPower);
}
ENDCG
}
FallBack "Diffuse"
}
This makes the glow grow to the infinite.
What am I doing wrong?

Answer 1

Extending my comment slightly:
You can't use _Time.y in this case, as it is the elapsed time since the game started, thus it will increase over time.
You can use _SinTime.y instead, which represents sin(_Time.y). This means that oscillates between the values -1 and 1. You can use this and assign (maybe a scaled version of _SinTime) to your variable: _GlowPower = C * _SinTime.y
More on build-in shader variables: http://docs.unity3d.com/Manual/SL-UnityShaderVariables.html

Answer 2

For doing a pulsing glow... I'd have a script 'outside' the shader and send in a paramater (_GlowPower) calculate in c# script like this....
glowPow = Mathf.Sin(time);
Then you only need to calculate it once. IF you put it in VErtex shader... it does it once per vertex, and if its in surface shader... then once per pixel = performance waste.
you can send variables to your shader like this... (very handy)
material.SetFloat(propertyName, valueToSend);
So you could send, time, strength, glow or whatverer you want.
If you really need to do a glow calculation per vertex or per pixel, then use
_glowPow = sin(_time);
inside the shader.