DeBry Ersa External Relative Strength
Here is a expert signal for those members who have the SPYGLASS add-on from Jon DeBry 

Long 

A:=ExtFml( "ERSA.ExternalRelativeStrength")>50; 
B:=ExtFml( "ERSA.ExternalRelativeStrength")<50; 
i:=Cum(A>-1 AND B>-1)=1; 
x:=BarsSince(i OR A)<=BarsSince(i OR B)=0; 
X=0 AND Ref(X,-1)>0 

Short 

A:=ExtFml( "ERSA.ExternalRelativeStrength")>50; 
B:=ExtFml( "ERSA.ExternalRelativeStrength")<50; 
i:=Cum(A>-1 AND B>-1)=1; 
x:=BarsSince(i OR A)>=BarsSince(i OR B)=0; 
X=0 AND Ref(X,-1)>0 

My ERSA settings were 40,30,20,10 

This is a simple crossing above and below the 50 line for signals, 
find other ways to exit your positions once you are in the trade