diff --git a/scripts/main.jl b/scripts/main.jl
index 8c35b49..dd0a440 100644
--- a/scripts/main.jl
+++ b/scripts/main.jl
@@ -9,42 +9,44 @@ using GLMakie
 using .Constants
 using .Visualization
 
-"""
+model_type = Observable(:gray_scott)  # or :fhn
+
 # GSParams
 N = 128
 dx = 1.0
-Du, Dv = Observable(0.16), Observable(0.08)
-F, k = Observable(0.060), Observable(0.062)
-
-param_observables = (
-    Du=Du,
-    Dv=Dv,
-    F=F,
-    k=k,
+gray_params = (
+    Du = Observable(0.16),
+    Dv = Observable(0.08),
+    F  = Observable(0.060),
+    k  = Observable(0.062)
 )
-"""
 
 # FHNParams
 N = 128
 dx = 1.0
-Du, Dv = Observable(0.016), Observable(0.1)
-ϵ, a, b = Observable(0.1), Observable(0.5), Observable(0.9)
-param_observables = (
-    Du=Du,
-    Dv=Dv,
-    ϵ=ϵ,
-    a=a,
-    b=b
+fhn_params = (
+    Du = Observable(0.16),
+    Dv = Observable(0.08),
+    ϵ  = Observable(0.05),
+    a  = Observable(0.7),
+    b  = Observable(0.8)
 )
 
-#params_obs = Observable(Constants.GSParams(N, dx, Du[], Dv[], F[], k[]))
-params_obs = Observable(Constants.FHNParams(N=N, dx=dx, Du=Du[], Dv=Dv[], ϵ=ϵ[], a=a[], b=b[]))
+params_obs = Observable(Constants.GSParams(N, dx, gray_params.Du[], gray_params.Dv[], gray_params.F[], gray_params.k[]))
+#params_obs = Observable(Constants.FHNParams(N=N, dx=dx, Du=Du[], Dv=Dv[], ϵ=ϵ[], a=a[], b=b[]))
 
-"""
-lift(Du, Dv, F, k) do u, v, f, ki
-   params_obs[] = Constants.GSParams(N, dx, u, v, f, ki)
+# Rebuild when model or sliders change
+function setup_param_binding!(model_type, gray_params, fhn_params, params_obs)
+    if model_type[] == :gray_scott
+        lift(gray_params.Du, gray_params.Dv, gray_params.F, gray_params.k) do Du, Dv, F, k
+            params_obs[] = GSParams(N, dx, Du, Dv, F, k)
+        end
+    else
+        lift(fhn_params.Du, fhn_params.Dv, fhn_params.ϵ, fhn_params.a, fhn_params.b) do Du, Dv, ϵ, a, b
+            params_obs[] = FHNParams(N, dx, Du, Dv, ϵ, a, b)
+        end
+    end
 end
-"""
 
 lift(Du, Dv, ϵ, a, b) do d_u, d_v, eps, aa, bb
     params_obs[] = Constants.FHNParams(N=N, dx=dx, Du=d_u, Dv=d_v, ϵ=eps, a=aa, b=bb)
diff --git a/src/fhn_solver.jl b/src/fhn_solver.jl
index 8afee64..2dbc3cd 100644
--- a/src/fhn_solver.jl
+++ b/src/fhn_solver.jl
@@ -35,7 +35,7 @@ function fhn!(du, u, p::FHNParams, t)
     du .= vcat(vec(fu), vec(fv))
 end
 
-function step!(U, V, params_obs::Observable; dx=1)
+function step_fhn!(U, V, params_obs::Observable; dx=1)
     """
     p = params_obs[]
 
@@ -75,6 +75,6 @@ end
 
 function multi_step!(state, n_steps, heat_obs::Observable, params_obs::Observable; dx=1)
     for _ in 1:n_steps
-        heat_obs[] = step!(state[1], state[2], params_obs; dx=1)
+        heat_obs[] = step_fhn!(state[1], state[2], params_obs; dx=1)
     end
 end
diff --git a/src/gray_scott_solver.jl b/src/gray_scott_solver.jl
index eeb0ed8..8fcb29d 100644
--- a/src/gray_scott_solver.jl
+++ b/src/gray_scott_solver.jl
@@ -6,7 +6,7 @@ using Observables
 using .Constants
 using .Laplacian
 
-function step!(U, V, params_obs::Observable; dx=1)
+function step_gray_scott!(U, V, params_obs::Observable; dx=1)
     lap_u = laplacian5(U, dx)
     lap_v = laplacian5(V, dx)
     diff_u = params_obs[].Du
@@ -40,6 +40,6 @@ function step!(U, V, params_obs::Observable; dx=1)
 end
 function multi_step!(state, n_steps, heat_obs::Observable, params_obs::Observable; dx=1)
     for _ in 1:n_steps
-        heat_obs[] = step!(state[1], state[2], params_obs; dx=1)
+        heat_obs[] = step_gray_scott!(state[1], state[2], params_obs; dx=1)
     end
 end
diff --git a/src/visualization.jl b/src/visualization.jl
index 792a87f..930f288 100644
--- a/src/visualization.jl
+++ b/src/visualization.jl
@@ -47,7 +47,33 @@ function param_box!(grid, row, labeltxt, observable::Observable)
     end
 end
 
+function rebuild_param_boxes!(grid, model_type, gray_params, fhn_params)
+    empty!(grid)
+
+    if model_type[] == :gray_scott
+        param_box!(grid, 1, "Du", gray_params.Du)
+        param_box!(grid, 2, "Dv", gray_params.Dv)
+        param_box!(grid, 3, "Feed", gray_params.F)
+        param_box!(grid, 4, "Kill", gray_params.k)
+    else
+        param_box!(grid, 1, "Du", fhn_params.Du)
+        param_box!(grid, 2, "Dv", fhn_params.Dv)
+        param_box!(grid, 3, "ϵ", fhn_params.ϵ)
+        param_box!(grid, 4, "a", fhn_params.a)
+        param_box!(grid, 5, "b", fhn_params.b)
+    end
+end
+
+function step_model!(U, V, model_type, params_obs, heat_obs)
+    if model_type[] == :gray_scott
+        heat_obs[] = step_gray_scott!(U, V, params_obs)
+    else
+        heat_obs[] = step_fhn!(U, V, params_obs)
+    end
+end
+
 function build_ui(U, V, param_obs_map::NamedTuple, params_obs, heat_obs)
+
     reset!(U, V, heat_obs)
     fig = Figure(size=(800, 800))
     gh = GridLayout(fig[1, 1])
@@ -61,6 +87,7 @@ function build_ui(U, V, param_obs_map::NamedTuple, params_obs, heat_obs)
     spoint = select_point(ax.scene)
 
     # Controls
+    dropdown = Dropdown(fig[0, 1], options=["Gray-Scott" => :gray_scott, "FHN" => :fhn])
     fig[2, 1] = buttongrid = GridLayout(ax.scene, tellwidth=false)
     btn_step = Button(buttongrid[1, 1], width=50, label="Step")
     btn_start = Button(buttongrid[1, 2], width=50, label=run_label)
@@ -142,6 +169,12 @@ function build_ui(U, V, param_obs_map::NamedTuple, params_obs, heat_obs)
 
         heat_obs[] = copy(U)
     end
+
+    on(dropdown.selection) do sel
+        model_type[] = sel
+        setup_param_binding!(model_type, gray_params, fhn_params, params_obs)
+        rebuild_param_boxes!(param_grid, model_type, gray_params, fhn_params)
+    end
     return fig
 end