In the tech tree, Circuit Networks is a prerequisite for Turrets.

However, if you look in the code this is is not mentioned.
type = "technology",
name = "turrets",
icon = "__base__/graphics/technology/turrets.png",
effects =
{
{
type = "unlock-recipe",
recipe = "gun-turret"
}
},
unit ...

Ah, thanks! I think I tried something like that, but must have made a small typo or something, and went in circles after that trying to make it work.

This is a basic programming question, if i look for example at the game's files:

type = "technology",
name = "bullet-damage-1",
icon = "__base__/graphics/technology/bullet-damage.png",
effects =
{
{
type = "ammo-damage",
ammo_category = "bullet",
modifier = "0.1"
}
},
prerequisites ...