Two terminal light barrier system

A radiant energy barrier system, particularly for automatic doors, having radiant energy emitter means as well as radiant energy receiver means that interacts with said radiant energy emitter means and generates an electric signal when an energy path between said emitter means and said receiver means is interrupted or cleared, comprising:

• a power source for providing power to said barrier system;
• a two-terminal network in which said emitter means and said receiver means are interconnected, said netework having two terminals;
• an output circuit means in said two-terminal network having an input connected to said receiver means and an output connected by a series resistor to one of said two terminals, said output circuit means producing an output at different voltage levels in response to said energy path being interrupted or cleared to changing the potential difference between said two terminals;
• a first and second power supply line connecting said power source to said two terminals of said two-terminal network;
• a precision resistor connected in series with said first power supply line remote from said two-terminal network;
• and
• monitoring means for sensing a voltage drop across said precision resistor as an indication of the potential difference between said two terminals.