TLP18x Photocoupler GaAs Ired & Photo Transistors Toshiba TLP18x Photocoupler GaAs Ired and Photo Transistors are small outline couplers that are suitable for surface-mount assembly. They each consist of a photo transistor, optically coupled to a gallium arsenide infrared emitting diode. The TLP184 is also connected inverse parallel and can operate directly by AC input current. Toshiba TLP18x Photocoupler GaAs Ired and Photo Transistors are smaller than a DIP package, making them suitable for high-density, surface-mounting applications such as programmable controllers.