Opto-Isolator with Logic Input (TTL and CMOS) (MEC201E)

This circuit was included in my book Robotics, Mechatronics and Artificial Intelligence (sold out) published in the United States. It shows how to implement a Opto-Isolator with logic input to be used as shield for microcontrollers. Applications include mechatronics or robotics projects with microcontrollers or as a building block for simple projects.

Figure 1 shows how an LED can be used to drive an opto-isolator from TTL or CMOS outputs. The value of resistor R depends on the logic and the power supply voltage as given by Table 1.

The LED and the LDR must be installed inside an opaque enclosure to avoid the influence of ambient light. Remember that the resistance of the LDR falls when the input logic level is high (1).


Figure 1 - Opto-isolator for TTL logic.
Figure 1 - Opto-isolator for TTL logic.






Note: Complete devices using LEDs as emitters, and devices such as Schmitt triggers, SCRs, transistors, Darlington transistors as receivers, and other configurations can be designed.