Opto isolator

In the OPTO-Isolator used above, I am not getting flow of ckt.

According to my understanding, in case of PS8821-2, RD ( of MC ) will connect to SCL( of TPS2384 ) and MC_SCL ( of MC ) will connect to SDA_I( of TPS2384 ).

Is it correct? And why we do so?

  • \$\begingroup\$ You have five opto-isolators in the above circuit. They look like they're wired up OK. Which one is not allowing a flow of current? \$\endgroup\$ Nov 22, 2014 at 10:59
  • \$\begingroup\$ I am not getting connection of opto isolator .. how it connects and when...its related to basic working principle of opto-isolator. \$\endgroup\$ Nov 22, 2014 at 11:03
  • \$\begingroup\$ Which of the five optoisolators? Can you explain the problem more clearly, please? \$\endgroup\$ Nov 22, 2014 at 11:05
  • \$\begingroup\$ PS8821-2, for this opto-isolator...i want to know the flow of circuit. ( 1st from top ) \$\endgroup\$ Nov 22, 2014 at 11:12

1 Answer 1


For the upper opto-isolator in that circuit, when MC_SCL is low, the LED will light and thus cause the phototransistor to conduct. The phototransistor will then pull current through R1 and make SCL low. Similarly with MC_SDA, R2 and SDA_I. When the pins are high, the LED will be off and R1 and R2 will act as pull-up resistors, making the outputs high. That'll make an isolated one-way I2C connection. The centre opto-isolator handles the I2C communication from SDA_O back to MC_SDA.

That part of the circuit looks correct to me, provided the resistors have the correct values.


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