Design issue Serial Comm. Isolator with Level shifting

[geobabu]

4n35 baud

Hi,
I am trying to implement a isolation in between a serial communication system. One side its have a RS232-TTL(MAX232 circuit) level terminal & other side contains a 3.3V based microcontroller whose power is directly driven from a transformer less capacitor based power supply. I tried a setup a circuit that mentioned in the attached file ,but it doesn't helped helped me. Please help me to rectify the error in this circuit.
Thanks&Regards,
Geo

 

[jdtoronto]

Um, I am not quite sure I understand your issue. Answer a couple of simple questions:

1.. The 3.3V on the right hand side of the schematic is coming from a transformerless power source. (YES/NO)

2.. I (HAVE/HAVE NOT) a 5V DC source to power the stuff on the left hand side of the isolators.

Answer those and then I can give you some choices that WILL work.

John

 

[FvM]

As one issue, R5 must be reduced to e.g. 1k to allow circuit operation.

 

[geobabu]

Hi...Thanks for the replies...

@jdtoronto
Yes, the 3.3V is generating from a transformerless power supply.
The Left hand side VCC is generating from a 5VDC Power supply.

So I think we need to isolate both sides inorder to share the data. I tried with the circuit. But I didnt get any data. The communication is happening in 57.6Kbps baud rate. One thing is , when I placed a CRO(for testing I used an isolated 3.3V supply) prob across the C-E pin of 4N35 in the Rx part (Also in RXD line), I can see some Sq.waves according to the Key press in the other terminal. This made me more confused.

@FvM
I will try with 1K.

Thanks& Regards,
Geo

 

[FvM]

4N35 with 10K load resistor is possibly too slow for 57 kBaud. You may want to check the operation at a lower speed first or carefully adjust the LED source and transistor load resistors.