I'm an electrical engineering students and for a school project I need to understand a circuit schematic. First, I will provide some context. The school project I'm working on is the modification of a sniffer in order to add the decoding of another protocol. The one already implemented is a proprietary serial protocol very much like the Dali protocol. The Dali protocol is used for Lighting. In my case a control device send frames to a security lighting. The sniffer is plugged in between. Frames looks like that:

The Idle state is high and it uses biphased code. First I want to investigate the hardware to see how the first one was implemented and what kind of modification will be needed but I'm pretty overwhelmed by the amount of complexity of the schematic so I will try to understand it one small part at a time. Here is a part of the schematic:

enter image description here

The full schematic is 4 pages long so I don't know if it would not be too much. The circuit was designed by a deutsche engineering team.The High Level voltage is 24 V and the low level is <= 4.5 V .The DALI_line_1 is the input which transmit the frames.DALI_line_OUT_1 is the output that will send the frame to the security lighting. The DALI_RX_1 is connected to the pin of a microcontroller that will read logical level to decode the frames and print it on a terminal.

I'm currently interested in understanding the part commanded by the optocoupler V50. This is I assume the part that produce the logical level. And this is what I'm currently trying to understand. I have basic electronics knowledge so I know what every component does, but I don't understand why they are assembled the way they are.

  • I don't understand what the capacitors C65 and C64 are used for since they will block the current.
  • What is the middle branch with R91 and C64 supposed to do?
  • When the optocoupler is on, will the current flow through the base of the transistor V53 or to the ground through R89? For me the BE junction doesn't have any resistance, so the current will keep going through the base.

I'm pretty lost so any help is welcome. Thanks in advance.

  • 1
    \$\begingroup\$ Please refer to component reference numbers (and not "the capacitor") and, I'd advise that you make those clearer on your schematic. Ditto "the middle branch" and "transistor". Nobody here can read your mind so if you require that I would recommend stack.exchange.psychics. Feel free to laugh. \$\endgroup\$
    – Andy aka
    Commented Sep 12, 2019 at 12:15
  • 1
    \$\begingroup\$ Max known info useful. | Why is the circuit truncated? Is full circuit available?| What do the inputs/outputs do - we could search DALI etc but you should be able to tell us. Links? | Circuit seems strange and undesigned - I MAY be wrong but it looks like something done by someone who didn't really know their stuff - OR by a grand master :-). | Knowing a bit more about DALI I/O line use will help. | Is DALI_RX_I and OUTPUT? | C64 R91 pass higher frequencies better so reduce high freq gain so add low pass filtering of sorts. ... \$\endgroup\$
    – Russell McMahon
    Commented Sep 12, 2019 at 13:05
  • \$\begingroup\$ | C65 suggests that RX has a DC component. | What feeds in from bottom right via V5 etc. \$\endgroup\$
    – Russell McMahon
    Commented Sep 12, 2019 at 13:05
  • \$\begingroup\$ I edited my post to add more informations \$\endgroup\$ Commented Sep 12, 2019 at 14:29

1 Answer 1


C64,65 are probably to slow down the switching of V53, this is a slow protocol and nothing is to be gained by providing lots of high frequency gain. R91/C64 are a classic snubber network that adds loss at high frequency.

I think there is a missing resistor between a supply rail and the gate of V8 as I see no way to pull this up in that schematic.

When the opto is on, current flows in R87 sufficient to develop about 0.7V across the BE junction of V49, this pulls down the gate of V8 so as to limit the current in the opto to approximately .7V/825 ohms = ~1mA as long as the bus is above the 5.1V (V51) + opto coupler LED forward voltage, so maybe 6.5V or so? This assumes my missing resistor really is missing and not just missing something obvious.

There appears also to be some current sensing stuff over to the right that can also shut off the opto under some conditions I cannot be arsed to figure out.

Stupidly complicated for what it does!


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