Isolated pulse detector

[Sylvain Munaut]

Hi,

I have two wire from which can came a pulse. The pulse voltage might be anywhere from 6V DC -> 50V DC (about ...).
I'd like to get from that a nice 3.3V pulse, and be isolated from input.

Wire 1
o
|
---
| |
|R|
|1|
---
|
|-----,
| |
--- |
| | |
|R| |
|2| \_|_
--- ^ \
| / \ Zener
| ---
--- |
\ / |
v |
--- |
| |
| |
|-----'
|
|
o
Wire 2


With the led being a optocoupler input.

Should that work ? Is this common, or a better way ?



Sylvain

 

[Joerg]

Hello Sylvain,

The zener and second resistor is often not necessary. An opto coupler
with a plain diode input characteristic limits the voltage across it by
itself. You might want to include a reverse diode across the opto
coupler input if you suspect that ringing might occur on this line.
That's always a good idea and cheap.

If the coupler has too narrow a current range you may have to replace
the resistor with a current source but that can be easily achieved with
a couple of transistors and resistors.

Regards, Joerg

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