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Help with optocoupler optocoupler connect to schmitt trigger 74HC14

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Danesh_SA:
Hi everyone,

I design a circuit that read 24V digital input .I want to use schmitt trigger after optocoupler ,since the Schmitt trigger is used to correct optocoupler  output waveform fed into MCU.
I realize that some Industerial boards used RC filter between optocoupler and 74HC14 ,whey it used?
Since i don't see any reliable circuit that interface 74HC14 and an optocoupler  ,I design the circuit below,does it perform right ?

Zero999:
Why not use the H11LXM, which has the Schmitt trigger function built-in?
http://www.farnell.com/datasheets/1874889.pdf?_ga=2.224244446.41188871.1577703349-157924787.1559479246&_gac=1.128826494.1576332778.Cj0KCQiArdLvBRCrARIsAGhB_sxJgbXZ1y39P0BlsCDX6_cxmAxk4dNR_hlDPrTptGxtAqRJ3XH9tWgaAoybEALw_wcB

An RC filter can be used to reject unwanted high frequency signals. It can be as simple as connecting a capacitor across the IR emitter.

Danesh_SA:
Dear zero999,
Actually I use LTV-814S which is an AC optocoupler ,therefore i want to add 74HC14 in opto's output to have staible signal,could you send me schematic of this?
and explain about RC filter and values in this application?
Thank!

StillTrying:
If it just 50Hz-60Hz switching and as long as there's enough current in the opto's LED, those values look about right to me, The opto and the hysteresis would do some filtering. About 2mA should flow in the indicator red LED.

fcb:
Are you trying to detect the presence of 24VAC?  The present design will give you a series of 50/60Hz pulses, which you'll have to interpret correctly further down the chain.

If you want to create a filter then just add a capacitor across the opto-coupler transistor - calculating this is straightforward.

74HC14 datasheet (Nexperia) shows a 'positive going threshold voltage' of 0.7V (minimum, Vcc=2.0V, any temp) and 1.7V at Vcc=4.5V, so lets assume we want to stay below 1.1V (Vcc=3.3V).  Lets also assume that your phototransistor can get down to 0.3V when fully turned on and can discharge the new capacitor in that time.  As 50Hz might be your worst case frequency, we need to hold the capcitor to <1.1V for at least 20milliseconds.

Plug these values into one of the many online calculators: http://mustcalculate.com/electronics/capacitorchargeanddischarge.php?vfrom=0.3&vto=1.1&vs=3.3&r=4700&time=0.02

And you get 13.7uF for your capacitor.  So perhaps start by prototyping with a 22uF or change your 4K7 to higher value.

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