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What I'm trying to achieve is taking a 24 to 30 VAC signal from an HVAC thermostat and converting it to possibly a 5 VDC or 12 VDC Trigger Input to a what I believe is a optocoupler/isolator type relay. The relay then closes the COM and NO contacts turning on a 12 VDC LED (Labeled either FAN, Heat, and/or Cool). I'll post a photo of what I'm working with.

diagram

optocoupler diagram

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  • \$\begingroup\$ To clarify, you want to trigger (energize) an isolated relay(s) when there is 24Vac to 30Vac voltage across the secondary windings of a transformer, and when there is close to 0Vac the relay should be de-energized (in it's default state)? Is the goal to make a PCB, Stripboard assembly, or use off-the-shelf electronics? \$\endgroup\$
    – Ernesto
    Mar 22, 2021 at 2:47
  • \$\begingroup\$ (Off the shelf electronics)Sorry so I have a mains 120VAC 40VA to 24VAC transformer which powers my thermostat, then I want the tstat to send the 24VAC signal to the ac to dc device that can be accepted with the right voltage so it will trigger the relay (without causing 60hz humming) then turning on a 12vdc led that's shared from the 12v source to power the off the shelf optocoupler \$\endgroup\$
    – Nic Savage
    Mar 22, 2021 at 3:14
  • \$\begingroup\$ This is what I'm using It requires 5vdc to 12vdc to trigger the octocoupler relay. amazon.com/dp/B00LW2GA5Y/… \$\endgroup\$
    – Nic Savage
    Mar 22, 2021 at 3:22
  • \$\begingroup\$ Would suggest the optocoupler solution with bridge rectifier since some have very wide tolerable input from trigger to max current which allows a wide detection range with a correctly sized serial resistance, and provides isolation. \$\endgroup\$
    – Damien
    Mar 22, 2021 at 6:01
  • \$\begingroup\$ @NicSavage Please see community guidelines where it states this forum is not about asking for buying recommendations link With that said, you could use off-the-shelf SSR part with socket ED06E5 Crydom with socket DRSED Crydom. I'd recommend spending the time to learn and follow answer given below \$\endgroup\$
    – Ernesto
    Mar 22, 2021 at 17:42

2 Answers 2

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Relays may jitter with a half-wave triggered coil current so a full-wave low current diode bridge gives 120 Hz pulses which need a filter cap.

However, I assume 24Vac is intended to activate the Opto input which is inverted active low so an NPN transistor is needed.

schematic

simulate this circuit – Schematic created using CircuitLab

Variations of R2C choices but this is easy to drive and pulls up R prevents noise false triggers.

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  • \$\begingroup\$ Any known products that can do this pre built or can you post what materials you would use, bridge rectifier or reccomended diodes? And sorry I'm still learning the lingo R2C? Thank you for such fast response time and amazing support! \$\endgroup\$
    – Nic Savage
    Mar 22, 2021 at 6:49
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I also thought TonyStewart's rectification methord. You can use 1N4002 diode which can hold 70 V rms, yours 40. Capacitor need over peak voltage. Use 100 V one(yours 60)

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