It is possible to set Vin(off) voltage for optocoupler. There is explained here:

TLP280-4 Optocoupler on 12/24V inputs

And in the appnote for TLP290 opto-coupler that can be downloaded from here:


So, Is it possible to set Vin(on) threshold voltage? I need to implement this:

ON voltage level > 18.0 VAC/VDC
OFF voltage level < 4.0 VAC/VDC

There is no explained in appnote for that opto-coupler. While input signal is 24.0 VAC/VDC. Additional requirement is keeping

Input impedance = 6.8 kOhm

  • \$\begingroup\$ Doing this would require hysteresis, which requires feedback, which a basic optocoupler can't provide. \$\endgroup\$ Jul 20, 2015 at 13:32
  • \$\begingroup\$ I think @user3583807 means with the input between those values the the output is undefined. \$\endgroup\$
    – Jasen
    Jul 20, 2015 at 13:42

2 Answers 2


While you could implement a "clever" passive circuit that did what you want as well as possible, it would be at the mercy of device parameter variations AND would depend on what you define as "off" and "on" at the output. Current transfer ratio varies by a factor of 2:1 for the "rank BLL" part and by 8, 3, 3, 4 for the other variants.

  1. As your Vin on/off levels have an 18/4 = 4.5:1 ratio then with a BLL part you could notionally do what you want passively with BLL part - see at end.

  2. Or (better distinction between two voltages) - place two back to back zener diodes in series with opto-LED. eg 2 x 5V6 zeners in opposed series will not pass useful current until Vin ~ >= 7V or so. So for Vin = 4 VAC I ~+ 0 mA.
    For Vin = 18 VAC I-n = (18 - 7V - 2V_vopto) / R series.

Input resistance can be controlled with series/parallel input resistance to suit.


simulate this circuit – Schematic created using CircuitLab


  1. The above is really misusing an optocoupler for something it is not designed for.

Using a comparator or similar circuit which can be designed to switch ar desired voltages allows far greater control.

Using BLL part with CTR = 200% to 400%.

Define on /off boundary = say 12 mA at CTR = 200 So Ioperate = 6 mA at CTR = 400

Adjust Rin so at 18V I in >= 12 mA so >= Vin x 2/3 At CTR = 400 Voperate >= 9V

So at CTR = 200, 4VAC < 12V operate limit and at CTR = 400, 4 VAC < 6V operate limit.

In practice make voperate slightly under 12V.


Yes, you can do something like put a couple series 6.2V zener diodes and a 6.8K resistor. Also put a 10K resistor across the opto input.


simulate this circuit – Schematic created using CircuitLab

The current will not be very high, so R3 has to be pretty high considering the low CTR of this part. Maybe ~20-30K.

  • 1
    \$\begingroup\$ Why do I bother ? :-). You must have posted that about when I started . \$\endgroup\$
    – Russell McMahon
    Jul 20, 2015 at 14:17
  • \$\begingroup\$ Please continue to bother! \$\endgroup\$ Jul 20, 2015 at 14:20
  • \$\begingroup\$ :-) ........................... \$\endgroup\$
    – Russell McMahon
    Jul 20, 2015 at 14:42

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