In SX Microcontrollers, SX/B Compiler and SX-Key Tool, g_daubach wrote:

Chris,

usually, I'm using CNY17-4 optos here, which are available in various packages. In my designs, I normally use 3.3kOhm input resistors for 24V input signals. To my experience, a CNY17-4 with 3.3 kOhm even switches when the input voltage is as low as 5V. For an 11-15 Volts system, you might consider using an input resistor of 2.2 or 1.5 kOhm with the output circuit as shown in Jon's example.

Right now, I'm working on a motor controller for a concrete saw. The controller is connected to two PWM-controlled motors (each of them "eating" up to 15A @ 24V) via a cable that also contains leads for end-sensor switches and motor tacho pulses with all these signals switching between 0 and 24 Volts fed into an SX via opto isolators. You can imagine that due to the high pulsed motor currents, a lot of noise is induced into the sensor lines. Without capacitors in parallel to the opto's LEDs, I got false end-sensor readings and "pseudo" tacho pulses frequently. Since the caps were installed, I never again had a problem like this.

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