Learning to Use an Optocoupler or Optoisolator

An optocouplter or optoisolator is a cool little device that allows you to completely separate sections of an electrical circuit. From what I understand, the MIDI protocol requires the use of optocouplers in all devices. I want to use an optocoupler for separating a circuit powered by USB (5V) from one powered by a 7.2V RC Car battery. The idea behind this is that I want to protect the USB based circuit(and my computer) from the large amperages, inductance, higher voltages and other things going on in the RC Car side of the circuit, but still want to be able to control it using a USB device. I am writing this after successfully using a Fairchild 4N25 Optocoupler to deliver pulse width modulation from a USB based circuit to the 7.2V circuit in order to control a servo.

The following are a series of descriptions of ideas that I have learned while working with an optocoupler over the last month.

1. Dont waste your time making or purchasing a breakout board for a package size that is large enough to directly solder wires to.

The leads are far enough apart. This didnt need a breakout board made for it.

This is the way I should have done it in the first place

The Fairchild 4N25SM has large enough pin spacing despite being surface mount, that you can easily solder wires directly to the pins for experimentation. I made a breakout board for this part and after a week found out the that optocoupler was not preforming as it should have. I think I got the pins on the optocoupler too hot while soldering them to the breakout board.

2. I'm not sure about other optocouplers, but the 4N25 works on the major principal of sinking current.

Before actually working with the part, I did not understand that you have to configure the optocoupler in such a way that when the signal you are trying to reproduce goes low, the optocoupler should turn ON. This is echoed in Paul Hill's Speed Control Examples Using the 4N25 as well as the test diagram that is shown in the Fairchild datasheet:

See how the output is the inverse of the input according to the diagram on the right?

In all of the configurations of the 4N25 I have always seen output tied in paralell to the transistor sides collector(pin 5). The transistor side of the 4N25(pin 4, 5, and 6) reproduces a signal by always being high except for when the LED gets turned on. When the LED in the 4N25 turns on, it causes the input output to go LOW because there is a direct path opened to ground.

3. Pin 6 (the base of the transistor) is used to tweak the sensitivity of the optocoupler.

When doing research for working with the 4N25 I always saw diagrams of the base not being connected at all, or being connected to ground by a very large resistor in the 100k and above range. Initially I did not connect pin 6, the base to anything. However, after sending a pulse width modulation signal through the optocoupler to control a servo, I noticed the servo would not hold its position without rapidly jerking a small amount back and forth. I tested the output of the optocoupler with two servos and noticed the same behavior.

I then hooked a large range potentiometer between the base of the transistor(pin 6) and ground. As I ran the resistance on the base of the transistor from zero to around 80K ohms, I could visibly see the jerking motion on the servo increase and decrease to smooth operation.

4. A simple test setup for an optocoupler can be constructed using a single power source, LED, resistors, and a pushbutton switch.

The diagram to the left is what I used to test the 4N25; You can click it for a higher resolution schematic. When I supplied power to the LED side of the optocoupler(pins 1, 2, 3), I would see the dimly lit LED turn OFF. The LED would turn back on again when I removed power from the LED side of the optocoupler.

The final diagram of the configuration I am currently using to smoothly control a servo as of writing this can be seen below: