Optocoupler System

This article explains to connect an Optocoupler System.

This system is used to isolate the two power sources. Typical applications include medical where the patient needs to be isolated from possible power supply faults and surges to avoid electric shocks. Those systems are used in EEG an ECG machines.

The amplifier is usually powered by rechargeable batteries.

The circuit can work with just one 1.5 V power supply.

Parts: optocoupler, 8 pin wire wrap socket, 1 kohm resistor - 5, 10 kohm - 1, 1 Megohm potentiometer - 2 (second potentiometer could be just a variable resistor to save money), wire wrap wire, insulated wire, power supply (3 V or 1.5 V can be implemented with AA/AAA/C/D batteries), matrix board, battery harness.

Tools: USB Oscilloscope, wire stripper, pliers, wire wrap tool.

Optional parts: Solder.

Optional tools: Soldering iron, multi-meter.

I used the old PSpice simulation software to reduce drawing time.

The input should be battery-powered to prevent lighting power surges or other power surges from entering the input and injuring the user.

Biasing the output is very a good idea because the power from the input photodiodes is very small.

Ro is used for output short circuit protection.

Ci is a Bipolar capacitor.

The output circuit is similar to a BJT NPN bipolar transistor.

The output signal is inverted and is smaller than the input signal. However, testing will prove that the system has a gain of -1.

There could be attenuation parameters in the inaccurate PSpice model that I used.

You do not need high-power resistors for this circuit that I used.

I used one 3 V power supply instead of two because I did not have an additional 3 V battery harness.

The input biasing resistor Rb1 needs to be a very precise variable resistor. I only used the potentiometer because I did not have any other components. You can try using a precise trimpot. It took me a long time to adjust the Rb1 value because I did not use a trimpot. The value was too low too high to prevent output signal clipping.

The value of Rc1 does not need to be accurate. You can use any variable resistor that you want. You can even replace Rc1 with a fixed resistor after measuring the resistance that is needed to keep the output at half supply voltage.

I used a cheap $25 USB Oscilloscope from eBay.

The first step was adjusting the output potentiometer, Rc1 so that the output voltage is half the supply voltage.

The second first step was adjusting the input potentiometer, Rb1 so that the input signal does not saturate. The second potentiometer has a minor influence on the output signal biasing value.

I set the input of my signal generator to a minimum amplitude. The system has a gain of -1. That means the input signal is inverted.