Opto-Isolated Transistor Drivers Microcontroller Interfacing

In part 1 we looked at a number of bipolar transistor and MOSFET driver circuits. They had a flaw of having to be electrically connected to low voltage digital circuits. See Part 1.

With opto-isolators we can sever this connection including the use of higher-voltage power supplies totally isolated from the low-voltage digital circuits. In fact we can even change the polarity of the higher voltage supplies without regard to the digital circuit common negative grounds if needed.

Q1 is a NPN bipolar photo transistor and both the LED and transistor are often one physical device. Again an important note is the output has no electrical connection to the input and can isolate several thousand volts. Also see More sample circuits.

• Added Nov. 16, 2014
• ULN2003A Darlington Transistor Array with Circuit Examples
• Tutorial Using TIP120 and TIP125 Power Darlington Transistors
• Driving 2N3055-MJ2955 Power Transistors with Darlington Transistors
• Understanding Bipolar Transistor Switches
• N-Channel Power MOSFET Switching Tutorial
• P-Channel Power MOSFET Switch Tutorial
• H-Bridge Motor Control with Power MOSFETS
• More Power MOSFET H-Bridge Circuit Examples
• Build a High Power Transistor H-Bridge Motor Control

While most opto-isolators use a LED infrared emitter the detector can include photo SCRs, Darlington transistors, and even digital circuits.

The output transistor of a 4N25 is still a low power device, so we must use it to drive higher power components for greater loads. This is usually higher power bi-polar transistors or power MOSFETs.

What can we use opto-couplers for?

The 4N25, 4N26, 4N27 and 4N28 devices consist of a gallium arsenide infrared emitting diode optically coupled to a monolithic silicon phototransistor detector.

Applications:
General Purpose Switching Circuits
Interfacing and coupling systems of different potentials and impedances
I/O Interfacing
Solid State Relays
Power supply regulators
Digital logic inputs
Microprocessor inputs

The 4N29, 4N30, 4N31, 4N32 and 4N33 devices consist of a gallium arsenide infrared emitting diode optically coupled to a monolithic silicon photo-Darlington detector.

This series is designed for use in applications requiring high collector output currents at lower input currents.

Here we drive a NPN Darlington power transistor. With a gain of 1000 very little base current is needed to be switched though the 4N25. The TIP120 has a maximum base current (Ib) of 150 mA while the 4N25 has a maximum collector current of 150 mA. R2 can be 5600-10,000 ohms.

Same as above except we use a PNP Darlington. In this case the base of the TIP125 is switched to ground though a 1000 ohm resistor. Note the 24-volt supply is electrically isolated from the digital circuits.

Here we switch a N-channel MOSFET to "sink" the load.

Once again we sever the 12-volt supply totally from the digital circuits. In fact we could have done this with all of the circuits on this page. Here we switch on a P-channel power MOSFET to "source" the load. R6 = 10K and BATT2 is limited to 30 volts because of the 4N25 or the Vgs rating for the MOSFET, which ever is smaller.

• New Nov. 2014
• Using the ULN2003A Transistor Array with Arduino YouTube
• ULN2003A Darlington Transistor Array with Circuit Examples
•  
• Using the TIP120 & TIP120 Darlington Transistors with Arduino YouTube
• Tutorial Using TIP120 and TIP125 Power Darlington Transistors
• Driving 2N3055-MJ2955 Power Transistors with Darlington Transistors
•  
• Using Power MOSFETS with Arduino YouTube
• N-Channel Power MOSFET Switching Tutorial
• P-Channel Power MOSFET Switch Tutorial
•  
• Using PNP Bipolar Transistors with Arduino, PIC YouTube
• Using NPN Biploar Transistors with Arduino, PIC YouTube
• Understanding Bipolar Transistor Switches
•  
• How to build a Transistor H-Bridge for Arduino, PIC YouTube
• Build a High Power Transistor H-Bridge Motor Control
•  
• Build a Power MOSFET H-Bridge for Arduino, PIC YouTube
• H-Bridge Motor Control with Power MOSFETS
• More Power MOSFET H-Bridge Circuit Examples

• Basic Triacs and SCRs
• Constant Current Circuits with the LM334
• LM334 CCS Circuits with Thermistors, Photocells
• LM317 Constant Current Source Circuits
• TA8050P H-Bridge Motor Control
• All NPN Transistor H-Bridge Motor Control
• Basic Triacs and SCRs
• Comparator Theory Circuits Tutorial

• Constant Current Circuits with the LM334
• LM334 Constant Current Source with Resistive Sensors
• LM317 Constant Current Source Circuits
• Introduction Hall Effect Switches, Sensors, and Circuits
• Using Ratiometric Hall Effect Sensors
• Pulse Width Modulation Power Control for Microcontrollers
• Introduction to PIC12F683 Programming
• Basic Transistor Driver Circuits for Micro-Controllers
• Opto-Isolated Transistor Drivers for Micro-Controllers

• Added Nov. 16, 2014
• ULN2003A Darlington Transistor Array with Circuit Examples
• Tutorial Using TIP120 and TIP125 Power Darlington Transistors
• Driving 2N3055-MJ2955 Power Transistors with Darlington Transistors
• Understanding Bipolar Transistor Switches
• N-Channel Power MOSFET Switching Tutorial
• P-Channel Power MOSFET Switch Tutorial
• H-Bridge Motor Control with Power MOSFETS
• More Power MOSFET H-Bridge Circuit Examples
• Build a High Power Transistor H-Bridge Motor Control

• Comparator Theory Circuits Tutorial
• Arduino Projects Revisited Revised
• Schematic for Following Projects
• Programming ADS1115 4-Channel I2C ADC with Arduino
• Arduino uses ADS1115 with TMP37 to Measure Temperature
• Connect Arduino to I2C Liquid Crystal Display
• Arduino Reads Temperature Sensor Displays Temperature on LCD Display
• Arduino with MCP4725 12-bit Digital-to-Analog Converter Demo