Optically Isolated Input Board

The objective of this project was to provide a generic optically isolated input board for use with a range of input devices. Specifically, the plan is to use these boards with both our USB IO Board and our Ethernet IO board connected to our Home Control System (HCS).


Each of these boards is designed to provide 8 optically isolated inputs, accepting a 12V input signal, though the current limiting resistors on the inputs should be changed to accept higher or lower voltages.

We have standardised on a 4-pin PCB mounting connector on all of the I/O boards.

The pin designations used throughout our home are:

  1. A 12V dc feed to the switch, sensor, etc.
  2. Ground connection or 0V relative to the 12V UPS.
  3. This pin is not always used but these are linked across all of the connectors. For the PIRs, this pin carries a 12V signal from the twilight sensor, which is used by the night light controllers.
  4. The 12V input signal to this input board.

Vero board layout
I've laid out the design on Vero board using Microsoft Powerpoint.

This board is connected to the IO board via a 10-way ribbon cable and connectors, to enable the swap in new ones if required.

In this design the output stages of the ILQ74 devices are configured to work in 'pull-down' mode. This means that they are inverting the inputs but, this is not generally an issue because the PIRs used are normally closed contacts and the software can handle signal inversion anyway.

Optical Isolation

Optical isolation is achieved using two quad opto-isolators. The ILQ74 IC is used on this board. The opto-transistors are connected to +5V from the I/O board via a 12KΩ resistor and when the input is activated, the input to the I/O board is pulled low. This thus inverts the signals but most of the sensors are active low anyway. Either way, the software can handle the normal state and trigger states.

The circuit is covered in more detail in the input design section.

The input board connects to the input board via a 10-way connector. The pins are:

  1. +5V
  2. GND
  3. Input 1
  4. Input 2
  5. Input 3
  6. Input 4
  7. Input 5
  8. Input 6
  9. Input 7
  10. Input 8

first prototype vero board
This is the first prototype we built and shows the general layout and connectors. Later boards are tidier!


  • These boards provide a cheap and easy way to interface the 12V signals found in our house to a wide range of I/O boards.
  • Given a bit more time, I will consider getting a PCB designed to improve the layout and reduce the size of this board but, since I only plan to use 3 or 4 in our home, I can't justify this yet.
  • Our next version will have a fuse for each input line, so that a short on one input does not affect all the others.

Further Reading

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