Technical Note – How to virtually increase the number of digital outputs

Application of FLIR AX8

A customer using an AX8 wanted to trigger on the digital output if either one of two temperature conditions were met. Each stage would be triggered by a different condition and would trigger a different digital input on the receiving device. 

Problem
The AX8 only has a single digital output. This posed the question, could a single digital output be used to send multiple digital signals that could be differentiated by a system receiving them?

Solution
The solution to this problem was found by using the features on the AX8’s web-based interface. This allows a user to select regions of interest (ROI) on the image stream. Each ROI can be used to trigger the digital output based on user defined parameters. However, the receiving digital input still does not know which ROI has been triggered.

The crucial feature that can allow a system to differentiate between each ROI is the ability to change the pulse length of each ROI trigger. Figures 1 and 2 below show two pulses being captured by an oscilloscope.

Now we can send a different signal depending on which ROI rigger condition has been met. We now just need a receiver that can tell the difference between the two. This can be implemented in software.

Using an Arduino board (and my primitive programming skills), I wrote a simple script that is activated when the digital output of the AX8 is detected. The code then waits to see if it was a short or long pulse. A short pulse would indicate region 1 and a long pulse would be indicate region 2.

In figures 4 and 5, the two regions of interest have been drawn and set to trigger over 27 degrees. The output of the script is programmed to indicate when either the left or right ROI has been triggered. My thumb provides the sufficient temperature increase to activate the digital ouput.

Conclusion

The example above is very simple but proves the limitation of one digital output can be overcome and leaves huge potential for refinement. With an inexpensive microcontroller, a single digital output

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