Emergency Stop
The big fear of every developer of a microcontroller or computer-driven control system is that the computer or controller could crash while it is in the middle of controlling some-thing and that the output signal will remain on ‘full throttle’. In this scenario motors could continue to spin faster and faster or a heating element could become red hot, without the system taking any corrective action. In reality, any control system needs some sort of emergency stop, which will turn everything off the moment something goes wrong. Microcontrollers or computers will usually have a spare TTL output, which can be used for this purpose. By adding a few lines of code to the program, this additional output can be made to toggle high and low periodically. This can save a lot of trouble and dam-age. Should the computer or controller crash, then the toggle signal on this output will stop as well. The circuit then, does little more than check whether this toggling (TTL-) signal is still present. The...