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Turning tiny, boring designs into fun tasks

Posted: 12 Jan 2015 ?? ?Print Version ?Bookmark and Share

Keywords:sensors? speed control unit? switch? resistor? Analogue Comparator?

Interfacing on-off sensors and dealing with bouncing and flickering can turn into almost a boring and routine job. But in rare occasions we need to go beyond wiring a couple of cables.

For one application we needed to use one speed control unit (figure 1) under some slightly different operating conditions inside a controlled environment. Firmware sources were not available, so our only entry point into the system was the sensor terminals block. The need to "fool" the control unit by tweaking its sensor signals made this application a good candidate for exercising skills and having some fun.

Figure 1: System overview.

The sensor operates like a very low impedance contact switch. Power for the sensor is provided by means of a 560 pull-up resistor in series with the internal regulator. Sensed state information comes in both, pulse width and pulse-to-pulse time interval. Our goal was to extend the pulse width, t1, without overlapping more than two consecutive pulses (figure 2).

Figure 2: Modification of the signal.

There were several relatively simple solutions, but we wanted our device to act like a small, battery-less, pluggable dongle with some "intelligence" inside.

The main constraint in the design was the need to be pluggable and un-pluggable without disconnecting the sensor. The small set of requirements served the challenging and fun part of the design: battery-less operation, low power consumption, fairly precise timing (millisecond resolution), small size, and ability to distinguish between self-driven LOW output from sensor-driven LOW.

Ahem! Why not using a microcontroller? Why not mix some analogue and digital modules? The resulting conceptual design idea is shown below in figure 3.

Figure 3: Conceptual design.

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