Attiny44 speed sensor circuit

Posted on Feb 7, 2014

My final project, cycling rollers, requires a method of sensing the rotational speed of one of the rollers. Typically speed sensors on cycles work by detecting the passing of a magnet attached to a spoke on one of the wheels using a Hall Effect sensor or a reed switch. I intend to use a similar principle to determine the road  speed of the rolle

Attiny44 speed sensor circuit
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rs by placing a magnet on one of the rollers and counting the number of rotations in a known time. As an initial step towards this, I want to establish that I can use a Hall sensor in this way by having it illuminate an LED as it passes. This feature will be incorporated in the final design to allow a user to determine the speed sensing is operational and will allow me to start developing with the Attiny44. The first step in the design was to select the current limiting resistor for the LED. Once I understood that the maximum allowable current was 40mA and that a voltage drop of 1. 8V was needed across the LED a minimum 80R resistor was needed. I mistakenly believed that a 10K resistor could be used as I did not initially understand that a current of order of 10mA is needed for the LED to illuminate. Ohms law indicates a resistor around 320R, hence the selection of 500R. I reviewed the data sheet for the Allegro A130x series of Hall sensors in the FabLab inventory and determined they were suitable for my need. One feature of their operation was the output signal was centred on Vcc/2 swinging up to Vcc and down to Gnd depending on the polarity of the magnet passing the sensor. This meant that I needed to use an Analogue to Digital Convertor on the Attiny44 to read the input signal which in turn determined that a bit on Port A would be required for the input signal. I reviewed the data sheets for the Hall sensor and the...

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