Creating a track speedometer – Part 3 (Sensor evaluation )

Photoresistors

I’ve ordered some photoresistors from Adafruit.com (and my 2×16 LCD screen). Materials arrive on 9/16/16.

Never used them before, but I’ll get familiar with them.

  • Dark resistance is quite high (20K ohms or greater)
  • Light resistance is somewhere in the (1K – 10K ohms range)
  • Either analog input pins will be needed or a tuned comparator circuit will be used off board
    • Look at expense, dev Arduino has resources, but was hoping to use a cost reduced version for the real thing
    • UPDATE: I found a cost reduced Arduino at $1.60 each on eBay.

Simple evaluation circuit

I’m expecting the trip point to be somewhere around 30% of Vdd, but will update this article to reflect when I find out.

sensor-circuit-2

Let’s breadboard it!

I ended up using 2.4K resistors and ditching the pot in the circuit. I created a simple auto-calibration routine to set the trip point at the mid-point value on the ADC of the arduino.

Software updates

I’ve built┬áthe LCD port expander as I need the “select” button to perform the calibration sequence for the photoresistors. Working on the calibration routines now.

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