An Electronic Design reader asks:
"I have a data logger that works from 0 to 5 V dc that I'm using in a race-car application. I want to be able to convert ground distance to a 0- to 5-V dc scale. I can create any hardware needed for this application. I was thinking in terms of having a wheel on the ground and a sensor or trigger that will step voltage from 0 to 5 V. I can calibrate it to find that x voltage is equal to y feet, for example. Would an optical sensor work? I want to record a distance of 0 to about 350 ft. It would also be great to be able to get a speed out of this (0 to about 30 mph). I have a time frame on the logger, so I guess I could calculate speed from that."
Answers:1. Use a counter for converting distance to voltage. Use an optical/magnetic sensor to convert the rotation of wheel to pulses, feed to counter(binary), feed this output to DAC for voltage out, finally to your data logger.
2. For speed measurement, you can feed same pulse to timer (pulses/unit time) and feed same via DAC to logger.
This way you can get information on both speed and distance.
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You could spend a lot of time developing a basic wheel revolution counter, but it may be less costly in the end to buy a data logger meant for go-carts. These should have the inputs you need. If you want to pursue this, use a magnet or magnets and a reed switch to get your 0 to 5-V signal into your logger.
I race a F500 car in the SCCA (Sports car club of America). A cheap speedometer I use is a bicycle speedometer with a speed reduction gearing of about 3:1 for a top speed of about 120MPH. If you are only going up to 30MPH, you could use the bike speedo without the gear reduction.
Do you have any solutions to this design problem? Please e-mail your ideas to Lisa Maliniak, eMedia Editor.
Readers seeking help with their own design problems are encouraged to e-mail Lisa Maliniak, eMedia Editor, for posting in future editions of Electronic Design's Helpline.