# Calculate the speed of an object

Discussion in 'Electronic Basics' started by Tom, Sep 20, 2006.

1. ### TomGuest

I need to build a device to measure the speed of a pine wood derby car.
One idea was to use two photo censors connected to a programmable logic
controller with the output going to a simple display. Is there an easier
way to accomplish this?

2. ### CheapscateDaveGuest

Hi Tom,
The Photo sensors are the way to go. I use an old laptop, printer port
or com port for input and a simple basic program for timing. The
display is already there.
Dave

3. ### Ken MoffettGuest

Have you Googled "pinewood derby timer" ? 505 hits. 24600 hits if you
leave off the quotes.

Just asking.

Ken

4. ### Phil AllisonGuest

"Tom"

** Errr - a laser / radar gun ??

Lots of other uses for one of them too.

See them for under US\$300 on line.

........ Phil

5. ### Greg HansenGuest

Depends on what you want to measure. What you described would measure
average velocity. You might be interested in maximum velocity or
velocity versus time or versus position, which would be different problems.

Maybe you could dispense with the PLC if there's a good way to divide
one analog signal by another, like using voltage-controlled resistors in
an inverting amplifier (I'm not one of those electronics cognoscenti).
Preset one voltage (or turn a pot) to represent the distance, and use an
integrator to get a voltage representing time between the two photocells.

6. ### ChrisGuest

Yup. And the very first link shows project 14, "The Worlds Least
Expensive Pinewood Derby Timer". That will work just fine, but I'd use
a regulated +5V wall wart instead of the unregulated 6V supply shown in
the link. I'm not comfortable about an unregulated +6V (which may peak
as high as 9VDC) attached to a printer port input pin with a 10K
resistor between.

Use an old laptop, and amaze the kids.

Good luck
Chris

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