Speed trap

[dr pepper]

I'v ebeen asked to make a speed trap for model cars.

My initial thoughts are 2 red lasers and phototransistors 100mm apart, a pic 16f628 and measure the time between the breaks, a bit of calculating and end up with a speed, and maybe a scale speed.

I noticed there are microwave doppler shift radar modules on ebay, do these have any outputs other than motion deteted, can you use them like the old ones where you can put together a circuit to detect target speed?

 

[JimB]

Do you have a link to the doppler radar units?

JimB

 

[dr pepper]

Yup,

**broken link removed**

Edit: I didnt see the above ad before, all the ones I saw were from china and very cryptic, however the ad says suitable for vehicle speed measurement and the pic shows an IF o/p from the module.
I guess that answers my question dunnit.

 

[JimB]

What a dinky little module.

I dont know what size model cars you are proposing to measure the speed of, the only thing to be aware of is to have enough metal in the car to reflect sufficient microwaves back to the module to give a sensible output.

Out of the IF terminal you will get an audio frequency proportional to the speed of the approaching (or departing) object.
It is an easy calculation to determine the speed from the frequency, problem is, I dont know what it is off the top of my head!

JimB

 

[jbeng]

Back when I worked for a radar manufacturer, we used 31.389 Hz/MPH for X-band at 10.525 GHz.

 

[dr pepper]

Yes Mr Beng thats what I worked it out as.

The cars are 1/32, only a couple of inches wide, and they are all plastic, 'cept for the dc motor.
However the radar modules are designed to detect people, and they are certainly not metal, in fact we are largely water which absorbs microwave energy to an extent, so I was thinking maybe these modules can 'see' plastic.

Fibreglass boats are fitted with a radar reflector if they want to be seen, having said that they work up to 40 miles on x band, we are talking about 20 meters.

 

[Dick Cappels]

The model cars are going to go much slower than the life-size cars, therefore the doppler shift will be proportionally smaller. Your laser & PIC solution is looking better and better.

 

[4pyros]

You can put foil tape on the inside of the plastic bodies to have them seen by radar.

 

[dr pepper]

Yes good idea, I have some security window foil.

Scalextrix used to sell a radar speed trap that used microwaves, they surely would have used infra reds if it was a better way to do it.

Edit: Make that hotwheels not scaley