I'm building a tabletop race track for micro RC cars. The track width is about 14 cm, and the cars are about 4 cm large.
I'd like to add a RFID reader on the finish line, and passive RFID tags to the cars, so that it would be possible to have lap times and order position of the various cars.
But I'm struggling to find a proper RFID tag that would be small enough but at the same time provide enough range to cover the whole track width.
A possible candidate is this one:
small enough to be put under or inside the car, but since the range is about 3cm, and assuming that I put the reader underneath the finish line (on the center of the track, widthwise), I would only cover about half of the width of the track (3cm on one side, 3cm on the other side).
Another option would be to put more than one reader.
Alternatives to RFID would be IR, but with IR I won't be able to recognize which car is which.
No better idea, but there is ONE important parameter you did not consider. The RFID readers do not give this specification, either. That is "how much time does the reader need to sense and read the RFID. You are trying to get an RFID from a moving target. Will your car be in range for enough time to be sensed and have the ID properly read by the reader?
Seems like only you can test this, as the sellers do not consider this parameter relevant.
Basically a 125KHz RFID takes about 40mS to read. Depending on the way the software is written it could take up to twice that time to get a reading. But 80mS is still fast enough for you.
End of quote.
So, will your car be over the reader for 40 -80 mS?
thank you for pointing me out this aspect..I really didn't consider that.
I'd also like to add that the number of cars is limited (I think 5 or 6) so probably there is something clever I could do with some other kind of sensor to recognize only those 6 possible values..don't know..
These are typically used for animal tag reading... the RFID chips they sense can be tiny (e.g. 3mm glass tube). Some are capable of sensing up to 20-30cm.
As @Paul_KD7HB said, I'm not sure how fast they are so that might be a limiting factor.
what about color sensing?
Assuming the 6 cars will have different (and primary) colors, I could use a pair of color sensor (one on each side of the track along the finishline)
I have never got anywhere close to that distance from a glass encapsulated RFID token. It is more in the range of 1 to 3cm.
The problem is the size of the pick-up coil, it is just too small for an great distance.
You can always wind your own pick-up coil so that it covers the whole width of the track. However mounting any RFID token on or near metal will detune it and reduce the range.
Yes but those are not the glass encapsulated RFID tags used for injecting under the skin of animals I was talking about. Those tags are massive of course you can read those big tags at that distance.
I'm starting to ditch the idea of RFID,
another idea I came up with, is:
the finish line is an illuminated led strip. Underneath every car I put a simple photoresistor connected to an Arduino and a 433Mhz transmitter. Every time a car passes the line, the photoresistor catches the light, and sends its own ID to a main microcontroller that collects data.
Transmission latency should not be an issue since every car will suffer the same latency
if this is not doable (I guess mainly due to the size of the car..I need to find a really small transmitter and arduino PCB) then I will try the color sensing route
I’ve made incremental encoders with reflective optical switches with pretty dense line spacing. With only 6 cars you could use a very simple code to differentiate each car. Just mount the switch in the track surface and print the lines on a tag on the bottom of the car.
The optical switch has an LED and a photodiode. They are mounted side by side such that the LED light is reflected back to the photodiode from a nearby reflective object. That distance can be a few mm or more.
If you had one pointing upward from the track and put a label on the bottom of the car with alternating light and dark lines, the signal from the LED/photodiode pair would be a number of pulses corresponding to the pattern on the label.
You would assign a unique number of lines to each car and count the pulses when it passed over the sensor. The car would have to pass over the sensor and if the label went all the way across then that would determine where the car had to be to sense it. Or you could have multiple “lanes” with multiple sensors.
