Special IR Circuit

[cpt591]

Hello, I am building a school project that uses a PIC to control a traffic signal.  In my model I need to create a way to recognize when emergency vehicles are present.  I am currently using a simple IR circuit accros the streets to detect when cars are present.  But I need something that can distinguish between regular cars and emergency ones.  I was thinking an IR circuit might work.  Just place TX in vechile and RX anywhere in the vicinity.  But I can not find a simple circuit to do this with.  I am not set on IR, so I would appreciate any ideas.  Thanks.

[hamoodyjamal]

Hi,

Im sorry i dont have any idea in that feild. But, it would be great if you can post the schematic for the circuit to detect when cars are present on the street.

Thanks ;D

[MP]

This is correct. There is nothing in the emergency vehicle which causes traffic lights to change other than the strobe that flashes when their emergency lights are on. There is a sensor close to the signal light fixture at the intersection which will sense this strobe. When the emergency vehicle is near enough to trigger it, the circuit overrides the computer sytem that normally controls the timing for the intersection and allows the emergency vehicle's lane to pass on a green light.
If you put one on your car to get through traffic, you will get a ticket.  ;D ;D

MP

[cpt591]

I am trying to attatch my IR circuit but I dont know if it is going to work.  So if you want it give me your e mail and I will send it.  Thanks for everyones replies.  I should have specified that I am working with a small model.  The street is only four inches wide, and the cars will be hot wheels.  So a strobe system might work,  but it could prove to be hard for such a small model.  I think the sound may work.  Does anyone have a good schematic for this? Thanks.

[izach]

since you are working with a pic there are a number of ways one could solve your problems

!) inductance pick up: your emergency vehicle could have a magnet connected under it , a reed switch could be positioned under the "road surface" when the emergency vehicle passes the reed switch it will pull a input on your pic low an the traffic light will change.

2) rf transmission: your emergency vehicle transmits a rf signal to the robot"aerial
the reciever pulls a input on your pic low and the traffic light changes.

3)ir  : your emergency vehicle transmits a coded ir signal that is picked up by the
  ir sensors on the side of the road(the ones that tell a car is there) this code is
  decoded by your pick and if the signal is correct it will change your lights..


but for all intense and purposes i think the first one will do for your project

[cpt591]

Oh my god, that is a great idea! I dont know why I didnt think of something like that.  But that is exactly the sort of idea I was looking for.  Now I dont suppose you have a schematic of a circuit like this?  I guess I could probably figure it out.  But if you do have one that would be great also.  Thanks again.

[izach]

a circuit for which one would you like

1  2 or 3 ?

[hatman]

Most trafic lights work on an inductive loop.

I work on these sometimes. It is a simple loop of wire embedded where a cars metal bulk is most likely to chage the inductance. Its fairly simple to build. and even a BMX can set them off if in the right spot.

I like the reed switch idea, You would have to put the switch back a fair way from the lights so that the light would go green in time for the vehicles in front of the cop car ect. to pass through and in case of traffic jams where another vehicle may obscure the switch.

IR does not have alot of range. But I work on systems all the time that have IR beams as the sensors for gates. They NEVER FAIL over distances of around 3-4m or less. not even in rain and hail. The trick is to have a tiny delay so that you test to see of the beam is broken a short time after it is intialy broken.

Becarefull where you place a the beam however as you dont want to miss the car. I have seen a car miss a safety beam on a tank proof gate! The car was crushed almost flat!

Nowdays when installing these beams we use two beams that cross eachother.

[cpt591]

The first one please.  Thanks again to everyone for all the help.

[izach]

the circuit here with is realatively simple  all that has to happen is an input on the pic  is brought low the pic senses this and changes the lights accordinly.

in the circuit R1 pulls the input high all the time while sw1 is not short when sw1 is short due to the magnet passing it i will pull the input low this is sensed by the
pic.

pic_magreed_switch.pdf