So I thought I'd play with technology to spoil the photos of license plates and share the fun with the world.
Can't jam the radar but I could theoretically make it harder to get a photo of my plate. They want to have really clean photos to stand up in court.
I used to build IR cameras for a living, so I know that all digital cameras are sensitive to
near IR radiation. IR LEDs transmit on wavelengths from 850nm to 1100nm. These are what are used
for IR remotes, etc. Invisible to the eye but visible to silicon digital camera sensors.
Visible light is is 390-750nm. Silicon is used as sensors, and silicon absorbs these wavelengths.
Some filters can be applied to cameras to filter out this wavelength, to improve the
sensitivity of the camera to visible. However sharp filters are almost physically impossible
and very expensive, they require many precisely controlled layers of deposition.
These are also used as invisible illuminators for security cameras, so we know cameras see them.
First I will leave them illuminated all the time, rather than try to catch the flash with a photocell some other nonsense. That is how photographer's flash extenders work, they sense the flash and make another flash fast enough that the shutter is still open, but the sensor is a lot closer and things are much more controlled than the outside environment. Speed cameras must have a pretty fast shutter speed (I calculated this below) too. The reaction time would be short, the flash hard to detect.
There are some phone apps to warn you of these cameras, like this one. Radardroid Lite
Since IR is invisible, this should be legal. I can't cover the plate, license plate covers are illegal in my state. The IR LEDs must be on the edge of the plate, close enough that some of the numbers can't be photographed due to the "bloom". Just a little glare to make it hard to read and the ticket will be blocked.
what the heck I'll try it anyway. I'm a bit unsure that the lights will be bright enough and this may all be a boondoggle.
There is a possibility that flashing at a high rate might be better to throw off the exposure control
of the camera. However I don't want to risk the lights being off during the time the photo is
taken, so the flashing will have to be fast. I considered flashing them in banks, but that wouldn't
have the effect of messing up exposure control. I think an all on - all off flash would be most disruptive
to photography. I will have to experiment with what works best.
Speed cameras must have long focal length lenses and short exposure times to get sharp photos
of moving cars from far away. That's why they need a flash, they need all the light they can get.
I'm guessing they need exposure times of 1/250 to 1/1000 to capture the moving car.
Flashing the LEDs at 1kHz would mean that 50% of the time I'd miss being on when the
photo was taken. IR LEDs are designed to modulate at 30-40kHz for remote control.
So modulating at >5KHz will keep the LEDs on for the photo time and will easily be
in the response time of the LEDs.
So I bought 40 IR LED's from Digikey of various wavelengths. I sorted for the highest
power with reasonable field of view, and of course, low cost!!
I already had 10 IR SFH415 U diodes from the IR sleeper and TV bgone projects.
I also bought these others from digikey, all were less than 50 cents apiece.
I wanted a mix of wavelengths but i especially wanted the 850nm ones, because they
will be closer to visible and will put more energy into the camera. Maybe I should
have bought all 850nm, but I thought a mix for experimented would be best to start.
|1||10||67-1001-ND||DIODE IR EMITTER 5MM IR CLR LENS||0||0.48500||$4.85|
|2||10||475-2871-ND||EMITTER IR 850NM 5MM RADIAL||0||0.48400||$4.84|
|3||10||475-1458-ND||EMITTER IR GAAS 950NM 5MM RADIAL||0||0.31300||$3.1|
As you can see, they didn't break the bank! So far I've spend <$20. The traffic tickets were $40 each.
Here is a sample of what they do to a digital camera when illuminated!
I can't show you what they look like to my eye, but they look dark and off.
I'm going to experiment with illumination, photos and placement when the parts come.
Got a license plate frame, plan is to drill it out and insert the LEDs. Got black to
make them less obvious. Got one that had a wide border along the bottom that will
be close to the numbers. This one has a cheezy "carbon fiber look" that has a
shiny woven look that will disguise the LEDs a little better. It is thick plastic and
hopefully will drill. Here is a photo with it on an old license plate
Pilot Automotive WL-714-CF Carbon Fiber Look Car License Plate Framehttp://www.amazon.com/gp/product/B002VKO60C/ref=as_li_qf_sp_asin_il_tl?ie=UTF8&tag=workingsilico-20&linkCode=as2&camp=1789&creative=9325&creativeASIN=B002VKO60C
This was the results of my initial comparisons between the three LED types
IR LED wavelength comment brightness
OED-EL-1L2 950 clear lens, high view angle Highest
SFH 4556 850 glows dull red to the eye Medium
SFH 415 U 950 dark and innocuous looking Medium
Generally, the clear LED was brightest to the digital camera, and the 850nm diode
which should have had a wavelength advantage, wasn't appreciably brighter.
Worse yet, it had some visible wavelength dull red glow. Not bright, but
it would be visible in the dark and raise suspicion. Too bad the clear LEDs are
a little more obvious but seem to be the best.
In this photo the LEDs are in order left to right, SFH 4556, SFH 415 U, OED-EL-1L2, SFH 415 U.
You can see the OED-EL-1L2 is the brightest (third from left). The 850nm diode has no
particular brightness advantage over the 950, which surprised me since it should have several
times the absorption by the camera. It's visible glow is probably going to kill the use of this one.
OK, for the OED-EL-1L2, the peak current is 1A, but the power dissapation is only 150mW
and the DC current allowed is 100mA. The forward voltage is 1.2 to 1.6V
That means that I want to shoot for 150mW to put the LEDs at their max light output.
100mA DC times 1.2-1.6V Vforward = 120mW to 160mW. So I have to keep the average
current to 100mA. My prototype will power the LEDs from 5V Arduino output but later
I may make a 12V simple circuit to do the oscillation. 5V/100mA is 50 ohms. But since
I'm flashing at 50% duty cycle, I will need 25 ohm resistor in series. The above photo was
takin with 100ohms, so I will be twice as bright as that.
Obviously the Arduino can flash an LED at that kind of current. The TV bgone uses a 2n3904
bipolar to switch the current, without any series resistor. I also have a pile of 2n7000 mosfets
that will save me some resistors. They can take 350mA. This was the tv bgone circuit. As
as the gate draws no current. Everyone should buy a bag of these, for 0.17 for 100.
To avoid a series resistor, I could drive the LED with a low duty cycle high current pulse.
The 2N7000 has a Rds(on) of about 2 ohms at 0.5A.
The LEDs could handle 1 amp peak (remember from above). 5V - 1.5V (Vforward) = 3.5V across the transistor. 3.5V/2 ohms = 1.75A. Too high for the LED without any series resistance. But if I put two LEDs in parallel, I will split that current and be at 0.875A per LED. This is not a perfect solution because the
LEDs may have slightly different forward voltages, and will not share the current equally. But best I can do.
Now I have to keep the LEDs from dissipating more than 150mW on average.
0.875A * 1.5V = 1.31 W peak. 150mW/1.31W =11.5% duty cycle.
SoI could flash LEDs at 10kHz, with 11.5% duty cycle, two LEDs per 2N7000 transistor, no series resistors. 5V supply.
To save components, I could bring the duty cycle up to 50%, and load on more LEDs in parallel.
I could put 10 LEDs on at 50% duty cycle, 0.50 * 1.75A * 1.5V / 10 = 131mW per LED.
I'm in trouble on the 2N7000s. they only want 1.4A peak current, and I'm doing 1.75A.
I mocked up an array of LEDs and resistors and plugged it into 12V for one last full
scale test before building up the license plate final version, and got a surprise.
While this was on, it started to smell hot. The resistors were getting really warm. Doh! I have 1/2W 100 ohm resistors, so that I put (12V-1.5V)/100ohms=105mA through the LEDs, their max rating. However since I'm running from the 12V auto supply directly, that puts 10.5V * 1.5mA = 1.1W through the resistors. Before I had been running off 5V during tests and the resistors did not get so hot. This simpleton project is becoming more complicated because I'm trying to make a cheap solution.
OK, brain finally started functioning. Many LEDs will be in series with a resistor
to set the current, this will prevent dissipating too much power in the resistor
and generally be more efficient, fewer components, all good.
6 LEDs in series like christmas lights, 6*1.5V=9V. 12-9V=3V across the resistor.
100mA*3V=300mW, which is in spec for the 1/2 watt (500mW) resistors. Might even
be able to squeeze in a 7th LED. 3V/100mA= 30 ohm series resistor.
Tested out strings of 6 LEDs in series with 30 ohms. Gets nice and bright, doesn't get hot, doesn't waste power.
OK, here is the complete circuit drawn up. Strings of 6 LEDs will be used.
I think I can pack the LEDs in and solder them into chains. So the big structure at the bottom will look like two rows of alternating staggered LEDs with no interruption in the middle for the ground.
There will be a tight cluster in the bottom center of the plate, and a smaller cluster at each side which is nice and close to the numbers.
Drilled a hole in the license plate frame as a test (see link above) and it drilled pretty well, nice clean hole.
The challenge is going to be making nice neat regularly spaced holes or the whole thing will look amateur.
I put one of the clear IR LEDs that won the brightness test, but it looks really obvious in the frame. Maybe somebody with a tricked out car might not care, but it really looks like a light, even when it is off. The solid color SFH 415 U look far better and less suspicious. I'm going to have to use those, i'd never have the nerve to put the others on the car. Forget the nice shiny OED-EL-1l2s.
Hard to get a good photo to show what i mean, but here it is...
Obvious Clear LED (shown off, which is how it looks to the eye). I thought the pattern would hide it.
Nearly invisible black IR LED. It is a bump of course and the picture isn't great. But you get the point.
I was planning to use my CNC machine that I've been writing about to drill the holes. Sorry if not everyone who reads these happens to have one. However the CNC machine stage is small enough I'd have to do the job in three passes, and I got impatient with the setup and just did it on the drill press. I clamped a bar to the press stage to make all the holes even distance from the edge of the frame, but freehanded the spacing. I was eager to move this project along and get to some outside trials. I did OK but not great in spacing the holes and kinda goofed on getting the placement of the arrays even on both sides. No harm done but a production version would have to be neater. The plastic license plate frame drilled nicely though, it was a good choice for the project.
Here is a picture of the holes I drilled. I put the LEDs in clusters to make hot spots in the photo, rather than in a whole line around the plate. The thought is that unevenness and tight hot spots will be best to ruin the photo.
Next to glue in the LEDs and wire them up! A note about LEDs since a few non EEs seem to be reading this post. LEDs aren't light bulbs. They have a polarity and must be hooked up the right way or they don't light up. This is how you tell which way to put them in. Note that not ever LED follows the long lead convention, but the flat side is a reliable indicator of polarity.
Here is a shot of the LEDs getting glued into the main phaser bank in the middle. Remember that they are in strings of six, so the pattern of anode and cathode is important to be able to easily solder them into strings. I will show the pattern later. This is about the best I've been able to capture the look of the LEDs in the frame. They are hard to see.
Next.... Ramping it up!