Convert Pulse to DC Voltage - Ideas Needed

[batman29]

I would like to comply with international regulations about rear fog lights on vehicles and this requires that rear fog light can only be activated when the dipped beam headlight is ON, not without it.

I know I have some gaps in my thinking to fully understand the electronic behavior of LED when the LEDs are driven by PDC. Please be tolerant if my reasoning is faulty.

The back sensor ECU of the vehicle produces PDC (pulse signal) 0V-8V, 40 kHz pulse signals, emitted approx. every 125 ms). That is what it says in the electrical manual for this vehicle. According to the previous data it means 40,000 flashings per second. The vehicle ECU takes 0.125 s to release a batch of pulses and therefore a batch of pulses consist of 40,000 x 0.125 = 5000 flashing.

The standard red LED has these characteristics: 20mA (LED), 2V (40 mW). Calculations proceed like this: 2 = 0.02 (R), R = 100 Ω (each LED). A number of 4-LED will use the 8V at each flash.

Because the flashings are positive and the current can be either 0 mA or 20 mA the LED will light only when the led is hit with the flash. I may be wrong and please correct me; human eyes will be not able to see the light flickering. I may be also wrong but there is no reason (apparently) to average the current because the LED will be hit with current either 0 mA or 20 mA not anything else.

 

[vk6kro]

The pulses emitted are at the frequency of 40 KHz, but they cannot be emitted continuously. This is because the receiver must be able to listen for echoes from other vehicles etc without interference from the transmitter.

More likely, a brief burst of 2 or 3 pulses is transmitted and then there is nothing while the receiver listens for echoes.

This is the problem if you want to use these pulses to light a LED or a lamp. Because there will be very few of them and long periods of listening, there is very little power available to do anything.

You can eaasily settle this for your own satisfaction. Just put an oscilloscope on the transmit output and observe the waveform.
Or, connect a LED with an appropriate resistor across the output and see what level of brightness you observe.

 

[batman29]

(Pulse signal, 0V-8V, 40 kHz pulse signals, emitted approx. every 125 ms)

Making sense of the statement above in addition to your description of how the signals are transmitted to the sensor heads (rear bumper) I may say the following:

The corner/back sensor ECU has 20 pins and only 11-pins are engaged all the time. 5-pins are connected to 5-wires that route to the rear of the car. The pulses travel through 4-wires and returned to the ECU by only 1-pin wire (with information about the obstacle distance when the car is doing a parking maneuver)

From reading again the information above it appears that the ECU releases a batch of pulses and then waits for 0.125 seconds to send another batch of pulses and so on. While 4-wires are waiting 0.125 seconds any response from the sensor heads are channeled together into a different 1-wire and sent to the ECU for evaluation. In summary the LEDs are energized (0 mA-25 mA) every 1/8 of a second. Is this reasoning correct? Will 1/8 of a second be enough to prevent noticeable flickering?

The reasoning above may have some weaknesses but I would like to be theoretically certain that it could work before dismantling the rear bumper and the rear side quarter panel of the car. Just experimenting (e.g. oscilloscope) will need the rear of the car dismantling in addition to the custom built set of LEDs (Red) to replace the 25W bulbs.

 

[vk6kro]

I suspect that what you have is a few pulses 25 uS wide emitted 8 times a second.

The problem is not limited to flickering. The problem is that there just isn't enough power to run any lights or LEDs.

You may have something entirely different to this and the only way you can find out is examine the output of the ultrasonic transmit transducer.

One way to do this would be to hold an ultrasonic receiver transducer close to the transmit one and examine the output of the receive transducer with an oscilloscope. This would not require you to remove the bumper of the car.
The oscilloscope would have to be near full gain, but you should get something.

 

[batman29]

OK I understood the inability of the sensor wires of carry enough power to light the LEDs.

Will the power of the pulses DC be enough to drive a miniature relay?

 

[jim hardy]

if you're determined to use the ultrasonic sensor

i'd place an ultrasonic receiver nearby and let it drive a relay to switch, say, a GE-4522 aircraft landing lamp. let the ultrasonic receiver trigger a timer circuit so the light won't follow your 8pps ultrasonic. that way you haven't intruded at all in the car's electronics.


http://www.aircraftspruce.com/menus/el/replacementlampsflashtubes.html

that'll require a wire to power the lamp because it draws about twenty amps.
run it yourself, under the car so you'll be happy with the installation.

that link carries LED high power lamps too but note they're pricey.

 

[vk6kro]

OK I understood the inability of the sensor wires of carry enough power to light the LEDs.

Will the power of the pulses DC be enough to drive a miniature relay?

No, it won't.

 

[batman29]

May I kindly ask you to provide your comments (e.g. Pros, Cons and Workability) of the approach suggested by our member (Jim Hardy)?

 

[sophiecentaur]

I will repeat what I wrote earlier on. If you have a supply of 12V with sufficient current capability then use a detector (diode and amplifier), a relay driver and a relay. That will do exactly what's needed. It will be the least 'invasive' solution and easily removed if you don't want it any more.
As Jim Hardy says, you can even use the ultrasound signal to avoid any direct connection to the 'electronics'. If it were up to me, I'd far prefer to do the hard work in the comfort of an indoor work bench and minimal effort on the car itself.

 

[batman29]

Many thanks

My background in electronic components is minimal; however I think I am able to follow your instructions. Certainly I will not be able to construct (myself) any ultrasound transmitter/receiver but I may order an already made ultrasound receiver to be located in the rear bumper. I am able to branch a line from a 12V source.

Reading the literature some people have built a transmitter and its corresponding receiver that will switch on a relay that will close a circuit to use a 12V source of current. I may say that the head of the sensor in my vehicle not only sends ultrasound beams but also receives the bouncer beams. Therefore the sensor head acts as a transmitter/receiver.

The problem I see from this exercise is that the rear fog light mostly is switched on when the car goes forward and the sensor head although it releases ultrasound to the air the beams are not bounced back (no obstacle) and therefore the best ultrasound receiver will not receive anything to be activated. I may be wrong here but I have the suspicion that the receiver will not work. If the ultrasound beams from the transmitter spray in all directions then the receiver may work.

Assuming that my logic is faulty I will appreciate it if someone can recommend a good brand of ultrasound receiver and where to contact.

A few months ago I designed (on paper) the use of RF transmitter (remote) to activate the rear fog light by draining 12V line from the 12V socket available in the cargo area of the car. I bought, very cheap, a Chinese remote kit (control unit ready to be used + transmitters) with a set of coloured wires to be connected directly to the 12V load. I have to use 2-transmitters one in the front to operate the rear fog light lamp (switch) and the other in the rear to operate the rear fog light. The problem with this arrangement is that I have to press 2-buttons to activate the system and I need only 1-button to perform the job. There are also some logistics troubles as the results of having 2-control units and also the transmitter is very small (about 1-third of the smaller mobile telephone), etc.

 

[jim hardy]

let's get real simple here...

what does this rear fog light do?

every car I've owned, including my '49 Buick, had back-up lights. Nowadays they're usually incorporated into the taillight assembly, though in '49 they were separate.

so if the object is to provide more illumination behind you when backing up,,,,,
the wire to your existing backup light will have capacity to drive one of those 45 watt LED aircraft landing lights and it will be energized when you're in reverse.

Back in the days when cars had four headlights (1960's) I used to put 4522's in the inboard two for mega-high beams. But the factory wiring wasn't heavy enough for those 250 watt lamps so i added separate wire and a relay.

in lamp terminology PAR means the glass is round parabolic reflector, the two digits following are its diameter in eighths of an inch.
The GE 4522 landing light is a PAR45, ie 5&5/8 inch diameter.
An old quad car headlight is a PAR46 , 1/8 inch larger. The PAR45 with a piece of inner-tube wrapped around it fits right into an automobile quad-headlamp ring.

The 4522 lamp illuminated cowpastures a half mile away . If that LED landing light has anywhere near comparable lumens, it will really impress your friends.

one of those LED landing lights should give you no electrical problems, just you got to mount it. You shouldn't even need a relay.


but be sure it can't shine at other drivers. i LOATHE those Xenon headlamps in the hands of imbeciles who won't dim them.

over and out,,, old jim

 

[sophiecentaur]

Many thanks

My background in electronic components is minimal; however I think I am able to follow your instructions. Certainly I will not be able to construct (myself) any ultrasound transmitter/receiver but I may order an already made ultrasound receiver to be located in the rear bumper. I am able to branch a line from a 12V source.

Whatever you choose to do, make sure that you put a low value of FUSE in series with the wire you take off the positive supply, wherever you connect it. I should keep it to 5A or less. That will ensure that any mistakes you make in wiring or construction will only result ina blown fuse!
Have fun.

 

[jim hardy]

@ sophie :: amen.