# Pulse to dc circuit?

After finding out that connecting 12V, 25 Watts bulbs into a PDC (8V) source will produce catastrophic results in my vehicle, it has come to my mind, just for curiosity, what would happen if LED (light emitted diodes) were used instead of filament /incandescent bulbs? Diodes may work with 3V-5V source and very small amperes. PDC seems to be suitable for LED lighting, isn’t it? Any comments will be highly appreciated. If this is possible I will not need to cut wires at the other end (i.e. front)

vk6kro
In that case you go from "certainly won't work" to "might work if you are lucky".

Once again, you have very short pulses of power and a long time when there is no power, so you need to store the power while it is there and use it when it is not there.

You need to find out how much power this circuit can deliver.
One way would be to put a 47 ohm resistor across it and see what the voltage drops to.
It is 8 volts now. If it drops to 7 volts, you could calculate the internal resistance of the circuit.
Like this:
The current in the 47 ohm resistor would be 7 volts / 47 ohms or 149 mA.
The internal resistance then has 149 mA flowing in it and 1 volt across it.
So, the internal resistance is 1 volt / 0.149 amps or 6.7 ohms.
Say so if you don't understand this. It is important.

This will tell you how much current you could use to light up some LEDs.
The duty cycle is still 1:5000 so you have to divide this 149 mA by 5000 to get an average current.
This is a uselessly small current of 29 microamps in this example. You wouldn't even see that it was on.

This leaves you with the chance of pulsing the LED. You can pulse a LED and your eye sees a bright light but holds the image for a short time. I don't know how well this works, but it might be your only hope.

... PDC seems to be suitable for LED lighting, isn’t it? ...

No.

I would go into more detail, but you seem determined to mess with the car's computer system regardless of our advice.

Best of luck to you. I'm done.

I am conversant with Ohm’s law and Kirchhoff’s law so I understand your DC calculations. I have also found some information on the web where PDC is used to drive LED (experimental stage). There is also a company that produces LED lamps driven by PDC.

I would like to make clear that by working only in the rear bumper (where the sensors are located) I am not touching at all the ETACS-ECU of the car neither cutting any wires at the front. The job will be just to detach the sensor head and replace it by a number of LED (7 or 8) which will be soldered (circular) in the manner that is done the little LED torches that are bought in places like Wall-Mart.

My concern now is to find out how to specify the LEDS (e.g. mA, V, and mW) so as a whole they can deliver 1700 lumens or 136 candelas that is equivalent to a 25 Watt 12V bulb luminescence lamp.

I must point out that I am very grateful for your comments that have distilled understanding in this problem. All the advice given has been taken very seriously by me and if I have persisted with the subject it is to find alternatives solutions to the same problem.

vk6kro
The main conclusion from the calculations is that there really isn't enough power available in the PDC to run any lamps or LEDs.

Why don't you run the lights from 12 volts taken from the tail light wires?

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.

(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.

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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
Gold Member
Dearly Missed
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.

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.

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
Gold Member
2020 Award
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.

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
Gold Member
Dearly Missed
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 gotta 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
Gold Member
2020 Award
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
Gold Member
Dearly Missed
@ sophie :: amen.