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Pulse to dc circuit?

  1. Jan 29, 2008 #1

    I need to find a way to convert a pulse that can be in any amplitude from 0 to 5v
    and output DC voltage at the same amplitude.

    any ideas? :)

  2. jcsd
  3. Jan 29, 2008 #2


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    To... convert?

    Do you just want a circuit that measures the amplitude of the pulse and adjusts a regulator to produce the same voltage, or what?

    Please try to describe your intended circuit in more detail.

    - Warren
  4. Jan 29, 2008 #3

    any circuit that his input will be 1 pulse and his output will be a dc voltage in the same amplitude and will stay that way until the circuit will get another pulse in his input.
  5. Jan 29, 2008 #4


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    So, does this signal always return to 0V between pulses?

    - Warren
  6. Jan 29, 2008 #5
    you can say that...:S

    ahm...itll only be one pulse and then...nothing, till the next pulse.:)
  7. Jan 29, 2008 #6


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    You are describing a peak hold circuit. You can make one easily with a diode (and be sure to account for the forward voltage drop of the diode), or you can add an opamp to eliminate the diode drop error, assuming the opamp is fast enough for your pulse. You can add a reset input to the opamp circuit to dump off the voltage that is held on the output...




  8. Jan 29, 2008 #7


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    You could also use an analog switch to sample and hold the peak value, if you knew when it was going to happen. A second switch would be used to dump off the peak held voltage from the small storage cap.
  9. Jan 29, 2008 #8


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    "Nothing" is not a meaningful term. I think you mean "zero volts."

    I'd suggest you use something like a peak detector. ADI's PKD01 seems appropriate:


    You can wire up its integrated comparator to automatically reset the peak detector each time a pulse comes along over some small threshold voltage. You might need to couple it with a one-shot to control how long the reset is asserted, or you might be able to use something like an RC delay. Either way, it's maximally a two-chip solution. You should look around at other peak detectors and see if you can find an even easier solution.

    - Warren
  10. Dec 22, 2011 #9

    I am new to this forum and I hope someone will give direct answers to my query.

    1- I have 2-wires that carry pulse DC to 2-sensors (parking sensors). The car’s ECU converts 12V car battery current to 0V-8V PCD (40 KHz pulse signal, emitted approx every 125 ms).
    2- I would like to replace the 2-sensors (parking sensors) with 2-bulbs (12V, 25 Watts) and use the same 2-wires that carry PDC to light the bulbs

    a- Can I do this?
    b- Will the 25 Watt bulbs light?
    c- Will the 25 Watt bulbs light at the same intensity (lumens) that a 12V DC source?
    d- Will the 25 Watt bulbs burn with use & time?
    e- How can I do the 25 Watt bulbs to light (i.e. same lumens as 12V DC) when the bulbs are connected to 0V-8V PDC?

    Please help
  11. Dec 22, 2011 #10


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    You can use the same wires, but you would need to rewire the other end of the circuit to supply 12 volts.

    At present you are getting a brief burst of 40 KHz pulses at 8 volts peak and you need to convert this to 12 volts at 50 watts, but this just isn't possible. Not easily anyway.
  12. Dec 22, 2011 #11


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    A peak detector, a relay driver and a relay?
  13. Dec 22, 2011 #12
    Many thanks

    I do not want to cut the 2-wires (under the car console) that carries PDC to the rear sensors and solder 2-wires 12V to them and thereafter using the 2-cutted PDC wires to carry 12V current under the car floor harness.


    Can I just solder the 2-wires 12V onto the present 2-wires PDC. This means that the 2-wires will carry 12V current and sometimes 0V-8V PDC to the rear or both. The car’s system activates the PDC when the car engages reverse gear. However I wish to light 2-bulbs (25 Watts each) at the rear when the car is or it is not engaged "Reverse".
  14. Dec 22, 2011 #13


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    No, there is no 12 volt supply to power the relay or the lamps.

    Just a (presumably) low powered 8 volt pulsing circuit. It drives an ultrasonic transmitter which would be low powered. How do you get 12 volts at 50 watts from that?

    At least one of the wires needs to bring 12 volts at a decent power level to the back of the car. If the ground is an option for the return path this would leave one wire spare.
  15. Dec 22, 2011 #14


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    We posted at the same time.

    Can I just solder the 2-wires 12V onto the present 2-wires PDC. This means that the 2-wires will carry 12V current and sometimes 0V-8V PDC to the rear or both. The car’s system activates the PDC when the car engages reverse gear. However I wish to light 2-bulbs (25 Watts each) at the rear when the car is not engaged "Reverse".

    No, you probably can't do this without a thorough knowlege of how the present system works.

    However, you could borrow 12 volts from the trailer power outlet if the car has one. This is powered all the time.

    You probably shouldn't be doing this. If you want lights then you should get an Auto Electrician to fit them for you. In most countries, lights on a car are controlled by regulations and you might see flashing blue lights if you get it wrong.
  16. Dec 22, 2011 #15
    I am able to locate a 12V wire under the car’s console. I will branch it into 2-wires to match the 2-wires PDC and prevent any over heat that may happen in the carrying wires until they reach the rear of the car. So each wire will light a 25W bulb.
  17. Dec 22, 2011 #16
    The final manipulation of the details will be done by an auto electrician. My main concern rests into finding out if the approach is physically/theoretical feasible. If the approach is feasible, then I will discuss the details and the proper use of tools to do the job with the expert.

    I have study the electrical circuits (in details) of my vehicle. The ECU corner/back sensors produces the PDC (a phenomena that is new for me) and send 8-wires to the rear; 4-wires carry the 0V-8V PDC and 4-wires bring the information from the sensors into the ECU. Once the 4-wires with PDC leave the ECU these go through a number of connectors (female & male) until the wires reach the sensors itself. The wires travel as part of the floor harness.

    The lights at the rear that I am talking about are called “rear fog lights” which are legal in Europe, Australia, and Canada. In the USA car may have it or not.

    The old method of implementing “rear fog light” is to run an orphan wire under the car carpet floor, which implies detaches (damage) fasteners, drilling holes on the car chassis, etc. At the end of the job the car will rattle because the end product will not be perfect.

    What happen if a bulb (25W) is connected to this 0V-8V PDC? Will light or not?
  18. Dec 22, 2011 #17

    jim hardy

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    the RMS of an 8 volt square wave is 8 volts.

    OOPS EDIT make that 4 volts.. for 8Vp-p

    sooo ..If the driver has enough current capability, you'll deliver (4/12)^2 power to 12 volt bulbs. they'll be mighty dim.

    presumably the ECU only activates the backup sensors when you're backing up

    do it right and run a switched & fused wire , affixing it to frame with suitable adhesive and cable ties.

    and be sure that your "12 v wire under the dash" has nothing to do with airbags.....

    and it's rude to burst into a thread
    Last edited: Dec 22, 2011
  19. Dec 22, 2011 #18
    and it's rude to burst into a thread!!

    I am very sorry. It was not my intention. Just I thought the topics are related and why to open a new thread to talk about similar subjects. Sorry again

    Using your formula the 25 W bulb will light with a power of 11.1 watts which I think is not "so mighty dim". Side marker bulbs are just 5 Watts.
  20. Dec 22, 2011 #19


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    It was an old thread, but this was on-topic, so don't worry about it.

    If you have a pulse for 25 μS then nothing for 125 mS, then this has a duty cycle of 1:5000, meaning that the 8 volt pulse is effectively 1.6 mV . (8 volts / 5000 = 1.6 mV). If there are several 25 μS pulses, then the voltage will rise slightly (by 1.6 mV per pulse) but still not enough to be useful.

    But it gets worse. The 12 volt lamps will have a resistance of only 3 ohms total (12 volts / 4.166 amps) and the Parking Distance Control will not be designed to cope with such a low resistance load.
    At best, it will just drop the voltage internally and give a few millivolts out.
    At worst it could be damaged or destroyed.

    The results of feeding 12 volts into the output of the PDC are certain to be catastrophic. A new one will cost you hundreds of dollars.
  21. Dec 22, 2011 #20
    Modifying or interrupting this wiring may have unintended and possibly negative effects on the vehicle's operation. Automotive computers have become intricately linked to just about every part of the car. You will have a much easier time simply adding a new, independent circuit.

    Not with any appreciable brightness, if at all.
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