Advice requested for connecting transistor

Discussion in 'The Projects Forum' started by adam2014, Jul 13, 2014.

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  1. adam2014

    Thread Starter New Member

    Jul 13, 2014
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    I've connected an Acewell 7659 digital dashboard to a kit car, but I've been having trouble with the speed interface. The car has a Variable Reluctance sensor but the dash is only compatible with Hall Effect (a square waveform which alternates between +5V and -5V: the dash registers a pulse every time the voltage crosses 2.5V).

    I've connected a VR to Hall converter but as the dashboard speed connection is outputting +5V I've been advised this may blow the converter if I connect it to the output. The dash has a strong pull-up resistor so adding a resistor doesn't work.

    I've attached a circuit diagram of how it's all connected (in the diagram there's a zener diode connected between the output and ground - a friend suggested this, but it didn't work: without the dash connected it gave the correct output but when the dash was connected the output hovers around 4V).

    What I need to know is how to connect a transistor between the dashboard and converter so as to protect the converter. I think this is probably a pretty basic connection but I'm not too good on electronics. I'm guessing I need two or three resistors but I have no idea what ratings.

    Any help very gratefully received.

    [​IMG]
     
  2. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    That makes no sense.

    VR sensor feeds the VR-to-Hall Converter, VR-to-Hall Converter feeds the Dashboard.
    The signal is going from converter to the dashboard.

    You, for some unknown to me reason, saying that dashboard is sending a signal to converter.
     
  3. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    If you are trying to buffer one device from the other. Find out the frequency of the signal you need to pass through the buffer.

    Is the signal still square wave from -5 to +5 volts?
     
  4. adam2014

    Thread Starter New Member

    Jul 13, 2014
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    Correct.

    The dashboard also has the facility to work with a reed switch, so the input wire reads +5V and when switched to ground registers a pulse. I don't have enough knowledge to know what happens to this +5V supply when it's connected to a normal Hall sender, but I've been told connecting it to the converter could blow it. I hope this makes a little more sense?
     
  5. adam2014

    Thread Starter New Member

    Jul 13, 2014
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    Yes, the signal is the Hall Effect square wave as mentioned. The frequency changes with the speed of the car.
     
  6. shteii01

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    Feb 19, 2010
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    Find out the maximum frequency. This is a design requirement.

    If you want a simple buffer, I would use operational amplifier instead of transistor.
     
  7. shteii01

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    Let me get this right, and correct it me if i get it wrong. The dashboard uses the same connection to receive a signal and to send a signal?
     
  8. adam2014

    Thread Starter New Member

    Jul 13, 2014
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    Forgive me if I've used the wrong terminology: I am not an expert (to put it lightly) when it comes to electronics. There are two wires coming out of the dash which connect to a speed sensor. One negative and one positive which reads 5V. Logically the information can only flow one way, into the dash, so it must sense when the 5V signal alters?

    I calculate about 140 pulses per second maximum.
     
  9. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    Well. If information goes only one way... then you don't need the buffer. The converter sends the signal to dashboard. Dashboard receives, it does not send anything out, so there is no need to protect the converter.

    Like I said earlier. What you originally wrote did not make sense. The converter sends out. The dashboard receives. Why would converter be damaged? What would damage it? Why would dashboard damage the converter when it does not send anything to converter?

    About the 140 pulses/s (pps?). Do you have the time for just one pulse? How long in time is just one pulse?
     
  10. Wendy

    Moderator

    Mar 24, 2008
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    The owners of All About Circuits has elected not to host discussions of automotive electrical system modifications/enhancements due to safety concerns, the potential of legal ramifications and the possible circumvention of vehicle regulations at the state and federal level.

    This thread is against the AAC forum rules, Chapter 6, as seen here:

    This can be found in our Terms of Service (ToS)

    Automotive modifications of any kind are strictly forbidden. Therefore, this thread will be closed.

    Please try to understand the reasons behind this action, and feel free to browse and use the forums.

    You might find answers to your questions in one of these forums:

    http://forum.allaboutcircuits.com/showthread.php?t=54400

    Another good forum that shares many of the same membership is http://www.electro-tech-online.com/ . Good luck.
     
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