Pressure-activated circuit

Discussion in 'The Projects Forum' started by anasasisxenophontis, Apr 18, 2010.

  1. anasasisxenophontis

    Thread Starter New Member

    Apr 18, 2010
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    I'm working on a circuit for an automotive application. It will use the voltage output from a manifold absolute pressure sensor to activate a relay and turn on a solenoid. I'm rather new to circuits, so I'm unsure of what diode and resistor to use in the circuit.
    I want the V-out to run through a zener diode so that the relay will only activate when the MAP sensor passes enough voltage to the diode. The input of the MAP sensor/relay side of the circuit is 5v @ 850mA.

    [​IMG]
     
  2. rjenkins

    AAC Fanatic!

    Nov 6, 2005
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    The output of the sensor is a varying analog signal. It is not suitable to drive a relay.

    You need to use a comparator IC (such as an LM311) to compare the senor output voltage to a reference value from a preset resistor (potentiometer).

    The comparator can then in turn control a transistor to switch the relay power.
     
  3. anasasisxenophontis

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    Apr 18, 2010
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    I have the sensor hooked up to vacuum lines and a multimeter in a mock-up of the real circuit, and the output of the sensor is undoubtedly a DC signal. I held a pressure of about 5psi on the line to the sensor for several minutes and the voltage did not fluctuate even a hundreth of a volt. The pressure rise in the actual setup will be linear DC, or at worst exponential, but certainly not analog.
     
    Last edited: Apr 18, 2010
  4. rjenkins

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    It is a sensor designed to send a signal to a measuring circuit (ie. an engine management system), it is not designed to feed any significant current to anything.

    Analog means the signal is represented by a varying voltage (or current) as opposed to a switched or digital signal.

    Reading the sensor signal with a digital multimeter requires it to supply something less than one millionth of an amp.

    A relay that works on 5V would need something like a tenth of an amp (100mA).

    A typical analog sensor can give possibly 10mA continuously. Any more and at best you lose accuracy and worst case you burn it out.

    I've found a datasheet for a typical automotive MAP sensor:
    http://www.datasheetcatalog.org/datasheet/motorola/MPX4250.pdf

    It has a maximum supply input current of 10mA and a design load current of 0.1mA so it should be operating with a load of about 47K Ohms.
     
    Last edited: Apr 18, 2010
  5. SgtWookie

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    Jul 17, 2007
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    The "tapping in" to the MAP sensor is meddling with the engine controls.

    It is my opinion that tampering with engine controls may result in safety and/or legal issues.

    We need for a moderator to weigh in on this before proceeding.
     
  6. rjenkins

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    From the schematic, it's an isolated sensor running on an independent power supply, not an existing engine component - I considered that before answering.

    If it is actually an existing engine sensor, then connecting anything else to it is obviously out of the question.
     
  7. anasasisxenophontis

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    Apr 18, 2010
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    As you can see, I never said anything about 'tapping-in' to anything in the engine harness. This is a stand alone circuit using a separate MAP sensor.

    I have measured the current of the input, and it is 850mA, which should be more than enough to drive a relay.
     
    Last edited: Apr 18, 2010
  8. SgtWookie

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    I agree; if it is a MAP sensor that is not connected to any other engine management systems then it might be OK.

    However, the 311 is not rated for automotive temperature use; it is only for 0°C to 70°C.
    The 111 is -40°C to 125°C, so it would be OK, if pricey.

    The LM2903 is another option; a dual comparator that is specifically rated for automotive temp ranges.

    However, this brings up another few possible issues:
    A) Firewall penetration.
    Where's the power going to come from? The power will need to be fused and conditioned. Typically, the fuse panel is in the passenger compartment. Penetrating the firewall has safety implications.

    B) Where will the output be used? If under the hood, that seems like it will be another engine management issue, as for what other reason would someone want the MAP signal to actuate a relay?
     
  9. Externet

    AAC Fanatic!

    Nov 29, 2005
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    Applying 5psi to the MAP sensor will not do a thing at all.
    It measures absolute pressure. That is, anything under atmospheric pressure; in other words, intake manifold vacuum, or if you prefer, 'negative pressure'
    +5 psi will peg it to full scale.
    Remove the pressure feed and suck to its port and then it will work.
     
  10. anasasisxenophontis

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    Apr 18, 2010
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    Actually, it takes 7.5psi (or 22.2psi absolute) to peg the sensor. You may be thinking of a 1bar MAP sensor that is unable to register anything above 14.7psi, but the vast majority of MAP sensors, including ones I use for testing, only return a fraction of the +5v supplied to it at 14.7psi. The sensor I'm using returns 3.33v at standard atmospheric pressure. It DOES register pressure above that value. It is this sensor feature that I'd like to take advantage of.
    Now back to my question...I understand that zener diodes will not pass current in reverse bias until they reach a certain voltage. Since the 'analog' voltage fed to it will be constantly creeping up as pressure rises, would it not reach that zener voltage and then pass current to the relay?
     
  11. Externet

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    Nov 29, 2005
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    As already told earlier, the current capability of the sensor (its internal resistance) output does not allow to drive a relay coil.

    Do your tests on bench, sucking and blowing into it, confirm the output varies accordingly, put a 10K instead of the relay coil and choose the zener for the operation/trigger point desired. All that should work. Until you use a relay coil. The current will flow into the relay, but will do nothing.
     
  12. anasasisxenophontis

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    Apr 18, 2010
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    When I took my readings, the pressure value that I chose to activate the solenoid produced about 150mA. Is that not enough to drive even a small relay?
     
  13. anasasisxenophontis

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    Apr 18, 2010
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    I've been reading about comparators, and if this is the more stable option, then I might as well go this direction. So this is what I understand... the comparator will take an input from a pre-set resistor as a reference voltage and another input from the MAP sensor. And if the voltage from the MAP sensor is greater than that of the pre-set resistor voltage, it enables the relay. Otherwise, it disables the relay. Is that about right? Also, this transistor that you mentioned, is this internal or the IC or would that need to be added externally to the circuit?
     
  14. JoeJester

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    Apr 26, 2005
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    What is the relay switching power to?
     
  15. anasasisxenophontis

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    Apr 18, 2010
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    A small pneumatic solenoid.
     
  16. retched

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    How small and what are the driving requirements?
     
  17. Externet

    AAC Fanatic!

    Nov 29, 2005
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    You are trying to make your life complex.
    You want to switch something 12V on at a certain pressure, buy a pressure switch

    http://www.amazon.com/NOS-15670NOS-Adjustable-Pressure-Switch/dp/B000A8N2CK

    You want it to switch something 12V on at a certain vacuum, buy a vacuum switch
    http://www.transmissioncenter.net/Vac3.jpg

    http://www.omega.com/pptst/PSW21_PSW22.html
    and forget about 5V MAF sensors, zeners, relays and the rest...

    They are used in cars to set the transmission TCC shift points, or to override the AC compressor at certain poor manifold vacuum under acceleration, to turn on a low oil pressure lamp... Abound at car parts stores.
    Turn its screw to set at wish and done.
     
    Last edited: Apr 19, 2010
  18. anasasisxenophontis

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    Apr 18, 2010
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    Wow, really... I feel retarded.
     
  19. retched

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    Lol. Well you learn something new everyday.

    But then again, google is your friend.
     
  20. anasasisxenophontis

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    Apr 18, 2010
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    Lol honestly. Solenoid, switch...done.
     
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