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Interfacing with 12-14 volt car signal — Parallax Forums

Interfacing with 12-14 volt car signal

generatorgenerator Posts: 7
edited 2004-09-09 03:55 in BASIC Stamp
What is the simplest way to monitor a 12-14 volt automobile signal with a BS1 stamp. Some suggest a voltage divider. Some suggest a 7805 regulator. I want the simplest but SAFEST way to monitor a switched line in a vehicle.

Sorry for the noob question. I've been out of the loop for some time.

Comments

  • allanlane5allanlane5 Posts: 3,815
    edited 2004-09-06 22:45
    I would tend to put a 22 Kohm resistor in series with the signal.
    The PIC has internal protection diodes which should shunt
    any excess voltage to ground, as long as you keep the
    current low.· Using a 22 Kohm resistor will keep the current
    low enough for the protection diodes to do their thing.

    Note that a car's 12V line is notoriously noisy, so you may
    need a low-pass filter (capacitor with resistor) to reduce
    some of the hash.
    ·
  • generatorgenerator Posts: 7
    edited 2004-09-07 00:00
    Is this true even when the voltage could reach levels as high as 14.5 to 15 volts. I was so under the impression that the stamp was very fragile when handling voltages out of spec. In fact when taking the indutrial design course that they dedicated one complete chapter to conditioning the signals for the stamp. They recommended using buffer ic's (i think they were op amps).

    When using this resistor in series with the signal line will the internal diode fail over time. What is the max voltage that the input can take before I let the magic smoke out?

    Also do you have any suggestions for cap and resistor values to filter engine noise?
  • KenMKenM Posts: 657
    edited 2004-09-07 00:13
    I am not sure about the diode in the PIC that was mentioned by allan, but from his previous posts I have read in this forum, I would trust what he said.....and the idea does sound feasable.
    An·alternative......, but not as simple as the resistor mentioned by allan.

    Get a 2n3904 transitor. Connect the emitter to ground.
    Connect the collector to the stamp +5 volt line THROUGH a 10k ohm resistor
    Connect the collector to the stamp input pin.
    Connect the base to the cars 12 volt line THROUGH a 47k ohm resistor.
    When 12 volts is present, the stamp input will read 0 volts, and visa versa.



    generator said...
    Is this true even when the voltage could reach levels as high as 14.5 to 15 volts. I was so under the impression that the stamp was very fragile when handling voltages out of spec. In fact when taking the indutrial design course that they dedicated one complete chapter to conditioning the signals for the stamp. They recommended using buffer ic's (i think they were op amps).

    When using this resistor in series with the signal line will the internal diode fail over time. What is the max voltage that the input can take before I let the magic smoke out?
    Post Edited (KenM) : 9/7/2004 12:19:07 AM GMT
    stamp12vmonitor.gif
    1005 x 1268 - 12K
  • generatorgenerator Posts: 7
    edited 2004-09-09 01:58
    Thank you both for your excellent suggestions. I have the circuit working. Thanks again.
  • KenMKenM Posts: 657
    edited 2004-09-09 02:01
    What was your final solution?
    generator said...
    Thank you both for your excellent suggestions. I have the circuit working. Thanks again.
  • generatorgenerator Posts: 7
    edited 2004-09-09 02:27
    I used the 2N3904 method. While I am sure the other method is acceptable, I think the safety features of the stamp should be kept as safety features and not as a usable feature. In a pinch I would do it but I think I can throw caution to the wind and spend the $0.29 cents on a transitor

    If I am wrong about the safety feature then please let me know.
  • Jon WilliamsJon Williams Posts: 6,491
    edited 2004-09-09 03:55
    If you're concerned about over-voltage you can, of course, use a divider. Connect a 10K to the + side of your circuit, connect it to a 4.7K which goes to ground -- the junction goes to the BASIC Stamp. At 12 volts across the divider you'll get about 3.8 at the junction. As indicated though, you're likely to get a bit more and yes, you probably want to filter the input, as well as the supply to the BASIC Stamp.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Jon Williams
    Applications Engineer, Parallax
    Dallas Office
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