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Opinions on how to kill a signal — Parallax Forums

Opinions on how to kill a signal

generatorgenerator Posts: 7
edited 2005-04-17 23:30 in BASIC Stamp
I am working on two devices that communicate speed of a rotating shaft. The first device has some form of hall effect sensor.
The line that communicates the speed caries pulses that go from 0 to 5 volts. The faster the shaft rotates the faster the pulses. This data line (single wire) is the only wire which is accessable to me. My question is this. Can I kill this signal temporarily by shunting it to ground (perhaps thru a resistor)? Is there any possibility of damage trying this? I know I can interupt the circuit with a relay but I am trying to make it as simple as possible and without having to cut the wire. I was hoping to be able to get one of my pins on my Stamp to go low and shunt the signal to ground.

Any suggestions are greatly appreciated.

Comments

  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-04-07 20:23
    generator,

    ·· I don't know about killing the signal, but you could kill the Stamp pin that way.· Making a pin and output while another device is outputting data to it can destroy the pin and/or the other device.


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  • Paul BakerPaul Baker Posts: 6,351
    edited 2005-04-07 20:32
    Ok a simple transistor should suffice:



    attachment.php?attachmentid=37516
    Notice the resistor, this is to make sure you do not burn out the signal line if it is being actively driven by your sensor (if you know for a fact it is an open collector out then the resistor isn't needed but doesn't hurt to be safe). The output is taken between the load and the collector of the transisitor.



    Post Edited (Paul Baker) : 4/7/2005 8:36:14 PM GMT
    bmp
    bmp
    loadshunt.bmp
    13K
  • Beau SchwabeBeau Schwabe Posts: 6,557
    edited 2005-04-07 20:34
    Perhaps if we understood a bit more as to why you want to "shunt" the signal?

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  • NewzedNewzed Posts: 2,503
    edited 2005-04-07 20:39
    Generator, does this "data line" go to a Stamp Pin?

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  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-04-07 20:51
    Beau Schwabe said...
    Perhaps if we understood a bit more as to why you want to "shunt" the signal?
    Beau,

    ·· I was· going to ask if he could just ignore the signal, but then figured, he must have a reason.· But you're right, it would be nice to know.



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  • KenMKenM Posts: 657
    edited 2005-04-08 04:25
    Please educate me or correct me if wrong, but the resistor is needed to protect the stamp output. With no emitter resistor and........
    1. No resistor shown between the base and the stamp pin (as shown)
    2. When the stamp pin goes·high you are·essentially putting 5·volts from a diode to ground and·will instantly exceed the stamp max output current.

    Of course a simple solution if there is no emitter resistor is to use a base resistor.

    Am I all wet or ????

    Paul Baker said...
    Ok a simple transistor should suffice:
    ·
    ·
    ·
    attachment.php?attachmentid=37516
    Notice the resistor, this is to make sure you do not burn out the signal line if it is being actively driven by your sensor (if you know for a fact it is an open collector out then the resistor isn't needed but doesn't hurt to be safe). The output is taken between the load and the collector of the transisitor.
    ·
    ·
  • Beau SchwabeBeau Schwabe Posts: 6,557
    edited 2005-04-08 04:59
    The B-E junction of the transistor forms a diode. The circuit the stamp "sees" is ...

    I/O Pin >------>|-----/\/\-----GND




    Conventionally you would have a resistor on the transistor base so the Stamp
    would see...

    I/O Pin >------/\/\------->|-----GND




    Depending on the value of the resistor, the resistor in Pauls circuit will also
    provide a shunt to ground to meet the original request. If this resistor value
    is too low and sinks more current than the stamp pin can handle, then it would
    be a good idea to place a resistor at the base of the transistor to the IO pin
    that will limit the overall IO current to a safe level...

    I/O Pin >--/\/\------>|-----/\/\-----GND

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  • Paul BakerPaul Baker Posts: 6,351
    edited 2005-04-08 15:25
    If there were no resisitors in the figure I previously presented it would be bad, so the part I said about no resistor if output is open collector is wrong. The circuit I presented has an equallibrium point where the voltage between the emitter and base is set by the transistor Vbe(on) this means that the current flowing through the emitter is Ie=(5-Vbe)/R and the current flowing through the base is· I b = Ie/(b+1)· (where b is the current gain of the transistor) which yeilds Ib= (5-Vbe)/(R+Rb). By appropriately setting the resistance value, the current sourced by the stamp pin is kept within acceptable limits.

    One problem with the circuit is that the sensor does not see a true shunt of its signal, but this is not desirable as previously mentioned.

    I too am a little perplexed why the shunt is needed in this application.
  • Beau SchwabeBeau Schwabe Posts: 6,557
    edited 2005-04-08 15:48
    This circuit won't exactly "shunt" the signal from the sensor, but it will prevent the Stamp
    from detecting it.

    If the Stamp I/O pin#2 is made LOW and an output, then the Signal from the sensor is allowed to
    be read at Stamp I/O pin #1

    If the Stamp I/O pin#2 is made HIGH and an output, then the Signal from the sensor is blocked
    and can not be read at Stamp I/O pin #1 (will always read HIGH)

    if you want this operation reversed, then reverse the DIODE and in this case,

    If the Stamp I/O pin#2 is made HIGH and an output, then the Signal from the sensor is allowed to
    be read at Stamp I/O pin #1

    If the Stamp I/O pin#2 is made LOW and an output, then the Signal from the sensor is blocked
    and can not be read at Stamp I/O pin #1 (will always read LOW)


    Signal from Sensor >------>|----o--------> Stamp I/O pin #1
                                |
                                o--/\/\--> Stamp I/O pin #2
                                   4.7K

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  • generatorgenerator Posts: 7
    edited 2005-04-17 04:12
    I am sorry for the late response: I have been away

    Anyway I appreciate the answers. ICeMAN & Chris, to answer your question:

    Imagine this scenario: An active sensor in one room and a Processing unit in another room. The active sensor has its own power source. The Processing unit has it own power source. The only common link between the two rooms is a data line and a common ground. The data line is used to count pulses. I want to disrupt the flow of the pulses midway in the simplest fashion. I could use a relay to interupt the data line but then that means the added expenses of building driver ciruits etc. I was envisioning a simple way to ground or shunt the pulse to ground by sinking it thru a resistor to a stamp pin. I like the transistor idea that Paul put up but I was hoping to do it with just one wire. Using a relay or transistor means additional wires to interupt the circuit rather than one wire to shunt the whole line to ground. I know my request seems insane but I would have to do this mod to many units and the easier it is the better.
  • Bruce BatesBruce Bates Posts: 3,045
    edited 2005-04-17 05:12
    Generator -

    Just another one part thought would be a 5 VDC reed relay with a built-in protection diode.
    Typical would be the Hamlin HE700 Series as shown here:
    http://www.hamlin.com/index.aspx?tabindex=2&tabid=8&mid=223 (click on picture)

    Regards,

    Bruce Bates
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-04-17 16:08
    generator said...(trimmed)
    I am sorry for the late response: I have been away
    Anyway I appreciate the answers. ICeMAN & Chris, to answer your question:
    Imagine this scenario: An active sensor in one room and a Processing unit in another room. The active sensor has its own power source. The Processing unit has it own power source. The only common link between the two rooms is a data line and a common ground. The data line is used to count pulses. I want to disrupt the flow of the pulses midway in the simplest fashion. I could use a relay to interupt the data line but then that means the added expenses of building driver ciruits etc. I was envisioning a simple way to ground or shunt the pulse to ground by sinking it thru a
    Okay,

    ·· You explained in more detail what you're trying to do, but I guess I am either missing something, or just don't see why the receiving end can't just be programmed to ignore the pulses when you don't want them.· For example...If you have a Stamp waiting for data on an input pin, and you want to stop doing that for awhile, you just ignore the pin.· The data can do whatever it wants, but you're not inputting it anymore.· I just don't understand the reasoning behind having to supress the signal.· That's what I was curious about...Why you'd need to do that exactly.· =)


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  • Paul BakerPaul Baker Posts: 6,351
    edited 2005-04-17 17:54
    Ok, I won't second guess your decision on a hardware inhibit. The best part for this application would be an analog switch, which is nothing more than an NMOS and PMOS mosfet connected in parallel with the gates tied together (and the gate signal to the PMOS· gate is inverted), this permits bidirectional signals through the switch depending on the gate control signal.·
  • KenMKenM Posts: 657
    edited 2005-04-17 18:21
    Generator,

    Do you want to eliminate the signal via software control based on various inputs?
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-04-17 23:30
    Paul Baker said...(trimmed)
    Ok, I won't second guess your decision on a hardware inhibit. The best part for this application would be an analog
    Paul,

    ·· I was trying to save everyone trying to come up with a hardware solution if it's not really necessary.· I just don't see why the signal can't simply be ignored by the receiving device.· Having the answer to this question might also provide key information to the way the signal might need to be interrupted.· There might be factors here we don't know without the whole story.· I tend to try to get the whole story before I offer solutions.


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