low voltage relay trigger

Discussion in 'The Projects Forum' started by ssnaples, Jul 14, 2012.

  1. ssnaples

    Thread Starter New Member

    Jul 14, 2012
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    I am having some trouble trying to trigger a relay. The only signal that i can find on my PCB has a very low voltage of around 0.7v when it receives a signal it jumps to around 1.4v. Here is the kicker.... the 0.7v signal and 1.4v signals are not constant. It seems to be some type of pulsating signal about every half second it goes from 0v to 0.7v and when triggered it goes from 1.4v to 0v every half second. So what I would like to do if possible is to trigger a relay somehow that can sense the voltage change and stays closed regardless of the pulse. Then when it goes back to 0.7v it would then open again. Sorry, I am a novice at any type of circuit. Any help in the right direction would be greatly appreciated. Thanks!
     
  2. wayneh

    Expert

    Sep 9, 2010
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    What are you really trying to accomplish? You may not need a relay or there may be an easier way to do the job.
     
  3. ssnaples

    Thread Starter New Member

    Jul 14, 2012
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    Basically what I have is a transmitter (i think it sends out an rf signal to the receiver when triggered). The receiver then responds with an audible alert of 3 beeps as well as a pulsating LED. I want to convert this audible/led signal to turn an antenna motor on/off. all i need to do to turn the motor on/off is to really turn the signal wire to the motor on/off. If you have any thoughts on a better way to do this, I am all ears. Thanks again for the help. I hope this was a bit clearer :)
     
  4. wayneh

    Expert

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    Once you turn the motor on, does it turn itself off, or do you need to control up/down and on/off?
     
  5. ssnaples

    Thread Starter New Member

    Jul 14, 2012
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    The motor is actually a car antenna that will start and stop on its on. When it is supplied with power, the motor will raise and when the power is cut it will go down on its own. I just need the signal (remote on/off)
     
  6. wayneh

    Expert

    Sep 9, 2010
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    By "power", you mean a voltage on a signal wire, right? The thing has separate wires for supplying power to the motor?

    And you need the signal wire to be either +12V all the time for up, or 0V for down? Does the remote send different signals for up and down, or does it toggle every time you press a button?

    Sorry to keep pressing with questions but no one can help if they don't know the details.
     
  7. ssnaples

    Thread Starter New Member

    Jul 14, 2012
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    You are correct. it is a signal wire that needs 12v to go up and 0v to come down. The motor has its own constant 12V supply. It toggles on/off with the press of a button
     
  8. WBahn

    Moderator

    Mar 31, 2012
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    So let's see if we are starting to get a sufficiently complete picture.

    You have a transmitter with one button.

    When you push the button, you want the antenna to go up if it is down or go down if it is up.

    When you are not pushing either button, you want the antenna to stay where it is.

    On the receiver, you have identified a signal that bounces between about 0.7V and 0V when the receiver is powered by the transmitter button is not being pushed. While the transmitter button is being pushed, it bounces between 1.4V and 0V.

    Here is what I am thinking.

    First cut (simple, but has some shortcomings that probably need to be addressed) is to have a latch circuit (a "toggle" flip flop - which can be implemented with a relay) that holds either 0V or 12V on its output (can be done by being open or closed, respectively). When it receives a command signal, it toggles state. The command signal is generated by detecting a pulse in excess of 1V at that point you've identified in the receiver.

    Here are the two problems I see right away: You need to keep it from issuing multiple commands each time it sees a pulse. In other words, there needs to be a mechanism whereby it behaves as though it recognized when the button is released. Here is a simple way to do that - once the command is issued, you ignore further pulses for some number of seconds. How long depends on how long you think someone might hold the button down for. The other thing you want to guard against is a bit of noise making the circuit think it has seen a pulse. This can be dealt with by requiring that the circuit see two (or more) pulses before it issues the command.

    You'll also want to consider what you want to have happen on power up and power down of the vehicle -- anything?
     
  9. ssnaples

    Thread Starter New Member

    Jul 14, 2012
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    Wbahn hit it right on the head. The problem is this pulse signal. How should I go about this?

    Create a circuit that is triggered if it detects a pulse in excess of 1.0v and ignore further pulses for say 3.0 seconds. Once the pulse drops below 1.0v the circuit is broken. It sounds like this should be fairly simple, but I have no clue as to how to set this up. I greatly appreciate all the help.
     
  10. wayneh

    Expert

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    Just thinking out loud here: There is such a thing as a missing pulse detector that you can build with a 555 timer. Drop the receiver signal across a diode, and it will be nearly 0V when the remote is off, and pulsing to about 0.7V when on. So "off" might be interpreted as continuous missing pulses, holding the 555 circuit in one state. When it sees "on" pulses, it would flip to the other state.

    I suppose a simple peak detector/integrator circuit (low-pass filter) would also give a voltage you could feed a comparator, to decide whether the remote trigger is on or not.
     
  11. ssnaples

    Thread Starter New Member

    Jul 14, 2012
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  12. wayneh

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    Yup, that even shows using the comparator to make a sharp cutoff between off and on.

    For the comparator step, you'll want a bit of hysteresis (via a feedback resistor) so that the comparator doesn't chatter as it's making the switch. With no hysteresis, you can get oscillation right at the point where the two inputs are at identical voltage. Also be aware that comparators often have an open collector output, meaning it can pull the output towards ground but it goes open in the high state. It needs a pull-up resistor on the output in order to go up to the supply voltage.
     
  13. ssnaples

    Thread Starter New Member

    Jul 14, 2012
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    Thanks for the input. I give it a go and see how it works out. Being the novice tinkerer that I am, is this something that you would be able to draw a schematic of?... just so that I get all of the bits and pieces in the correct order. If not, I understand and appreciate the direction.
     
  14. wayneh

    Expert

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    Here's the schematic from the tutorial. The stuff to the left is his clapper. That'll be replaced by your receiver signal. To the right he is using an op-amp as a comparator. You could also use an LM339 comparator, which you can usually find locally very easily. Just read the datasheet and it will show how to set it up as inverting or non-inverting (as shown), and how to add hysteresis (a 1M feedback resistor, for instance) either way. You'll be supplying it 12V (or pulling up the comparator output to 12V) instead of 5V in that schematic, so that the output can go up to 12V. Your reference voltage will be similar to what is shown, 1/11th of the supply voltage as shown.

    BTW, to set your reference voltage, you need a stable reference voltage, and automotive systems are not so good. You might want use a zener diode to establish a constant voltage source. I think I'd use a 7805 regulator to supply the circuit, but then use a resistor to pull up the comparator output to system voltage.
    [​IMG]
     
    Last edited: Jul 15, 2012
  15. ssnaples

    Thread Starter New Member

    Jul 14, 2012
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    Thanks wayneh, I have setup a circuit on a breadboard and am going to test it out as soon as I get some free time. Question for you though. Do I need to include the op amp in the circuit, or can I eliminate this and simply use the peak detector and comparator? It doesn't seem as though I need to increase the input voltage, I simply need a steady constant voltage and let the comparator do the work. Is this assumption correct? Thanks again, this website has been a lot of help.
     
  16. wayneh

    Expert

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    The op-amp is there wired as a comparator. You could use either an op-amp or a genuine comparator, whichever is easier for you. You don't need both, either will be fine for deciding between the on or off state.

    Personally, I'd use the LM339 comparator for one simple reason - the output can be pulled up to the 12v supply even if the IC itself is powered by a regulated 5V (which you'll need for setting the reference voltage anyway). The op-amp cannot put out a higher voltage than it is supplied, although I suppose there are easy ways around this too.

    Another reason to choose the comparator is that it can sense down to the ground rail and certainly at your reference voltage. That's not true for every op-amp (not for the TL082 available at Radio Shack, for instance), so you'd need to choose carefully.
     
  17. ssnaples

    Thread Starter New Member

    Jul 14, 2012
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    I tested out my circuit last night, and did not get the results I was hoping for. The incoming signal is pulsing from 0 to 0.7v before trigger and .7 to 1.4v after trigger. After the peak detector I am getting 0.6v constant before and after trigger. There is no change. When I measure the voltage at the LED (which is on) I get 6.7v. Something is definitely wrong in the wiring. The thing that confuses me the most is which rail to attach the various grounds to. Do I ground everything to the 12v battery, or to the source signal? I have attached a diagram of my circuit as it is on my protoboard. Any help in getting me in the right direction would be appreciated. This is my first circuit, so please be easy on me. I am confident and determined that we can make this work.:)
     
  18. wayneh

    Expert

    Sep 9, 2010
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    The top diode needs to be turned around, and the signal connected to its left-hand side (down one row).

    Why is the lower diode there? The current limiting resistor for the LED is 100K, that's too high. Try 1K. But you say it's lit? Shouldn't be, based on the diagram.

    Source and battery grounds should be connected.
     
  19. ssnaples

    Thread Starter New Member

    Jul 14, 2012
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    Thank you for all you input! I attached a revised version that I think pick up all of the mistakes that you caught. Thank you for hanging in there with me and all your advice. I will try it out this evening and let you know how it goes.
     
  20. ssnaples

    Thread Starter New Member

    Jul 14, 2012
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    Well it still doesn't appear to be functioning quite right. I adjusted the board as you had suggested. It seems like the peak detector is not working the way it should. I still get the 0.7v input and then after the peak detector I am getting a bouncing voltage of 1.5v to 2.2v. Could the capacitor be damaged? What size/type capacitor diode and resistor should I be using here? This seems so simple and straight forward, but I am unable to figure it out. Any help is greatly appreciated.
     
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