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Homework Help: Alarm circuit using conductive ink

  1. May 20, 2009 #1
    I am a graphic arts student attending a class of "New printing technologies", which
    aims to introduce us to RFID and printed electronics in general from a designer's point of view. However, we have been assigned homework which requires knowledge of electronics and apart from the conductive ink part, is about conventional electronic circuits. We have no training in electronics and I only know how a capacitor or inductor works from high school.

    1. The problem statement, all variables and given/known data

    First, electrical conductance is being introduced to us along with its units, as G=1/R=I/V.
    The problem follows. We have a conductive ink whose conductivity is proportional to ambient temperature according to formula G=kT (k is not given).
    We are then asked to make a circuit which will function as a fire detector. Then we have to connect this circuit to a fire sprinkler so that it can activate it.
    We can use capacitors, inductors and LEDs.

    2. Relevant equations

    Also mentioned above

    3. The attempt at a solution

    I imagine that when there is not a fire, ie when conductivity is below a specific threshold, I
    need to isolate the led/sprinkler from the rest of the circuit. When conductivity reaches the
    threshold the circuit is supposed to activate. I thought that a capacitor discharge could help with the activation but I don't know exactly how to connect the components to achieve such behavior. A friend of mine has told me that a transistor could function as a switch and that it might help, but I think that we are not supposed to use a transistor because it is not mentioned in problem statement. Moreover, nothing specific about the sprinkler is mentioned, eg how it activates etc, and no other information is provided.

    That's all I can think, I don't know if it counts as an attempt at a solution but I can't write down anything specific with the limited knowledge I have, only a vague idea and thoughts (which I'm not even sure if they're valid).

    I would appreciate it if you helped me, I don't know where else I can find help (the professor is out of the question as it is more likely that he confuses me more instead of helping me)

    Thank you very much for your time.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. May 20, 2009 #2


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    Staff: Mentor

    Welcome to the PF. The problem as stated would be challenging for an EE! So there must be some simplifying assumptions we can make, or some other info that we can use.

    For example, how is the sprinkler activated? Is it with a voltage-level input? (Like 0V input is off, and 5V input is on.) It's hard to believe you are only allowed to use ink, capacitors, inductors and LEDs. That's going to make it pretty hard to build much of a circuit. No voltage source, no resistors, no transistors?

    Also, most conductors (like the ink you are working with) have a positive temperature coefficient of resistance. That is, the hotter they get, the higher their resistance. So the k in your conductance equation will most likely be negative. Just something to keep in mind.

    EDIT -- oh, I see you already address the activation of the sprinkler, and that no info is given in the problem statement. Do you think you could get full credit making some assumptions about that input characteristic?
  4. May 20, 2009 #3


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    Staff: Mentor

    Specifically, if there are no voltage sources in the circuit, there can be no energy available to sense the ink resistance and do anything with it. It is possible to use a bunch of LEDs like photocells, and generate a very small amount of power. But that assumes the lights are on, of course, which is also not stated in the problem statement.

    I'd recommend pushing back on the person giving this assignment. The problem is under defined.
  5. May 21, 2009 #4
    Sorry for my delay answering and thanks a lot for your help! We have a rather big time difference so please forgive me if I don't answer right away.

    I suppose we will get full credit as I can't see how we could solve the problem without making the assumptions you mentioned. Moreover, this class gives me the impression that it works that way as most homework is ambiguous and under defined, as you pointed out.

    About how the sprinkler is activated, your suggestion is what i also had in mind. Zero voltage for off and some voltage for on should be fine and I think it will be acceptable as long as it works.

    You are absolutely right that the problem is under defined. What's worse is that the people in charge of this class are not well organized and it's not easy to get a clear answer from them. I will however attempt to ask them all the issues you mentioned. If I don't get a satisfactory answer, I don't have a choice other than to make all the assumptions mentioned by both of us.

    The energy could be provided by a voltage source. In previous assignments which did not mention anything about the energy source we were expected to use a battery. So I suppose that even if it's not suitable for real applications we can use a battery or some other dc voltage source. Judging by their behavior I got that they don't mind so much about the source.

    I guess that we can use resistors even if it's not mentioned in the statement. One reason that I can think against using a resistor is that maybe we are expected to use the ink itself as a resistor, but I don't know enough to assume that this can lead to a circuit that works correctly. I will attempt to ask them about that. About transistors, it is not mentioned in the statement but I will ask them about that as well.

    One thing to mention is that as long as we are not given any specific values like voltage activation, source voltage etc, I guess that just a design of how the circuit works in general would be sufficient. I don't think it's necessary to give specific values for eg capacitance etc. I have thought about a general plan in my mind, a battery connected with the ink and the sprinkler and LED on the other side. The problem of course is what to put in between.

    You are right about that, but the statement says that conductivity 'increases' as a function of the temperature. That of course does not exclude the possibility that the increase is negative, actually a decrease, but I think that they mean that k is positive. The way the statement is phrased is not clear if one thinks about what we mean by saying 'increase' in physics, but I'm not at all sure if they mean it that way or it's just the everyday use of the word. I will ask them about that as well, in case it affects circuit design.

    Again, thank you very much for your time and forgive me for the long post.
  6. May 24, 2009 #5
    I have asked for clarification on the issues we were discussing about and got some answers.
    We are free to choose any power source we like. A battery is acceptable.
    Resistors and transistors can also be used.
    The sprinkler can be activated in any way we see fit, as long as it is applicable in real circuits. So I guess the activation method you mentioned above is fine.
    Finally, k is positive.
    And generally, we can use whatever else we want and make any kind of assumption about how things work, as long as the final result works.

    I imagine that a transistor could be used, but the only thing I know about them is what I have been told by a friend, that they can function as a switch. Am I on the right track? I however don't have a clue about how I can use the transistor so as to make a correctly functioning circuit.
  7. May 26, 2009 #6


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    Staff: Mentor

    Ah, that makes much more sense. Glad the tempco is positive!

    So to get your thinking started more, look at the following links to get some info:

    Resistor dividers (use your ink for one resistor, and a real resistor for the other part of the divider, to get a voltage that varies with the ink's temperture):


    You can amplify that voltage, and maybe put it through a comparator, if that's allowed:


    The final drive to the sprinkler input can be a common-emitter transistor stage:


    Reading through those links should get you started. Post more questions as you get farther along.
  8. Jun 18, 2009 #7
    Thanks a lot for your help. I didn't have enough time, so I studied the material you suggested and tried to make a circuit using a circuit applet (http://www.falstad.com/circuit/). I tried to make the circuit work using all the components you mentioned but failed to understand how the transistor works. I however found that the circuit worked using only the voltage dividers and the comparator, without the transistor. Now I suppose that maybe this is not good enough for a real circuit, but at least it worked with the applet!
    My work was graded 6.5/10, not bad for something which I hadn't had any idea about before.
    Again, thanks a lot, I wouldn't have done the exercise without your help.
  9. Jun 18, 2009 #8
    Brousing around, I see conductive inks of both silver and carbon. I imagine the carbon based inks would have a positive temperature coefficient.

    I don't know how the problem is presented but it may be more directed to the methods of lithography. If so, deposition thickness is constained by the recommendations of the manufacturer. Trace width and length are the variables under your control.
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