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Need help Transistor Touch Sensor

Discussion in 'Sensors and Actuators' started by Any, Jan 27, 2015.

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  1. Any

    Any

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    Jan 27, 2015
    Hello!
    I am a new member and I need some help with a very small and easy circuit.
    In fact I want it to be as simple as possible.
    I have a battery, 3.0V and 30mA, a LED ~ 3.0V and also 30mA.
    My first question is: do I really need to put a resistor?

    I want the circuit to be activated with a simple finger.
    I saw that we can create a simple touch sensor with a Transistor NPN.
    I also saw that we can increase the output of the transistor by using a Darlington Pair(but I tryed and didn't notice any difference)

    When I power the LED with the simple battey, nothing more, this is working fine and the LED works pretty well.
    When I put the transistor and my finger, it also works, but the LED bright much less, why?
    I don't understand why the Transistor would drain some power or current? I am a total noob in electronics sorry.
    My circuit looks like that but I don't use any resistor and my source power is 3V.
    http://www.learningaboutelectronics.com/images/Touch_sensor_schematic.jpg

    I precise that I am using a random NPN transistor, I just baught the first I saw, this is a BC547.

    Does the light bright less because of the caracteristics of the transistor? Is there a transistor which correspond to my requirements?

    I would like to activate the LED with a touch sensor like this, but I want the LED to bright as much as if I power it directly with the battery, is it possible?

    I thought about temporary switch, but I really really want touch sensor.

    If nobody can help me, do you think there is any other forum or person who can help me?

    Thank you in advance. :)
    PS: Escuse my english, this is not my nattive language.
    Don't hesitate to correct me.
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Hi, let me answer one very important question first. You must always have something to limit the current through a LED. Typically you use a resistor.

    Next we get into some practicalities. It is far easier if the available voltage exceeds the LED voltage, both because it allows you to use a series resistor for current limiting, but also (and very importantly in your case) because there will be some voltage drop across any transistor used to switch the LED on and off.

    One other detail is that batteries are not typically supply a particular current. They will supply a current which varies with the resistance of the load. In addition its voltage will also drop as the load current increases.

    The practical upshot of this is that for very small (think hearing aid) batteries no resistor may be needed. I will assume this is not the case here though.

    If you can use a red LED you can more easily achieve what you are after because they typically require about 1.8 volts.
     
  3. Any

    Any

    5
    0
    Jan 27, 2015
    In fact, I want to use CR1216, which are pretty small.
    To theoraticly I don't need resistor right?
    How could I know which battery needs a resistor?

    Thanks
     
    Last edited: Jan 28, 2015
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,396
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    Jan 21, 2010
    Well, even with this battery you will likely overdrive the LED. This will shorten the life of the LED but it will probably not shorten it to less than the life of the battery.

    This still leaves you with the problem of switching the LED. It could probably be solved using a litre voltage power MOSFET. Using a high value resistor to hold the MOSFET off, touching a pair of contracts between the gate and the other power supply rail would probably switch the MOSFET quite easily.

    Beware that MOSFETs are static sensitive and you will need one with a low Vgs(th).

    I'm not in a position to draw a circuit diagram our recommend a most right now but I may be able to later today.
     
  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Hi there and welcome to Electronics Point :)
    You seem to think that because these numbers are the same, that you can connect them together and they will work. That's not necessarily true. Those numbers are voltage and current values, but they mean different things for the LED and for the battery.

    For the LED, 30 mA is probably the recommended or maximum continuous current that you're supposed to pass through the LED, according to the manufacturer. The actual amount of current the LED will draw at any time depends on how much voltage you apply across it. Here is a typical voltage vs. current graph from the data sheet for a white LED.

    Typical LED Vf vs. If graph.png
    This graph shows that as you increase the voltage across the LED (the "forward voltage", VF), the LED draws more current (the "forward current", IF). This graph shows that this particular LED typically draws 20 mA when 3.2V is applied across it, and 30 mA at about 3.4V.

    Since an LED's brightness is roughly proportional to the current flowing through it, LEDs are normally specified to operate at a particular current. That particular LED, a member of the Cree C503C family, is specified at 20 mA operating current. You can see from the graph that this corresponds to a VF of 3.2, but that graph is only for a typical device. Because of manufacturing tolerances, the actual VF can vary over quite a wide range - it may actually be as high as 4.0V! This is shown in this table from the data sheet:

    Typical LED characteristics table.png

    If the part that you happen to buy needs 4.0V before it will draw 20 mA, and you provide only 3.2V to it, it may only draw 10 mA, or 5 mA, or even less. And it will be noticeably dimmer than it should be.

    Conversely, if the part you buy will draw 20 mA at only 2.8V and you connect 3.2V to it, it may draw 30 mA or 40 mA or more! It will glow brightly, which is nice, but its lifespan will be greatly reduced. It may even overheat and go into thermal runaway (Google for more information).

    This is why LEDs are operated at a particular current, not a particular voltage, and it's why some kind of current regulation is used - either a current regulator, or a simple resistor.

    And it's not just manufacturing variations that affect the VF vs. IF characteristic - temperature affects it as well, and so does aging, slightly.

    I hope that helps you understand the relationship between voltage and current in an LED, and why you should not apply a fixed voltage to them.


    Now to the CR1216 cell. This is a lithium coin cell with a nominal terminal voltage of 3.0V and a rated capacity of 34 mAh (at least, that's what Energizer's one is rated for) with a terminal voltage drop to 2.0V.

    A capacity rating of 34 mAh is an energy rating, not a current rating, and it does not translate into a current rating of 34 mA. The cell will supply whatever current you draw from it, up to a certain limit. That cell is actually designed to be used at a much lower current drain.

    The 34 mAh specification means that if you draw 34 mA from the cell, it will provide that current for one hour. During that time, its terminal voltage will drop from 3.0V down as far as 2.0V, at which point it is considered exhausted or "flat".

    So if you want to draw 30 mA from that cell, firstly you will only get one hour of operation from it, and secondly, over that period, its terminal voltage will drop very significantly.


    Do you understand the potential problems with your idea?
     
  6. BobK

    BobK

    7,682
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    Jan 5, 2010
    Another case of battery abuse. If you look at the datasheet for the CR1216, it's rated capacity is at a draw of 0.046mA. 30mA would be 650 tines higher than this! It is also specified at 2.8mA pulsed for 2 seconds every 2 hours. Again way below the 30mA continuous that you want.

    http://data.energizer.com/PDFs/cr1216.pdf

    Battery capacities do not scale linearly when the current draw is well above the specifications. I doubt that the battery would put out 30mA for more than a few minutes.

    If you want to run an LED for any time off a coin cell, use a super bright LED and run it as about 1mA. These can actually be pretty bright at that current.

    Bob
     
    KrisBlueNZ likes this.
  7. Any

    Any

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    Jan 27, 2015
    I'm quite lost now...
    I really want to use this battery CR1216, so suppose I can't change it. I want to use the Smd Led 3014 or maybe 3020 but it doesn't change much things.
    The led will be powered only for few seconds everytime, like 20seconds will be the much longer time. Is there a way to power the led properly with this prerequisite. I really don't care about the life time of the Led or the battery, this won't be used for years. Maybe powered few times a day and few day in a month max. Should I use push button to power it as I want?
     
  8. BobK

    BobK

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    Jan 5, 2010
    I made a suggestion of using a more efficient LED, Steve suggested using a red LED and a MOSFET for the switch. This combination of ideas should get you to a workable solution.

    Edit: And a resistor to limit current. For a red LED with typical forward voltage of 2 to get 1mA you would want use 1KΩ.

    Bob
     
  9. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Personally I would try that and see how it goes. Use the battery's internal resistance to limit the LED current. Just connect a pushbutton between them. If the LED is a lot brighter than you need, add a resistor - try 10Ω to start with; increase if the brightness is still too high and vice versa.

    This isn't the "proper" way to do it; it may reduce the LED's lifetime and it will definitely reduce the battery's lifetime to a lot less than an hour, but this is exactly how it's done in cheapo miniature torches. It's not like you're designing a mission-critical product!
     
    Arouse1973 likes this.
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Re Kris' reply: It is also the principal behind LED "throwies". They can have a surprising life because the current falls as the batter flattens and the LEDs remain visible even at low currents
     
  11. Any

    Any

    5
    0
    Jan 27, 2015
    So to conclude, I can try to use a LED with a smaller voltage?
    Or use a MOSFET? Which I don't really understand, would you mind just explain to me a little?
    Doesn't MOSFET have a big resistance?

    I can't find any SMD super bright LED which consume 1mA...

    Or the push button right?
     
    Last edited: Jan 30, 2015
  12. BobK

    BobK

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    The superbright LEDs are still usually rated at 20mA, but are insanely bright at that level.

    MOSFETSs can have very low on resistance, 100mΩ or less. At 1mA, this would be a loss of only 100uV.

    Bob
     
  13. Any

    Any

    5
    0
    Jan 27, 2015
    And what should I look for in the specs of the Led to be sure it is super bright?
    Because for LEDs SMD 3020 or 3014 they are all the same.
     
    Last edited: Jan 30, 2015
  14. BobK

    BobK

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    Jan 5, 2010
    Unfortunately that is not so simple. The brightness and viewing angle are inversely correlated. So an LED with 10000mcd but a viewing angle of 15 degrees will look very bright, but only if you are looking straight at it. How is this LED to be used? Should the viewing angle be wide, or is a narrow angle okay?

    Bob
     
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