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12v relay on 3v ??

Discussion in 'Electronic Basics' started by ahonda55, Sep 2, 2005.

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  1. ahonda55

    ahonda55 Guest

    Hello , please help me , i have small rquest , i need a simple circui
    that drives a 12v relay with an input of 3v or less. i mean when th
    circuit input reachs about 3v the output of it becoms 12v and unde
    the 3v the output is 0v
    may be using transistor or using LM741 ( i don't know how)
    any ideas or drawings will be aprecciated

    Plz help ..

    thanks a lot :
  2. On Fri, 02 Sep 2005 03:35:13 -0500,
    What supply voltages are available to you? Where does the 3V signal
    come from?

  3. ahonda55

    ahonda55 Guest

    Thank you Jon for replying me

    I made an RPM for my car using LM2907 , it's output is 1 to 3 volt
    i need to make this output drive a 12v relay , but the main problem i
    that i need to drive the relay on exactly > 3v , i mean 3v , 3.1
    3.2 so on
  4. Ralph Mowery

    Ralph Mowery Guest

    What you need is a comparitor circuit. Most easially made by using an op
    amp. Put a 3 volt refferance (can be set by a pot) on one input and feed
    the 0-3 volts to the other input. I don't remember which way it is right
    off, but you should be able to find the simple circuit. When the voltage
    becomes greater than the refferance voltage , the op amp output will go from
    about 0 to the supply voltage. You can use this to drive a low power relay
    or more likely a transistor and then the relay.
  5. On Fri, 02 Sep 2005 19:35:26 -0500,
    Have you examined the various output configurations on the data sheet?
    It appears to me that the output transistor may very well be able to
    drive your relay (it handles up to 50mA and up to 28V on the
    collector.) I imagine that with the right voltage being provided on
    the inverting input (3V), you are just about there. (You can also
    adjust the generated Vs you are comparing with, if some other voltage
    threshold would be more convenient to use, I think.)

    What's the difficulty with the illustrated output configurations on
    the data sheet?

  6. All that is pretty much already in the LM2907. Take a look at it.

  7. Jamie

    Jamie Guest

    you don't specify the source voltage for the 12 Relay so i will
    assume that its 12 volts and you need 3 volts control signal to
    close it also, you didn't specify the amount of current the
    relay coil consumes? so i will assume it to be aprox. 250 ma's for
    using a 2N2222 type transistor.
    collector to one side of the relay coil, Emitter to ground.
    330 ohm resister in series feeding the base.
    the other side of the relay coil going to the 12 Volt source,
    and a simple 1N4001 type diode across the relay coil connections with
    the Cathode (line side) connected on the side that is connected to the
    12 volt source! this is for protection.

    apply the 3 volt signal to the other side of the resistor.
  8. ahonda55

    ahonda55 Guest

    thank you all dear friends :
    i have made the transistor - relay circuit , it is working very goo
    but there is a small problem , i wanted to adjust the relay ope
    i mean that i need to make the relay open on 3v or 2.5 etc
    i got a 100k variable risitor but it BURNED because of the rel
    what is the best way to make the open voltage for the relay variable
    i mean to control the 3v or less , where should i put the variabl

    thank you very very very much for helping me , and sorry may be i a
    confused cuz i am still biginner ;
  9. On Sat, 03 Sep 2005 19:35:23 -0500,
    Can you show us what you are doing now? Write up a description of how
    you are connecting up the LM2907? I think you have everything you
    already need in the LM2907 without burning up parts!

  10. ehsjr

    ehsjr Guest

    The single transistor circuit is no good for what you want.
    Go back and read Ralph Mowrey's post of 9/2. He correctly
    identified a comparator circuit, and anticipated you wanting
    to adjust the voltage. Here is a schematic and explanation.
    I went into a lot of details - I apologize if I mention
    material you already know..

    0 + 12 V
    | | | |
    | | [Relay] [Diode]^
    | | | |
    | - |\ | +-------+
    [POT]----| \| |
    | | \ /c TIP 120
    | |OP }---[R]---|
    | | / \e
    Input-- | -----| /| |
    | + |/ | |
    | | |

    R is a 2200 ohm resistor. OP is an op amp, like an LM324.
    (Many different op amps will work - the 324 is very popular)
    The plus and minus signs to the left of the op amp refer
    to whether the output voltage will be inverted as compared
    to the input voltage. - means inverted, + means non-inverted.
    The banded end of the diode, a 1N400x, connects to +12 (as
    indicated by the ^ symbol). The potientiometer can be 10K ohm
    or higher.

    When your input voltage is goes positive with respect to
    the voltage set by the pot, the output of the op amp
    goes positive (close to 12 volts). The transistor is an
    NPN, which means it needs positive on its base to turn
    on, so the relay is energized. When your input voltage
    goes negative with respect to the voltage set by the
    pot, the output of the op amp goes negative (close to
    0 volts) so your relay is de-energized.

    To set it up, apply exactly +3 volts to the input. Then,
    carefully adjust the pot until the relay just energizes.
    Once you're sure you have it set properly, you can use it.

    Later on, once you have this circuit working, it can be
    improved to avoid relay "chatter" which will occur if
    your +3 volt input single is not stable.

    The reason a transistor is included is that the op amp
    cannot provide enough current to energize most relays.
    The transistor needs only a tiny current from the op amp
    to control a large current for the relay.

    To discover which pins to use on the op amp IC,
    Google for the datasheet. For example:
    I found that by putting "lm324 datasheet" in the
    Google search box. On the search results, I clicked
    on the first one and got to the link above. On that
    page, I clicked on DOWNLOAD to see the datasheet.
    It shows that the LM324 is actually 4 op amps inside
    one chip. I strongly recommend you use the LM324
    If you use the first op amp in the IC, the output
    is pin 1, the - input is pin 2, and the + input
    is pin 3. +12 volts connects to pin 4 and Gnd connects
    to pin 11. Identifying the pins: Set the IC on a table,
    standing on its pins, oriented like this:

    14 13 12 11 10 9 8
    | | | | | | |
    | |
    ) LM324 |
    | |
    | | | | | | |
    1 2 3 4 5 6 7

    The end with the notch is always the pin 1 end.

    I would recommend you start with a breadboard like this:
    You can use it over and over again, experimenting with
    all kinds of circuits. Once you are sure you have a
    circuit right, you can put it on a PC board like this:
    or similar.

  11. I am curious why you think that the LM2907 doesn't already have the
    necessary components for the OP's need. It already includes an opamp
    that can be used as a comparator (with many examples provided on the
    data sheet) and a 50mA floating transistor at its output.

    By choosing the C and R connected to the node common to the
    non-inverting input and the charge pump at pin 3, the OP can select
    the desired F->V conversion desired and presented for comparison to
    the opamp. By selecting the resistor divider, the inverting input can
    adjusted to set the switch point. Another resistor can set the
    desired hysteresis, if any. Finally, the load (relay) is connected
    between the automotive supply rail and the internal floating
    transistor collector (emitter tied to ground, for example.)

    Under this circumstance, assuming that the transistor is capable of
    handling the relay (and I don't know that it cannot, at this point,
    with the data sheet specifically mentioning that it is designed to
    directly drive relays), it should be done with what's already there in
    the LM2907 except for a few external passives.

    What do I miss, here?

  12. ahonda55

    ahonda55 Guest

    Hey guys , you are impressing me with your replys. you are talking a
    if it is your circuit not my circuit , thank you very much thank yo
    , really thank you

    i made this .
    i know that LM2907 has a comparator in it , but there is two problem
    with me , the same problems with LM324 .
    1- i can't find the 8 pin version of the LM2907 here
    2- i have the data sheet of many IC's but i don't know how to realiz
    the circuits mentioned there
    i have LM741 , will it work ? if yes would anyone show me a circui
    for my usage
    i think i need so much time to "design" a circuit , so plz do it fo
    special thanks to ehsjr for his great work like all of you
    but i don't understand your diagram , i am not proffeccional like yo
    if you can plz draw a simple circuit for LM327 or LM741 the mos

    Thank you[/size
  13. Jamie

    Jamie Guest

    for your project , you can use a 741.
    Vcc = 12 volts from your source which i think is pin 7 but don't quote
    me on that.
    pin 4 goes to ground.
    Op-AMP output goes to your 470 R
    voltage divider on the - input of the OP-AMP set at your desired
    trip point. 2 Resistors in series makes the divider and to get
    3 Volts you need 1/4 of your 12 volts.
    something like 7500 for the high end and 2500 for the low end to
    ground, the tap connects to the - input.
    the + input should have a small Resister in series for noise reasons
    that will lead to your tach output signal..

    basically all you need to do is get the - input set at your trip point
    level and use the + input from the source ref of your tach..
    the 741 does not have a lot of gain how ever, the relay will help
    prevent chatter.
    that is the simplest way i can explain it.
  14. ehsjr

    ehsjr Guest

    I said nothing whatsoever about the LM2907.

    It already includes an opamp
    I don't know what you're missing, if anything. However, I
    doubt the OP can follow the datasheet and work out the
    circuit without help. The OP said "i am confused cuz i am
    still biginner"

    I think an explicit schematic with values and description
    was necessary, based on that. I think guessing about
    whether the chip can drive some unnamed relay is a bad
    thing, which could lead to destruction of the existing
    circuit. In fact, any changes to it could do that, due
    to errors that a beginner should be expected to make.
    The OP already experienced a burned out pot, although
    (I assume) not in the original circuit. Somebody proposed
    a one transistor circuit, apparently overlooking the 3
    volt requirement as the OP intended it. That is what led
    eventually to the burned out pot.

  15. No, you didn't. That's what confuses me. Here is a post from the op:
    Do you see the mention of the LM2907??
    Well, go back and read that post from the OP and the others and help
    me understand. I think an LM2907 is being used. But?? I'm willing
    to be wrong about that.

  16. You have not provided enough detail of your RPM circuit for any of us
    to be clear about what this extra circuit must do. But, as Jonathan
    has already suggested, it seems likely that you can achieve all you
    want from the LM2907 itself. Presumably you have looked at a datasheet
    and built your RPM circuit using that? For example, there's a 'Minimum
    Component Tachometer' described on page 7 and 8 of the datasheet here

    For me, one confusing part of your description is "it's output is 1 to
    3 volts." Do you mean that represents the rpm range you want to cover?
    Are you perhaps trying to activate an alert when the rpm ('speed')
    reaches a certain maximum level?

    It may well be that the simple relay driver you've shown (which looks
    fine on its own) will indeed do the job for you. But I can only echo
    Jonathan's last recommendation: draw your circuit and show us!



    I've never used an LM2907, but I have a couple of problems with the
    above datasheet.

    1) The +Vout figure of 67 Hz/V for the typical circuit shown at the
    top of page 8 seems wrong. Using the formula given on page 7, I make
    it 670 Hz/V. (That would also be a lot more practical for anything
    other than steam engines!)

    2) No component designations are given, which IMO is unacceptable
    carelessness in a datasheet by a major supplier. However, it's fairly
    obvious what R1, R2, C1, C2 are (which is how I worked out the 670
    Hz). But what is 'I2' in the formula for max input f? Is it a
    reference to 'Output Current'? If so, how is the formula applied in
    practice pleaser
  17. On Sun, 04 Sep 2005 19:35:14 -0500,
    Looked at it, but it isn't your complete circuit. I'd like to see the
    rest of what you are doing with the LM2907.
    So you are using the 14 pin version. I don't think this matters
    except for pin designations.
    They provide the equations needed. What is confusing you? Do you
    need a complete walkthrough?
    I think most folks don't mind teaching someone, if it may lead to more
    independence in the future. (At least, I hope so, since I'd like help
    myself!) But just being asked to 'do it for' you isn't the better way
    to influence.
    I'm no professional at this, by any possible measure of the term. But
    I was thinking something along these lines:

    : ,-----------------------------,
    : | |
    : | |
    : | |
    : | |
    : | +12V +12V |
    : | | | |
    : | | | |
    : | | | |
    : | )| --- D1 |
    : | )| RELAY / \ |
    : | +12V )| --- |
    : ,--, ,--, | | )| | |
    : | | | | | | | | |
    : gnd | gnd | | | +-------+-' |
    : | | | | | | |
    : ,--+------+-------------------------------------, | |
    : | X X | | | | | | | |+12V
    : | | | | | | | | | |
    : | COM | | | | | \ | |
    : | | | | | | / R4 | |
    : | ,--------------------' | | | | \ 220k | |
    : | | | | | | / | \
    : | | ,------------+---------------+-' | | | | / R2
    : | | |+ | | | | | | | \ 10k
    : | | |\ ,---+----, | | | | | | /
    : | '--|-\ | charge | | |+ | | | | |
    : | | >------+ pump | | |\ | | | | |
    : | ,--|+/ '-+----+-' '---|-\ |/c | '-------+--+
    : | | |/ o,-------' | | >----| | |
    : | | |- | ,-----',----------|+/ |>e | |
    : | | COM | | | |/ opamp2 | | |
    : | | | | | |- | | \
    : | | | | | COM | | / R3
    : | | | | | | | \ 10k
    : | | | | | ,-------------' | /
    : | | | | | | | |
    : | | | | | | X X | |
    : '-------------------------------------+------+--' |
    : | | | | | |
    : | | | | | LM2907N gnd
    : | | +------+, gnd
    : | --- C1 | |
    : |( --- 1000| |
    : magnetic |( | | |
    : pick-up |( | --- C2 \
    : |( | --- .1u / R1
    : | | | \ 100k
    : | gnd | /
    : | | |
    : gnd | |
    : | |
    : gnd gnd

    I do not recall what RPM you are looking for, to set your cross-over.
    So the values of components above are for illustration only and almost
    certainly will not really get what you want. But the basic ideas are
    there. What is missing is how to create Vcc, I suppose. But if a
    kind of ratiometric threshold is okay and if (and I don't know) the
    LM2907 can withstand the various voltages in automotive applications
    then you may just hook it directly to the car supply. Otherwise, you
    may need something to provide protection for the LM2907.

    C1 and R1 are designed based on knowing your RPM threshold and the
    voltage threshold you want to set (perhaps 1/2 of your Vcc) and the
    minimum current that the LM2907 can manage (140uA) to supply. C2 just
    filters this and sets the ripple voltage. Bigger can be better, but
    only up to a point. I included R4 so that some hysteresis would be
    added to overwhelm whatever ripple may be present after C2 filters it,
    and it needs to be designed to be small enough to do the job. But
    that will depend on the resistors, R2 and R3, which set the threshold
    voltage, across which you want the relay switched.

    Anyway, you will definitely need to fully disclose what it is that you
    are trying to achieve, what RPM or range of RPMs you want to cover,
    where you are getting your relay supply voltage, where you intend to
    get your LM2907 voltage, etc. It's just guess-work until then (and
    maybe even after that.)

  18. ahonda55

    ahonda55 Guest

    dear Jonathan , i want to thank you very very much
    i have some troubles and i will tell you all of them..
    i made the LM2907 tachometer circuit , and the output goes to th
    transistor-relay circuit , but there is the problem of the ripple
    the relay start bounceing before going ON or Off , i had used a 100u
    capacitor , it is ok but the reaction of the relay becomes little bi
    slow , what should i do to remove that ripple
    that's all about the LM2907 circuit which i will post in the bottom o

    the second aim of my dreams is (( speed switch )) , using LM2907 t
    make it open the 12v relay on a desired RPM and closes it when th
    RPM becomes low
    the problems i have are..

    i have the 14 pin version and i don't have the circuit for it , i jus
    have the 8 pin

    here is all supplies ..
    Input voltage 8v or whatever below 12v will work
    the relay is a small 12v rela
    i want to make the relay On when the frequency reachs 150 hert

    i want to know how to use the 14 pin instead of the 9 pin and will i

    and if you can help me plz send me any circuit drawings not the tex
    because i can't understand it ;) and sorry for that

    here is the circuit...

    and the relay photo by me camera ;

    Thank you very much and really sorry to ask all that help , but thi
    is the biginner me , need help 24/7 in something i ike to do
    Thank you agai
  19. Anyone? I'm curious to know whether it's the datasheet or me that's
  20. mike

    mike Guest

    One way out of your problem is to redefine it. If you use a counter and
    compare the counts, you can have a very accurate threshold determination
    anywhere you want. If you need analog output, a PWM can provide that
    from the counter. And all this is built into any number of cheap
    microcontrollers. One chip and you're done.

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