LM741 Op Amp Voltage Follower Help

Discussion in 'General Electronics Chat' started by drparkwood, Jan 27, 2012.

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  1. drparkwood

    drparkwood Thread Starter New Member

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    Hello,

    I am an extreme Novice at this, but I have a feed back issue with my motorcycle cruise control.

    I had to tap into the VSS (vehicle speed sensor, pulse generator) which pulsed 0v to 5v with the frequency of pulses dictating the speedometer indicated speed.

    When I tapped into this, I beleive the signal is weakenought and there is slight interferance on the original circuit. The recommendation I recieved was to install a voltage follower to buffer and strength the pulse signal.

    My question is to build a voltage follower is all that I need the Op Amp or are there any other components.
     
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  2. Audioguru

    Audioguru New Member

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    When a 741 opamp is powered by +12V and 0V then its inputs do not work from 0V to +4V and from +8V to +12V. Its output will not go lower than about +2V.

    When an MC34071 opamp is powered from +12V then its inputs work all the way down to 0V but do not work from +10V to +12V. Its output goes down to +0.3V.
     
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  3. jimkeith

    jimkeith Active Member

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    I would try a voltage follower--requires only (1) transistor (e.g. 2N3904) and (1) resistor (e.g. 1K)
     
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  4. Audioguru

    Audioguru New Member

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    An NPN transistor emitter-follower changes the voltage. Its output is about 0.65V lower than its input.
     
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  5. cumesoftware

    cumesoftware Senior Member

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    An op-amp is all you need. You can use an LM358, or the quad version LM324. These op-amps work well with single power supplies and the output can go down to 0V (both inputs can sense very close to 0V either). Just tie the output to the negative input and you are set.

    You can use the 12-13V coming from the battery to supply the chip. The LM358/LM324 will withstand up to 16V. Use an 100uF 25V electrolytic capacitor and a 100nF 63V polyester capacitor in parallel with the chip. This measure will filter (bypass) any oscillations on the supply.
     
    Last edited: Jan 27, 2012
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  6. Audioguru

    Audioguru New Member

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    No.
    Their output does not go down to 0.0V. With a 5.0V supply and a 10k load to ground its output low voltage is typically 5mV but could be as high as 20mV.

    A Cmos opamp has an output that can go down to 0.0V when it is not sinking any current.
     
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  7. BJT_user

    BJT_user Member

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    I just looked up the specs for the LM358 Low Power Dual Operational Amplifier, as Cumesoftware suggested, and it appears to be just what you need. It is very common and costs little, howerver, be aware that these are very low-power op amps, with their outputs only being able to supply around 20ma. This should not be a problem though, since all you are driving is the input of a pulse detector. You will need a few extra components, as Cumesoftware has stated. A few supply decoupling caps, and additionally, as it states in the datasheet linked above, a reverse voltage protection diode is needed because should the power supply for the op-amp ever become reversed in polarity, an unlimited current surge through the resulting forward diode within the IC could cause fusing of the internal conductors and result in a destroyed unit.
     
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  8. PaulEE

    PaulEE Member

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    I have one comment on this entire thread so far:

    www.ti.com/lit/ds/symlink/sn7407.pdf

    All you need is +5 for the chip...and pull-up resistor...unless the V_low_output voltage of 0.7v bothers you,...then add a diode and resistor to drop it to approximately zero.

    If the circuit that looks at the sensor output in the first place is transistor-transistor logic, 0.7v IS logic low, anyway.
     
    Last edited: Jan 27, 2012
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  9. cumesoftware

    cumesoftware Senior Member

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    Indeed, I didn't remembered that the circuit could be digital. A 0.7V at a TTL input is indeed logic low. The OP could give more details about the circuit. A detailed photo would be nice.
     
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  10. Audioguru

    Audioguru New Member

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    I was thinking that since the signal is for a tachometer then the output must go down to close to 0V for an accurate idle reading. but now I read that the frequency of the pulses, not their peak voltage determines the RPM reading.

    We don't know what the voltage limits are for the pulses and we don't know why there was interference when the extra circuit was added.
     
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  11. PaulEE

    PaulEE Member

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    Meanwhile, I'm sitting here thinking, "isn't a motorcycle a type of automobile?"...

    :)
     
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  12. Audioguru

    Audioguru New Member

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    My father's '58 Chevy had a mechanically pulsed speedometer. If it had a tachometer then it might have worked the same.
    Is this motorcycle also very old?
     
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  13. Wendy

    Wendy Moderator

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    Yes, a motorcycle is automotive. And trying to use a 741, an extremely old chip with some of the worst characteristics is a classic reason why we don't allow this to be discussed here.

    Something like a cruise control is especially dangerous, thing what would happen if it malfunctioned and took total control.

    The All About Circuits forum Administrative Team has elected not to host discussions of automotive electrical system modifications/enhancements due to safety concerns, the potential of legal ramifications and the possible circumvention of vehicle regulations at the state and federal level.

    This thread is against the AAC forum rules, Chapter 6, as seen here:

    http://www.allaboutcircuits.com/l_tos.html


    Automotive modifications of any kind are strictly forbidden. Therefore, this thread will be closed.

    Please try to understand the reasons behind this action, and feel free to browse and use the forums.

    You might find answers to your questions in one of these forums:

    http://forum.allaboutcircuits.com/sh...ad.php?t=54400
     
    #13
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