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For experimental - educative purpose, I'm trying to control one 3-color led with the following circuit:

enter image description here

  • Power: single rail, 5v.
  • Signal generator: triangular 0-5v, 500 Hz (in next step, signal generator should be replaced by an op-amp circuit, could be a rectangular generator followed by integrator).
  • operational amplifier: 741 in comparator configuration. v+ from signal generator; v- from variable resistor R1=100 K .
  • red led with R2=160 ohms resistor (in the future, circuit will be triplicated for each color of the RGB led).

In this way, the triangular signal is converted to a WPM (width pulse modulated) where the width of the pulse is controlled via the variable resistor.

Problem: using a 741 op-amp the led intensity does a "jump" when reaching the low/high limits, probably due to its operational margin levels (741 seems not designed for 5v power supply).

Do you think MCP6004 can be used instead of 741 in this project to solve this issue ? (another possibility is TL974)

Thanks a lot.

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    \$\begingroup\$ When will the 741 finally die? \$\endgroup\$
    – PlasmaHH
    Commented May 10, 2017 at 9:35
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    \$\begingroup\$ Start with the LM358 (dual) or LM324 (quad). Cheapest and vvvv common op amps. See if they will do. Go from there (they too are olde olde parts) \$\endgroup\$
    – Russell McMahon ♦
    Commented May 10, 2017 at 11:18
  • \$\begingroup\$ Interesting options, nice they support +-15V. LM324 is not classified as rail-to-rail (in manual I read Voh=Vcc-1.5) ¿could be this fact an issue for this circuit? \$\endgroup\$
    – pasaba por aqui
    Commented May 10, 2017 at 11:49
  • \$\begingroup\$ "741 seems not designed for 5v power supply" Gee, d'ya think!? -1 because you obviously haven't bothered to read the datasheet. We are not here to read the datasheet to you. \$\endgroup\$
    – Olin Lathrop
    Commented Aug 10, 2017 at 11:17
  • \$\begingroup\$ @OlinLathrop: Too much aggressive. Do you feel better if I write "741 is not designed for 5v power supply" ? \$\endgroup\$
    – pasaba por aqui
    Commented Aug 10, 2017 at 11:36

1 Answer 1

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The minimum supply rail on some 741s is recommended as +/-10 volts so this might immediately exclude it. I believe the ST uA741 can work down to a single +5V rail but there are much better choices of op-amp compared to this diminutive dinosaur.

My personal list of reasons not to use a 741 are listed below: -

  • Minimum recommended power supply rails are +/- 10 volts
  • Input voltage range is typically from -Vs + 2 volt to +Vs - 2 volt
  • Input offset voltage is typically 1 mV (5 mV maximum)
  • Input offset current is typically 20 nA (200 nA maximum)
  • Input bias current is typically 80 nA (500 nA maximum)
  • Input resistance is typically 2 Mohm (300 kohm minimum)
  • Typical output voltage swing is -Vs + 1 volt to +Vs - 1 volt
  • Guaranteed output voltage swing is -Vs + 3 volt to +Vs - 3 volt
  • Supply current is typically 1.7 mA (2.8 mA maximum)
  • Noise is 60 nV/sqrt(Hz) for LM348 (quad version of 741)
  • GBWP is 1 MHz with a slew rate of 0.5 V/us

The LM741A is slightly better but still a dinosaur in most areas.

The MCP6004 is suitable for the 5 volt supply but I would encourage you, line by line, to make comparisons with the 741 as I have listed above.

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    \$\begingroup\$ Suggestions: make a self answered (community wiki??) q&a "Why not to use a 741" we can then use to link everywhere, maybe even into the 741 tag wiki, should such exist. \$\endgroup\$
    – PlasmaHH
    Commented May 10, 2017 at 9:40
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    \$\begingroup\$ @PlasmaHH sounds a good idea. \$\endgroup\$
    – Andy aka
    Commented May 10, 2017 at 9:41
  • \$\begingroup\$ @Andyaka: thanks for your answer, In addition to discard the 741, I need to select an alternative. I'm thinking in MCP6004. Do you think it will be valid for this application ? (forum rules doesn't allows me to ask for a replacement suggestion). \$\endgroup\$
    – pasaba por aqui
    Commented May 10, 2017 at 9:45
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    \$\begingroup\$ I'll firstly quote my answer "I would encourage you, line by line, to make comparisons with the 741 as I have listed above". Second, I'll point out that this is what EEs do. Third, I'm not about to make guesses on some description that doesn't include a schematic. \$\endgroup\$
    – Andy aka
    Commented May 10, 2017 at 9:47
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    \$\begingroup\$ Yes it is R2R and the analogue input range is the full rail but your triangle waveform generator might be set wrong and exceed the over-voltage limits and do some harm to the inputs. It's a caution! \$\endgroup\$
    – Andy aka
    Commented May 10, 2017 at 10:22

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