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I draw a schematic which uses LM741 and IRF830. The main function of the schematic is to light up a few leds (which consumes about 100mA in total) when the night comes. Please have a look at the schematic and tell me if it is correct. I am a beginner and I am trying to draw a schematic to see if I understood some things about electronics.

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Dave Tweed
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beard999
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    One thing you don't understand is all the reasons not to use the 741... – Dave Tweed Apr 19 '18 at 18:37
  • @DaveTweed Can I use TL071 instead ? I saw that TL071 has the same pinout as LM741. I already built the PCB and I am making some tests with it. – beard999 Apr 19 '18 at 18:43
  • Almost as bad as using a 741 is using an op amp to do a comparator's job. I don't know if you're setting up the regulator to give a 12 volt supply, but assuming that, your gate drive will only be about 4-5 volts, which is too low for reliable operation. – WhatRoughBeast Apr 19 '18 at 22:19

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As I often say to my students, try it out and see how it goes. You can isolate the power circuit from the control circuit if you want to test them separately (and if you have a power supply).

I think you're on the right track! Have a bit of faith in your abilities!

Simon Marcoux
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    Have you never heard of hysteresis? That's what the positive feedback is for. – Dave Tweed Apr 19 '18 at 18:44
  • Regarding the voltage divider (R3 and R4) -- have you compared the minimum output voltage of the 741 with the threshold voltage of the IRF830? (I'm finding hard to believe that you're an instructor of some sort.) – Dave Tweed Apr 19 '18 at 18:48
  • Edited to reflect the current discussion. I didn't really look up what Op-Amp and FET were used before answering on a whim. I simply looked at the schematic briefly.

    Yes I have heard of hysteresis before. The circuit we gave to first year students didn't have them so my reflex was purely to compare the two layouts since both works, but one is over simplified for first year students.

     – Simon Marcoux Apr 19 '18 at 19:04
  • @DaveTweed I measured the voltage between GND and mosfet's gate and it is about 0.6V when the mosfet is off and about 4.9V when the mosfet is on. Using TL071. – beard999 Apr 19 '18 at 19:27
  • http://www.vishay.com/docs/91063/sihf830.pdf Minimum threshold voltage is documented between 2 and 4 volts and max VGS is 20 volts. It should therefore work properly. Is your circuit functionnal at the moment or there is a problem? – Simon Marcoux Apr 19 '18 at 19:41
  • @Simon Marcoux The circuit is functional at this moment. – beard999 Apr 20 '18 at 00:23