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I setup the following circuit:

enter image description here

When I measure the voltage between GND and INPUT of the LM7805CV I have a voltage of 7.9V (around 8V as per power supply). But when I measure the output it is only 1.4V instead of expected 5V.

How is this possible? I don't see why the voltage drops so badly.

enter image description here

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    \$\begingroup\$ The schematic looks OK. Lets see a photo of your setup. \$\endgroup\$
    – Transistor
    Commented Sep 22, 2018 at 21:19
  • \$\begingroup\$ done... I measured at the voltage reg directly and I see the 7.9 volts on the input but only 1.4 at the output (so I assume it is the voltage regulator?!?) \$\endgroup\$
    – sesc360
    Commented Sep 22, 2018 at 21:27
  • \$\begingroup\$ Please disconnect everything else from the regulator output, leaving only that output capacitor. According to the photo, at the moment you are trying to power an unnamed (perhaps ATMega328?) MCU and who knows what else isn't visible off the right side of the photo :-) Then re-test. Thanks. \$\endgroup\$
    – SamGibson ♦
    Commented Sep 22, 2018 at 21:32
  • \$\begingroup\$ Are you sure of the exact part number? I'm not finding any LM7805CV on the market. ST has an L7805CV, but no LM7805CV. (Best solution: give a link to the datasheet for the exact PN you're using) \$\endgroup\$
    – The Photon
    Commented Sep 22, 2018 at 21:34
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    \$\begingroup\$ The photo seems to have been taken at sunrise so much of it is in deep shadow. There are components connected that aren't shown on the schematic and we can't see what's connected behind the regulator. Nice, neat wiring though. \$\endgroup\$
    – Transistor
    Commented Sep 22, 2018 at 21:34

1 Answer 1

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You have miswired the ATmega328P MCU on the breadboard.

This is a crop from your photo, showing the problems:

Original photo from question, cropped & annotated to show the wiring problems

You have connected the Gnd rail to pin 20, which is the AVcc pin and should be connected to Vcc (through an optional filter). The MCU might have been damaged by this miswiring.

You have also connected the power & Gnd rails to pins 8 & 9 instead of the correct pins 7 & 8. Again, that might have caused internal damage to the MCU.

The miswiring (especially connecting AVcc pin 20 to Gnd) must be causing an excessive current inside the MCU, so that the regulator can only maintain that 1.4V which you measured on its output. This was confirmed since you updated that the correct 5V was measured on the regulator output, when the MCU was removed from the miswired breadboard.

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    \$\begingroup\$ Exactly what happened.... man............ 2 beer and uC working session is not a good idea \$\endgroup\$
    – sesc360
    Commented Sep 22, 2018 at 21:44
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    \$\begingroup\$ Maybe the PTC saved your beerbooboo \$\endgroup\$
    – Tony Stewart EE75
    Commented Sep 22, 2018 at 22:22

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