lm78xxpowervoltage
I setup the following circuit:
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.
Cables don't just have resistance, they have inductance. You've acccidentally built a boost converter; you have an LC circuit which is resonant at about the width of your pulse.
If you want to measure the resistance of the cable, use DC. Measuring the characteristic impedance is more complicated. Is that what you actually want.
Active low means that the function of that pin is enabled when the signal on it drops to 0V of below the Vi(l) level specified in the data sheet. For instance, if a chip as a #EN or nEN line, this means that the device will be enabled when the signal on the #EN or nEN line drops low.
As for the voltage drop, you need to examine the data sheet for the pin impedance and whether it is classifies as Open collector/Drain. Without knowing which devices you are using, it is difficult to comment further. If the input side of your second device has some impedance, then it will appear in parrallel with R2. A better way to achieve level convertion is to use a level shifter or a simple low VGs fet arrangement
Any FET may be used provided it satisfies the shift voltage and VGS(on) requirements.
Best Answer
You have miswired the ATmega328P MCU on the breadboard.
This is a crop from your photo, showing the 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.