Hey r/explainlikeimfive could someone explain to me how a pull down voltage circuit works? I tried googling it but I can’t make sense of the answers I’m getting.
Thank you in advance!
In: Technology
You want to have the input line to a chip at a known value even if you do not have an input signal. If you do not force it to a voltage is can float around and randomly change from high to low and back if the load draws very little current as a CMOS transistor gate do.
So if you have a chip and a switch that you can press to put is to lest say +5V how does the line go to 0V to indicate no button pressed? A line at +5V will remain at that level until charges are drained and with no direct connection to ground, you will only have small leak currents. The line is also sensitive to outside interference and it could change for low to high even if the button is not pressed by the capacitive effect.
The solution is a pulldown resistor. You connect the conductor with a resistor to the ground so there is a way for the charges to get away and it will as long as you do not connect it to +5V remain at 0V. There will be a current through it all the time when you press the button so the input will not be at 5V but a bit less but as long as that is in the range that it considers high it do not matter.
The value of the resistor will determine how the fact it returns to zero but a faster return also results in higher leak current and reduce voltage when you press the button. What you use is a compromise.
So you can use a simple close open switch and a resistor so the wire voltage depends on the position ow the switch
Another usage is when a wire can have input from multiple sources. Each chip could send a +5V signal for something but when they do not send the output is not 0V but disconnected from the wire so multiple chips can rise the wire to +5V. In this case, you need to pull it down with an external resistor or one inside the chip that reads the signal.
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