Basic Electronics

# Basic Electronics: pull up and pull down resistors

## pull-up and pull-down resistors

Basic Electronics: pull up and pull down resistors:- Many times, electrical circuits have “inputs” through which they receive an electrical signal from the outside (binary type) that has nothing to do with the power signal obtained from the source. These external signals can serve for a multitude of things: to activate or deactivate parts of the circuit, to send to the circuit information from its surroundings, etc.

The “pull-up” (and “pull-down”) resistors are normal resistors, only that They are named for the function they fulfill: they serve to assume a value for defect of the signal received at a circuit input when it is not detected by it no specific value (neither HIGH nor LOW), which is what happens when the input does connected to nothing (that is, it is “on the air”). Thus, this type of resistor ensure that the received binary values ​​do not fluctuate meaninglessly in the absence of an entrance sign.

In the “pull-up” resistors the value that is assumed by default when there is no external signal-emitting device connected to the input is HIGH and on the Pull-down is the LOW value, but they both pursue the same goal, so the choice of a “pull-up” or “pull-down” type resistor will depend on the particular circumstances of our assembly. The difference between one and the other is at their location within the circuit: the pull-up resistors are connected directly to the external signal source and the pull-downs directly to the ground (see circuit diagram2 below).

Let’s look at a concrete example of the usefulness of a pull-down resistor. Suppose we have a circuit like the following (where the resistance of 100 ohms is nothing more than a voltage divider placed at the input of the circuit to protect her).

When the switch is pressed, the circuit input will be connected to a valid input signal, which we will assume binary (that is, it will have two possible values: HIGH for 5V, and LOW for 0V), so the circuit you will receive one of these two concrete values ​​and everything will be ok. On the other hand, if the switch is released, the circuit will open and the circuit input will not be connected to nothing. This implies that there will be a fluctuating input signal (also called “floating” or “unstable”) that we are not interested in. The solution in this case would be place a “pull-down” resistor like this:

In this example the pull-down resistance is 10 KΩ. When the switch is pressed, the circuit input will be connected to a valid input signal, like before. When the switch is released, the circuit input will be connected to the pull-down resistor, which pulls toward ground (which is a reference always fixed).

Someone might think that when the switch is pressed, the circuit will receive the input signal but will also be grounded through the pull-down resistor: what really happens then? Here is the key to why the pull-down resistor is used and a direct connection to ground is not used: the opposition to the passage of electrons from the external signal exerted by the “pull-down” resistance causes these to always deviate at the input of the circuit. If we had connected the input of the circuit to the ground directly without using the pull-down resistor, the external signal would go directly to the ground without go through the entrance of the circuit because that way you would find less resistance (pure Ohm’s Law: less resistance, more intensity).

With a “pull-up” resistor the same could have been achieved, such as shows the following schematic. In this case, when the switch is pressed the exterior signal is diverted to the ground because it finds a direct path to it (so the circuit input does not receive anything – a “0” -) and when the switch is left without pressing is when the input of the circuit receives the external signal. Must have careful with this.

In the previous examples, we have used “pull-up” or “pull-down” resistors. 10 KΩ. It is a fairly common rule to use this particular value in electronics projects where work is done in the 5V range, although, in all In this case, if we want to refine it a bit more, we can calculate its ideal value using the Law Ohm from the current consumed by the circuit.