0
\$\begingroup\$

I am trying to understand the basics of electronics and I often come across the terms pull-up and pull-down resistors. From what I understand these resistors help the switching logic devices like transistors.

Could you help understand the basics of these components in simple terms.

Edit 1:

I am trying to build a small feed back system that uses a transistor as switch. The system uses a buzzer that makes a sound when the yellow and gray components meet. The problem that I am facing is that the buzzer keeps making a low volume sound even when the components are not touching. I was reading about pull up and pull down resistors and would like to know how to implement it here

enter image description here

Figure 1. Buzzer schematic.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 2. The OP's schematic redrawn by @Transistor using the CircuitLab schematic editor. (There's a button on the toolbar.)

\$\endgroup\$

3 Answers 3

1
\$\begingroup\$

It's very simple:

A pull-up resistor pulls the voltage up to the "high" logical level (5V, 3.3V or whatever is used as a "high" level) when the is no signal driving the input.

A pull-down resistor pulls the voltage down to the "low" logical level (0V or close to it) when the is no signal driving the input.

This is achieved because the input impedance of the device being pulled up/down is usually very high (and much higher than the resistor itself, so they won't form a voltage divider). Due to this there is almost no flow of current through the resistor, and the voltage drop is almost zero volts. This means, in turn, that the voltage (either "high" or "low") you are connecting the resistor to then gets passed "as is" to the input of the device being pulled up/down, which helps set a "default" input value when there is no signal driving it.

However, when an input signal appears, it will usually have a source impedance much lower that the pull up/down resistor, and will dominate he combination and set the state regardless of the resistor.

This diagram (source here) illustrates the behaviour described:

Pull up/down resistors

EDIT:

Regarding your specific use case, you'll need to pull down the input of the BJT. Use a resistor 20x R1 and it'll probably be fine.

\$\endgroup\$
5
  • \$\begingroup\$ Hey thanks for your answer, I have an electrical circuit in my mind that I needed help with. I have added it to the question, would like to know how to add these resistors here. \$\endgroup\$ Feb 15, 2020 at 11:08
  • \$\begingroup\$ You need to pull down the input of the BJT. Use a resistor 20x R1 and it'll be fine. \$\endgroup\$ Feb 15, 2020 at 11:13
  • \$\begingroup\$ hey thanks ! will try this and get back to you !, I would just like to clarify my thoughts. \$\endgroup\$ Feb 15, 2020 at 11:46
  • \$\begingroup\$ I would just like to clarify my thoughts. We add a pull down resistor so that any current created in the open circuit will get pulled down to the ground, But if the resistance of the pull down resistor is too high it will stop the flow of the current to ground and send it to the BJT instead ? wont this have a counter effect ? \$\endgroup\$ Feb 15, 2020 at 12:01
  • \$\begingroup\$ There's no current in an open circuit. The pull down resistor will set a "low" default state to the input instead of letting it float in an undetermined state when there is no input signal. There's no counter effect as long as you choose a R1 that doesn't divide the voltage at the base of the BJT (that's the reason for setting it at least to 20x R1). \$\endgroup\$ Feb 15, 2020 at 12:12
0
\$\begingroup\$

A pull-up or a pull-down resistor, when connected to a point in a circuit, provides that point with a weak default voltage which dominates unless some other stronger component forces it to something else.

A pull-up means it pulls the point towards some positive voltage (typically the supply voltage), for example +5V, while a pull-down pulls the point towards ground or 0V.

You could for example connect a pull-up resistor to the collector of a NPN transistor. When the transistor is off, the voltage at the collector is decided by the pull-up resistor, but when the transistor is on, it is stronger and forces the voltage down.

You can also use it with e.g. a switch where you want to set the default voltage output when the switch is in "off" position.

You might read the Wikipedia article for some more info: https://en.wikipedia.org/wiki/Pull-up_resistor

One important aspect is that since this is indeed a weak voltage source, you cannot use it to drive circuits that needs lots of power. In that case you will need to amplify the signal somehow, for example with an op-amp, or some other voltage follower circuit.

\$\endgroup\$
2
  • \$\begingroup\$ Hey thanks for your reply ! I would like to add a circuit to explain the problem I am having \$\endgroup\$ Feb 15, 2020 at 10:51
  • \$\begingroup\$ Sure, go right ahead! \$\endgroup\$ Feb 15, 2020 at 11:24
0
\$\begingroup\$

I am assuming that you are using a NPN BJT in the diagram. Please connect a resistor of about 10k(higher) between base and the ground(battery negative) of the circuit.

Connecting the pulldown resistor provides a known state instead of leaving it floating. Currently, When the yellow and Gray parts are not connected, the base of the transistor is floating.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.