I'm currently working on a project where I am making a wheeled robot using the Hack-e-bot library on the arduino. The circuit board which I have planned to put together through soldering will eventually look similar to this: enter image description here

Here is a schematic:enter image description here

I understand my circuit quite well but my mentor recommended using a capacitor for this and although I have got it on my schematic, I've never actually worked with capacitors before. Why should they be used in this project?

  • Using a capacitor for what purpose???
    – Majenko
    Jun 4, 2017 at 7:52
  • 1
    From the schematic, it seems to be a decoupling capacitor. Jun 4, 2017 at 8:13
  • 2
    C1 is connected to Vcc and GND of the microcontroller, but it is hard to see. Since motors are used, an extra larger capacitor might reduce the noise and voltage dips caused by the motors. @Utsav, could you redraw the schematic ? Use GND symbols and put them at the bottom; don't try to make the picture small; If possible, keep the VCC in the top; Put labels for the pins of J1 and J2; Write the values for the capacitors and resistors; Write the type/name of the microcontroller in the schematic; A good schematic shows clearly the path of the sensor signals via the microcontroller to the motors.
    – Jot
    Jun 4, 2017 at 9:50
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    I am assuming by the wording of the question that you are being advised to add another capacitor somewhere in your circuit in addition to the existing one? It's hard to know, since you are saying you have been advised to add a capacitor, yet your circuit already has a capacitor... Also, I really pity D1's chances in life...
    – Majenko
    Jun 4, 2017 at 11:02
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    Oh, and "here's a random picture of a circuit board, and here's a plate of spaghetti" doesn't really help us much. I am not sure what you hoped to achieve by showing us some other circuit board, and you really need to read a few tutorials on how to lay out a schematic properly so that people can read it (though kudos for not making us suffer another Fritzing doodle...)
    – Majenko
    Jun 4, 2017 at 11:05

2 Answers 2


A capacitor is a device that can store (and release) charge. It is made up of 2 metallic electrodes (or plates) that are separated by a thin dielectric material.

When you apply a voltage over the two plates, an electric field is created. Positive charge will collect on one plate and negative charge on the other. - build-electronic-circuits.com

In short, a capacitor lets the AC component through and blocks the DC component.

Capacitors can be used in the following ways:

Decoupling Capacitors

(Also called Bypass capacitors)

A decoupling capacitor’s job is to supress high-frequency noise in power supply signals. They take tiny voltage ripples, which could otherwise be harmful to delicate ICs, out of the voltage supply.

Noisy motor

Image reference http://www.seattlerobotics.org/encoder/jun97/basics.html

In a way, decoupling capacitors act as a very small, local power supply for ICs (almost like an uninterupptable power supply is to computers). If the power supply very temporarily drops its voltage (which is actually pretty common, especially when the circuit it’s powering is constantly switching its load requirements), a decoupling capacitor can briefly supply power at the correct voltage. This is why these capacitors are also called bypass caps; they can temporarily act as a power source, bypassing the power supply.

Decoupling capacitors connect between the power source (5V, 3.3V, etc.) and ground. It’s not uncommon to use two or more different-valued, even different types of capacitors to bypass the power supply, because some capacitor values will be better than others at filtering out certain frequencies of noise.
- sparkfun.com

Power Supply Filtering

DC power supplies can be quite noisy on their own. Take the output of a bridge rectifier for example, there is a lot of ripple on the output. Capacitors are a perfect solution for reducing the ripple/noise of power supplies.

bridge rectifier smoothing capacitor

Image Reference: http://www.electronics-tutorials.ws/diode/diode_6.html

Capacitors are stubborn components, they’ll always try to resist sudden changes in voltage. The filter capacitor will charge up as the rectified voltage increases. When the rectified voltage coming into the cap starts its rapid decline, the capacitor will access its bank of stored energy, and it’ll discharge very slowly, supplying energy to the load. The capacitor shouldn’t fully discharge before the input rectified signal starts to increase again, recharging the cap. This dance plays out many times a second, over-and-over as long as the power supply is in use.

Also see Keelan's answer over at electronics stack exchange.

Signal Filtering

You can also combine capacitors and resistors to form filters that target specific frequencies. - build-electronic-circuits.com

They can block out low-frequency or DC signal-components while allowing higher frequencies to pass right through. They’re like a bouncer at a very exclusive club for high frequencies only. - sparkfun.com

For example in an audio system you can target the high frequencies to remove them (e.g. in a sub-woofer). This is called a low-pass filter. - build-electronic-circuits.com

Low pass filter

Image Reference: Wikipedia

Back to your application.

Motors are notoriously noisy. Your mentor has recommended a capacitor to decouple the noise of the motors and protect the IC from voltage drops/spikes.

Side note: D1 in your schematic is a power indication LED. It needs a current limiting resistor in series with the LED to limit the current and prevent the LED from burning out.



Based on the schematic shown, the capacitor acts as a decoupling or smoothing capacitor. When the circuit components such as a motor require a sudden source or produce a sudden spike of energy, this would help filter out the noise and balance the power line.

More info regarding decoupling capacitor may be found on wikipedia here.

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