DC Motor speed controller circuit

You often need to dim the brightness of the led, led strip, or control the speed of the DC motor. We can make A simple DC dimmer using NE555 IC and a few more components. Using this circuit you can control the brightness of LED strips or the speed of a DC motor efficiently. You may think that we can use the potentiometer for this purpose. That is ok for low power applications. But, in the applications where high power is required such as a led strip or DC motor. There will waste a lot of power in the form of heat because a potentiometer is nothing but a resistor. So, that is why we will make a DC motor speed controller circuit or a DC dimmer circuit using 555 timer IC to efficiently reduce the voltage of any voltage source.

We can use the PWM function of 555 IC for dimming the voltage of DC. But first, let’s see what a PWM is?

PWM: Introduction

In digital electronics, we use PWM to achieve different voltage levels. Because digital electronics use 0 and a fixed volt to indicate 0 and 1. It can switch only between 0 and 1 but can’t stay somewhere in between. So we use the PWM technique to achieve different voltage levels in digital electronics.

PWM stands for “pulse width modulation”. In PWM we switch rapidly any circuit between 0 volt and high voltage (let 5 volts). By changing the “ON” and “OFF” time periods, we can achieve different voltages, and “ON” and “OFF” time is called the duty cycle. For example, if we are switching between 0 volt and 5 volt and on time is equal to off time then the duty cycle will be 50% and voltage will be 2.5 volts. Let’s understand it graphically.

PWM square wave signal


               VCC = ON state voltage

TON = ON time

               TOFF = OFF time

               TTotal = Total time period

Duty cycle and frequency of PWM signal

If duty cycle is 50% and VCC is 5 volt then average voltage will be

VAVG = 5 volt×50% = 2.5 volt

PWM signal

If duty cycle is 25% and VCC is 12 volt then average voltage will be

VAVG = 12 volt×25% = 3 volt

Circuit Diagram for DC motor speed controller or DC dimmer using 555 IC

The circuit shown below is capable of generating a PWM signal between 0 to 100%.

DC motor speed controller circuit

The components used in this circuit are-

ComponentsAmazon link for IndiaAmazon link for other countries
NE555 timer IChttps://amzn.to/3AmxZcIhttps://amzn.to/3AlKmG1
two resistors (1k)https://amzn.to/3qZwXABhttps://amzn.to/3GU5Qfw
one potentiometer (500k)https://amzn.to/3IwjBSfhttps://amzn.to/3qSv3BI
two diode (1N4001)https://amzn.to/3AolkpHhttps://amzn.to/3tPHvnI
two capacitors (0.1uf and 0.01uf)https://amzn.to/3qP4AVqhttps://amzn.to/3GMfxN7
one transistor (BD131 or BD139)https://amzn.to/3fOxgYshttps://amzn.to/3qQmnvB
power source and bulb which we are trying to dim.

The 555 IC is in astable mode in this circuit. It switches between 0 volt and 12 volts. We can change its duty cycle using a potentiometer. Transistor BD131 is used to drive a high power load as a switch because 555 is not capable of output high current.

Potentiometer: introduction

A potentiometer is short of a voltage divider that has three terminals. One of which we use to give it power, second is ground terminal and from third terminal, we take the variable voltage.

By moving the knob we can change the resistances and hence the output voltage.

Structure of potentiometer

Potentiometer structure

Here is the structure of the potentiometer. Its VCC and ground are connected to a layer of graphite, and VOUT (middle terminal) is connected to a wiper which position can be changed using a knob. As we know every conductor has resistance in it and it is directly proportional to its length. So, graphite will also have resistance per unit length. So if we connect a voltage source across it, there will be different voltages at different lengths. By moving the knob we connect the wiper to the different positions of the graphite layer and that way we get different voltages from VOUT.

Working of DC motor speed controller circuit

To understand the working of the 555 PWM generator, let’s replace the potentiometer with two resistors in voltage divider configuration.

Fixed speed DC motor controller circuit

What will be the output of 555 IC, it depends upon the voltage on pin numbers 2 and 6. Both pins are short in this circuit so, voltage on the both will be the same as voltage across the capacitor.

In the figure above you can see that the power supply given to 555 IC is divided into 2 parts internally using 3 5KΩ resistors. That is why we call this IC 555 timer IC. Suppose that we have powered the IC using a 5-volt source. Then it will be divided into two parts. At the point ‘a’ voltage will be two-third of supply voltage which is 3.333 volts in the case of 5 volts supply. And at the point, ‘b’ voltage will be one-third of supply voltage which is 1.667 volts in our case.

Point ‘a’ and ‘b’ are connected to comparator comp1 and comp2 respectively. Non-inverting terminal of comp1 is connected to the threshold pin of 555 IC and inverting pin is connected to node ‘a’. That means if the voltage at the threshold pin is above 3.33 volt then the output of comp1 will be high otherwise it will be low.

Inverting terminal of comp2 is connected to trigger pin of 555 IC and non-inverting pin is connected to node ‘b’. That means if the voltage at the trigger pin is below 1.67 volts then the output of comp2 will be high otherwise it will be low.

In dimmer circuit threshold and trigger pins are shorted and connected to capacitor C1, so if the voltage across the capacitor is above two-thirds of supply voltage then the output will turn off and if it is below one-third of power supply then the output will turn on.

PWM signal to control the speed of DC motor

Now if the charging time and discharging time of the capacitor are different we can generate the PWM signal. For this purpose, two diodes are used in this circuit. Now if we take a look at the circuit we can understand that capacitor is charging through R1 and R3 and discharging through R4 (pin 7 is discharge pin). By changing the values of resistors R3 and R4, we can change the charging and discharging time of capacitor). This is why potentiometer was used.

Caution:- Don’t short the resistor R1 for perfect 0 or 100% duty cycle. Because Pin 7 connects directly to the ground via transistor Q1 during off period. This way you can short the power source. You can see the internal structure of NE555 timer IC.

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