Adjustable DC Motor Speed Controller Circuit with Reverse Forward Facility

This post shows you how to build a circuit to control the speed and direction of a DC motor.

It uses a common chip called the IC 555 and a special switch.

A knob lets you adjust the speed by controlling how much power goes to the motor PWM.

A switch lets you change direction forward or backward.

The guide includes some formulas to help you understand how it works, but you do not need to be a math whiz to build it.

WARNING: Building circuits with motors can be dangerous.

Only do this with adult supervision.

What is a Adjustable DC Motor Speed Controller Circuit with Reverse Forward Facility

An adjustable DC motor speed controller circuit with reverse forward capability allows you to control the speed and direction of a DC motor using a simple electronic circuit.

This type of circuit is often used in applications where precise control of a motor’s speed and direction is required, such as in robotics or small electric vehicles.

Components Functions:

IC 555 : Configured as a PWM generator.

The 555 IC is set up in astable mode to generate a continuous square wave signal.

Diodes Two: Connected in a way to control the charge and discharge time of the timing capacitor.

This modification is likely used to create a variable PWM output.

Potentiometer: Connected to control the charge and discharge time of the timing capacitor.

This allows for the adjustment of the duty cycle of the PWM signal thereby controlling the speed of the DC motor.

Transistor: Connected to amplify the PWM signal generated by the 555 IC

It serves as a switch to control the flow of current to the motor.

DPDT Switch (Double Pole Double Throw): Used to control the direcion of the DC motor.

This switch allows the motor to rotate in both forward and reverse directions.

How the Circuit Works:

Parts List:

The 555 IC is configured in astable mode, generating a continuous square wave signal.

The diodes and potentiometer modify the charge and discharge times of the timing capacitor creating a variable PWM signal at the output pin of the 555 IC.

The potentiometer allows the user to vary the duty cycle of the PWM signal.

This variation in duty cycle directly affects the speed of the DC motor connected to the circuit.

The PWM signal from the IC 555 is then fed to the base of the driver transistor.

The transistor amplifies the signal and acts as a switch for the DC motor.

The DPDT switch is used to control the direction of the DC motor.

Depending on the switch position, the motor can be set to rotate in the forward or reverse direction.

Formulas:

Here are some useful formulas for the IC 555 PWM DC motor speed controller circuit:

Frequency of the 555 Timer (f):

f = 1.44 / ( R₁ + 2 * R₂ )*C

where,

• R1​ and R2 are resistances and C is the timing capacitor.

Duty Cycle D of the PWM Signal:

D = R₂ / R₁ + 2 * R2

where,

PWM Duty Cycle: A Pulse Width Modulation PWM signals duty cycle is the percentage of the cycle when the signal is high relative to the entire cycle duration.

Most commonly, it is stated as a percentage (0% to 100%).

Resistor Dependence: Resistors can be employed in some circuits for PWM signal production, however it is rare for them to have a direct link to duty cycle as shown by the formula:

D = R2 / R1 + 2 * R2

PWM circuits frequently employ microcontroller control, comparator control, or timers to figure out the duty cycle.

Charge Time (t1) of the Timing Capacitor:

t1 = 0.693 *( R1 + R2 )*C

where,

• t1: This shows how long it takes the capacitor C to charge from almost discharged (usually regarded as 0 volts) to a certain voltage level, which frequently reaches around 63% (1 – 1/e) of the supplied voltage source.
• 0.693: This is a constant that is roughly equivalent to ln(2), the natural logarithm of 2.
• It is a mathematical relationship that shows how long it takes the capacitor voltage to reach 63% of its ultimate value in relation to the circuits time constant RC.
• R1 + R2: This is the overall resistance that the capacitor observes when it is being charged.
• Because R1 and R2 are linked in series, the total resistance equals the sum of their individual resistances.
• C: This is the capacitors capacitance, which is commonly expressed in microfarads (µF) or farads (F).

Discharge Time (t2) of the Timing Capacitor:

t2 = 0.693 * R2 *C

where,

• t2: This shows how long it takes for the capacitor to discharge from a given starting voltage typically approximately 63% of the source voltage to a very low value that is usually insignificant in comparison to the starting voltage.
• 0.693: This is a constant that is roughly equivalent to ln(2), the natural logarithm of 2.
• It makes a connection between the circuits time constant RC and how long it takes the capacitor voltage to drop to about 37% (1 – e^(-1)) of its starting value.
• R2: This is an illustration of the resistance R2 that is linked across the capacitor as it is being discharged.
• C: This is the capacitors capacitance, which is commonly expressed in microfarads (µF) or farads F.

Total Cycle Time T:

T = t₁ + t₂

where,

• T: This is the total cycle time, or the amount of time needed for the capacitor to go through one complete charging and discharging cycle.
• t1: This is an illustration of the charging time as previously discussed.
• It measures how long it takes the capacitor to charge from almost discharged to a certain voltage level, which is typically 63% of the input voltage source.
• t2: This is the discharge time that was previously described.

It measures how long it takes the capacitors voltage to drop from a given beginning value typically the voltage attained after charging to a very low level that is usually insignificant in relation to the starting voltage.

Remember:

This formula makes the assumption that the capacitor will go through a full cycle of charging and discharging, starting at a low voltage reaching a certain charged state, and then discharging back to a low voltage.

The overall cycle time T is determined by the values selected for the resistors R1 and R2 and capacitor C in the charging and discharging routes.

For T, the formula yields an approximation.

More complicated charging/discharging scenarios or non ideal components may need the use of more sophisticated circuit analysis techniques for extremely accurate calculations.

Construction:

• Connect the IC 555 on the PCB.
• Connect pins 2 and 6 to create the timing capacitor charging and discharging path.
• Connect pin 7 to pin 6 to enable continuous operation astable mode.
• Connect two diodes in series.
• Connect the anode of one diode to pin 2 and the cathode to the junction of the potentiometer and the other diode.
• Connect the other end of the potentiometer to pin 6.
• Connect the junction of the two diodes and potentiometer to the base of the NPN transistor e.g. TIP122.
• Connect pin 3 of the 555 to the collector of the transistor.
• Connect the emitter of the transistor to the ground.
• Connect the motor to the collector of the transistor.
• Connect the DPDT switch to control the direction of the motor.
• Connect the common terminals of the switch to the motor.
• Connect one set of switch terminals to the collector of the transistor and the other set to the positive supply.
• Connect the positive and negative terminals of the power supply to the PCB.
• Ensure that the voltage level is suitable for both the motor and the IC 555.
• Adjust the potentiometer to control the duty cycle and consequently the speed of the motor.
• Use the DPDT switch to change the motors direction.

Testing:

• Power up the circuit and observe the motors speed and direction based on the potentiometer and DPDT switch settings.

Conclusion:

The potentiometer adjusts the PWM signal controlling the speed of the motor, while the DPDT switch controls the motor direction.

This adjustable DC motor speed controller circuit with reverse forward facility provides a versatile solution for controlling a DC motor with speed variation and bidirectional rotation.

References

Motor controller

Datasheet TIP122