Adjustable DC Motor Speed Controller Circuit with Reverse Forward Facility

The construction of a circuit for controlling a DC motors speed and direction is discussed in this article.

A common chip known as the IC 555 and an special switch are used.

You may change the speed by adjusting the amount of power that is sent to the motor PWM using a knob.

You can adjust your direction forward or backward by pressing a switch.

The article offers some formulas to help you understand how it works but you do not have to be an expert in numbers to build it.

WARNING: It can be risky to build circuits containing motors.

Do this only under the supervision of an trained person.

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 to control the speed and direction of a DC motor using a simple electronic circuit.

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

Components Functions:

The IC 555 is set up to generate PWM.

To produce a continuous square wave signal it is set up in astable mode.

The timing capacitors charge and discharge times are managed by two diodes which are connected.

A variable PWM output is generated by modifying it.

The timing capacitors charge and discharge times are adjusted by a potentiometer.

This makes it possible to modify the PWM duty cycle signals in order to regulate the DC motors speed.

A transistor is used to boost the PWM signal generated by the 555 integrated circuit.

It controls the motors current flow by acting as a switch.

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 modify 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 provided 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,

• D is the duty cycle
• R1 and R2 are the resistors resistances

PWM Duty Cycle: A Pulse Width Modulation 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 are used 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 are used in microcontroller control, comparator control and timers to figure out the duty cycle.

Charge Time (t1) of the Timing Capacitor:

t1 = 0.693 *( R1 + R2 )*C

where,

• t1 is the time it takes for the capacitor C to charge from nearly discharged (sometimes regarded as 0 volts) to a specific voltage level which often reaches about 63% of the voltage source that is given.
• 0.693 is a constant that is roughly equivalent to ln(2) the natural logarithm of 2.
• R1 + R2 is the overall resistance that the capacitor observes when it is being charged.
• C 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 shows how long it takes for the capacitor to discharge from a given starting voltage which is approximately 63% of the source voltage to a very low value that is usually insignificant in comparison to the starting voltage.
• 0.693 is a constant and is roughly equivalent to ln(2) the natural logarithm of 2.
• R2 is an example of the resistance R2 that is connected across the capacitor as it is being discharged.
• C is the capacitors capacitance which is commonly expressed in microfarads (µF) or farads F.

Total Cycle Time T:

T = t₁ + t₂

where,

• T is the total cycle time or the amount of time needed for the capacitor to go through one complete charging and discharging cycle.
• t1 is an example of the charging time as previously mentioned.
• t2 is the discharge time that is also previously mentioned.

It calculates the time it takes for the voltage of the capacitor to decrease from a certain starting value usually the voltage reached after charging to a very low level that is normally negligible in comparison to the initial 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 calculated by the values selected for the resistors R1 and R2 and capacitor C in the charging and discharging routes.

The formula provides an estimate for T.

More advanced circuit analysis methods are often required for extremely exact calculations in more complex charging/discharging situations or with less than ideal components.

How to Build:

To build a Adjustable DC Motor Speed Controller Circuit follow the below mentioned steps:

• Connect the cathode of D1 diode to one end of potentiometer and anode of diode D2 connect to other end of potentiometer
• Wire the potentiometers opposite end to pin 6.
• Connect the potentiometer and the junction of the two diodes to TIP122 NPN transistors base.
• Wire pin 3 of the 555 to the collector of transistors
• Connect the emitter of transistor to the ground.
• Connect the motor to the collector of transistors
• To regulate the motors direction connect the DPDT switch.
• Common terminals of the switch to the motor should be connected
• Connect one end of switch terminals to the collector of the transistor and the other end 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 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