Mamunar Rahoman

Project Title: Operational amplifier-based analog tachometer.

Project duration: Undergraduate, 2nd Year 2nd Semester.

Objective: The objective of this project is to design a system that responds linearly to a linear input. A fan propeller is configured such that its speed can be varied linearly, and the system provides a corresponding linear feedback signal. For this reason, the system is referred to as an analog tachometer, as it offers an approximate indication of the RPM of a rotating object.

Tool used: EasyEDA (for schematic drawing), No simulation was performed due to the unavailability of all component libraries in any simulation tool.

Figure: Implementation of the Analog Tachometer System

Abstruct: The primary objective of this project was to design and implement a system that provides a linear response corresponding to a linear input system. A motor with a propeller was configured for linear speed control using a simple potentiometer. A reflective material was attached to the propeller so that it could reflect the infrared (IR) light emitted from an IR sensor transmitter. The reflected light was detected by the IR sensor receiver, generating a pulse signal with a 50% duty cycle corresponding to the RPM of the motor propeller. Higher RPM produced a higher frequency and narrower pulse width.

This pulse signal was then fed into a frequency-to-voltage converter circuit, followed by an amplifier stage to scale the output voltage. The resulting output voltage was measured using a digital voltmeter module. Additionally, a circuit was integrated to control the number of LEDs illuminated proportionally to the motor speed, providing a visual representation of the RPM. The final system effectively functions as an analog tachometer, giving a linear voltage output corresponding to the rotational speed of the propeller.

Component List:

IR Sensor Module.
Frequency to voltage Converter.
Voltage Amplifier.
Voltage Level Indicator.

Figure: Schematic diagram of the Analog Tachometer System

Working Principle:

IR Module- Active infrared (IR) sensors both emit and detect infrared radiation. An active IR sensor consists of two main components: a light-emitting diode (LED) and a receiver. When an object comes close to the sensor, the IR light from the LED reflects off the object and is detected by the receiver. In this project, the IR module functions as a pulsating DC generator, producing a pulse signal corresponding to the rotation of the motor propeller.
Frequency-to-Voltage Converter- The frequency-to-voltage converter circuit generates an output voltage proportional to the input frequency. The KA331 IC, an 8-pin device, is used in this stage. Pin 8 is connected to a 9V supply, while pins 3 and 4 are grounded. The input frequency is applied to pin 6, and the output voltage is obtained from pin 1. The input pulse is shaped by an RC network (R7 and C3) and fed to pin 6. The timer inside the IC triggers a built-in comparator on the negative edge of each pulse. The resulting output voltage varies linearly with the input frequency, enabling accurate conversion from the pulse signal generated by the IR module.
Voltage Amplifier- The voltage amplifier stage uses a comparator circuit to monitor and amplify the voltage signal. A comparator compares two input voltages—an inverting (–) and a non-inverting (+) input—and switches its output accordingly. When the non-inverting input voltage is higher than the inverting input, the output goes to a high state (positive saturation). Conversely, when the inverting input voltage is higher, the output goes to a low state (negative saturation). This stage ensures the signal is properly amplified and conditioned for measurement and display, providing a clear voltage representation of the motor speed.

Learnings:

Gained hands-on experience with IR sensors and their application in pulse generation for rotational speed measurement.
Learned to design and implement frequency-to-voltage conversion circuits for analog signal processing.
Strengthened understanding of comparator-based voltage amplification and signal conditioning techniques.

Difficulties Faced: Initially, it was challenging to determine how to convert the pulsating DC signal from the IR sensor into a linear response corresponding to the propeller speed. After conducting research, we identified a frequency-to-voltage converter circuit, which successfully resolved this issue and enabled the system to provide a proportional analog output.

My Role in This Project:

Designed and implemented the complete analog tachometer system, including sensor integration, frequency-to-voltage conversion, and voltage amplification stages.

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