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I. INTRODUCTION
The 555 Timer, designed by Hans Camenzind, is a versatile IC that could function as a timer or a square wave generator. A relaxation oscillator can be implemented accurately by using a 555 timer and few additional timing components.
The philosophy of the 555 timer IC is that square waves signal can be generated due to the function of SR flip-flop with an additional discharge transistor to enable periodic square wave operation.
This project proposes the elimination of discharge transistor and SR flip flop through functioning the two comparators to ‘Set’ and ‘Reset’ each other to produce periodic square waveform by using only two comparators configured as a closed-loop dual comparator. As a result, higher power density, higher efficiency, and lower cost production can be realized due to fewer components.
The modified 555 timer circuit can be implemented using a total of 10 transistors, 17 resistors, and 1 capacitor. Compared to the conventional 555 timer circuit which is composed of 23 transistors, 15 resistors, and 2 diodes, the proposed relaxation oscillator is more than twice simpler and can be realized with lower manufacturing cost.
II. LITERATURE REVIEW
1.1 Differential Amplifier and Functioning
A differential amplifier is a circuit that amplifies the voltage difference between two input signals while rejecting any common mode voltage (voltage that appears equally on both input signals). A traditional differential amplifier is shown in Figure 1. It consists of two input terminals, each connected to a separate input signal, and two output terminals. The input signals are typically connected to the base of two transistors, while the collector of each transistor is connected to a common emitter resistor. The output voltage is then taken from the junction of the two emitter resistors. In addition to the basic structure, some differential amplifiers may include additional components such as capacitors and feedback resistors to improve stability and reduce noise.
1.1.2 Functioning
The differential amplifier functions by amplifying the voltage difference between two input signals while rejecting any common-mode voltage, which is any voltage that appears equally on both input signals. The functioning of the differential amplifier can be described in the following steps:
1. Input Signals: The two input signals are applied to the bases of the two transistors in the differential amplifier circuit.
2. Differential Amplification: The differential amplifier amplifies the difference between the two input signals, while rejecting any common-mode voltage. The amplification occurs because the two transistors are connected in such a way that their collector currents are proportional to the difference between the two base voltages. The differential amplifier amplifies this difference by a factor determined by the gain of the circuit.
3. Output Voltage: The output voltage is taken from the junction of the two emitter resistors, which is also the point where the two collector currents meet. The output voltage is proportional to the difference between the two input signals, and it is amplified by the gain of the circuit.
4. Common-Mode Rejection: The differential amplifier also rejects any common mode voltage that may be present in the input signals. This is because any voltage that appears equally on both input signals will produce an equal and opposite current in the two transistors, which will cancel out at the output.
Overall, the differential amplifier provides a high degree of amplification and noise rejection, making it a versatile and important component in many electronic systems. It is commonly used in applications that require high common-mode rejection, such as in instrumentation and communication systems.
1.2 Comparator
A comparator is a device, electronic circuit, or even software program that compares two things and determines which is larger. Here's a breakdown of the different types of comparators:
Electronic Comparators:
1.2.1 Proposed Comparator’s Architecture
It consists of only five transistors. When the inverting input voltage is higher than the noninverting input voltage, the output is LOW. When the noninverting input is higher than the inverting input, the output switches to HIGH. The simulation result confirmed the working principle of the proposed comparator circuit. Hence, the modified 555 timer circuit can be implemented using a total of 10 transistors, 17 resistors, and 1 capacitor. Compared to the conventional 555 timer circuit which is composed of 23 transistors, 15 resistors, and 2 diodes, the proposed relaxation oscillator is more than twice simpler and can be realized with lower manufacturing cost.
In the transistor comparator circuit you described, each transistor plays a specific role:
Differential Pairs (Q1/Q2 and Q3/Q4):
Current Source (Q5):
Other Transistors:
Overall, the transistors work together to create a high-gain amplifier for the differential voltage between the input and reference. This amplified voltage difference then controls the final output (Vout), indicating which input is larger.
1.3.1 BJT as a Switch
Transistors can be used as amplifiers of AC signals when their biasing voltage is applied in a way that it operates in the active region. By changing the biasing voltages accordingly, the transistor can also be made to function as an ”on/off” type solid state switch. This can be achieved by driving the transistor back and forth between it’s cut-off and saturation region without having to study the Q-point biasing and voltage divider circuitry required for amplification. In this case, a Common Emitter Configuration is used to demonstrate the function of a transistor as a switch.
1.3.2 BJT as an amplifier
1.4.1 555 Timer IC
The 555 timer oscillator or, as commonly referred to, 555 timer, is an extremely popular IC for timing-related applications. They are robust and versatile, as they can be used in any circuit which requires some sort of time control. It can be used to generate various types of pulses, to create time delays, and also for Pulse Width Modulation (PWM). The most common use of 555 timers is to generate clock signals for circuits.
Let us take a look at what's inside a 555 timer and how these electronic components work together to perform a variety of applications.
The first part of the block diagram is the voltage divider circuit. The voltage divider is made using three 5k resistors. As the resistors are of the same value, an equal voltage gets divided between the three resistors. The voltages across these resistors are given as reference voltages to the comparators.
A comparator is a special configuration of an operational amplifier. It compares the voltage on its inputs and outputs a high or low voltage depending on whether the voltage on the inverting or non-inverting input is higher.
The outputs of the comparators are given as inputs to a flip flop. An SR flip-flop is a memory element that can store and output a logic “0” or logic “1” depending on the two inputs SET and RESET or S and R, respectively. The outputs of the flip flop are Q and QBAR, where Q and QBAR are the complements of each other.
The output of this flip-flop is then given to the output driving circuit to raise the current levels and is then finally passed to the external output pin of the IC.
1.4.2 Working of the 555 timer
Understanding the working of 555 timers can be a little overwhelming at the first look, so to make the process a little bit easier, we’ve divided the circuit into two parts. Let's look at the first part of the circuit having the voltage divider and comparators.
The behavior of the 555 timers is controlled by the three input pins: Threshold, Trigger, and Control Voltage.
When the voltage on the threshold pin increases above the 2/3 Vcc reference voltage, we get a logic “1” at the output “A”. Otherwise, the output “A” is a logic “0”.
When the voltage on the trigger pin is less than the 1/3 Vcc reference voltage, we get a logic “1” at output “B”. Otherwise, we get a logic “0”.
Then comes the role of the second part of the circuit: SR Flip-Flop and the output driving circuit. There are two inputs to our flip flop: the output of the threshold comparator labeled as “A” connected to R, and the output of the trigger comparator labeled as “B” connected to S.
There are three operating modes of the 555 timers: Monostable, Bistable and Astable. Various combinations of capacitors and resistors are connected to the input pins of the 555 timers to switch between these modes. This allows us to create different applications with the 555 timers, just by rearranging the externally connected components.
This is also known as the "one-shot" mode. When triggered, the timer generates only a single output pulse and returns to its stable state. Uses include time delay generation, touch switches, pulse width modulation, and many more.
In this mode, the timer acts as a flip flop as it has two stable modes. We can store 1 bit of data using the timer. However, this is not a preferred method for storing data.
In this mode, 555 acts as an electronic oscillator. The output continuously switches from logic high to logic low as per the configured period. This mode is used for pulse generation, logic clock generation, LED, and lamp flashers.
The different operating modes of the timer allow for a vast range of applications. We’ll be looking at the working of these modes in detail in the upcoming tutorials. This was just an introduction to get you familiar with the internal working of the timer. It can be said that we’ve just scratched the surface of the true potential of these timers.
III. FORMULAE
IV. SIMULATION AND RESULTS
Waveforms
Comparator
Start up condition
Steady State
V. CONCLUSION
The modified 555 timer IC without discharge transistor and SR flip-flop was presented in this paper. The proposed interconnection between two comparators was introduced to increase efficiency and lower the production cost with fewer components. The configuration of a free-running oscillator using the modified 555 timer circuit was analyzed and formalized. Various parameters which affect the oscillation frequency were discussed and compared. The comparator circuit using 5 transistors for IC layout design was confirmed using a simulation result. The simulation and experimental results validate the proposed 555 timer circuit as a promising candidate for high frequency, high power density, and high efficiency relaxation oscillator. In future works, it is interesting to design the integrated circuit of the modified 555 timer for a wide range of future applications.
CONTRIBUTORS:
Syed Abubaker Bin Junaid (221EC257)
Ajit Kumar Yadav (221EE104)
Pranav Jois (231EC237)
Report prepared on May 9, 2024, 11:46 a.m. by:
Report reviewed and approved by Aditya Pandia [CompSoc] on May 9, 2024, 11:47 a.m..
