This PPM circuit, with the help of a special chip 555, is like a messenger for your electronics.
It sends information by creating short bursts of electricity pulses.
Unlike Morse code with dots and dashes, PPM changes the timing of these pulses to send the message.
The strength amplitude and length width of the pulses themselves stay the same.
But the spaces between the pulses get longer or shorter depending on the information being sent.
This way, the circuit can send complex signals using just tiny jumps in time.
Circuit Working:
Parts List:
| Category | Component | Quantity |
|---|---|---|
| Resistors | 4.7k | 1 |
| 10k | 1 | |
| Capacitors | Ceramic 1nF | 1 |
| Electrolytic 10µF 25V | 1 | |
| Semiconductors | IC 555 | 1 |
In pulse position modulation PPM, the amplitude and width of pulses remain constant but their positions change based on the sampled value of the modulating signal.
The diagram above illustrates the simplest PPM circuit using an IC 555.
In this circuit, each pulses position changes, and both the width and period of the pulses vary with the modulating signal.
By applying the modulating signal to pin 5 Control voltage pin, the upper threshold voltage UTP changes to 2Vcc/3 + Vmod.
An increase in Vmod results in an increase in the UTP level and consequently an increase in pulse width.
Conversely, a decrease in Vmod leads to a decrease in the UTP level and pulse width.
Therefore, the pulse width varies.
The pulse width W is calculated by the below mentioned formula.
The formula for period T is explained below.
The constant space between pulses is given by 0.693 R2C.
This circuit finds various applications, for instance it is utilized in communication systems for voice or data transmission.
Formulas and Calculations:
Due to its ease of usage and adaptability, the 555 integrated circuit is frequently utilized to produce PPM pulses.
When triggered in this arrangement, it usually functions in monostable mode, generating a single output pulse with a fixed width W.
Using a 555 IC, the following formula may be used to determine the pulse width W in a PPM circuit:
W = – (R1 + R2) Cin [ (Vcc – UTP) / (Vcc – 0.5UTP) ]
where,
- W is the pulse width in seconds
- R1 and R2 are resistors connected to the 555 IC control voltage pin 5
- Cin is the capacitor connected to the 555 IC trigger pin 2
- Vcc is the supply voltage to the 555 IC
- UTP is the upper threshold voltage of the 555 around 2/3 Vcc
How the formula functions:
Through resistors R1 and R2 the formula basically calculates how long it will take the capacitor Cin to charge from the trigger level UTP to the threshold level 2/3 Vcc.
The pulse width and this charging time are correlated.
The RC charging circuits time constant is shown by the phrase (R1 + R2) * Cin.
The correction factor (Vcc – UTP) / (Vcc – 0.5UTP) takes into consideration the unique features of the 555 IC.
Important Things to Keep in Mind:
In a PPM circuit, the capacitor cins charging time dictates the pulse width.
The pulse width is directly influenced by the component values and configuration of the 555.
Although the method gives a theoretical calculation, component tolerances and other parameters may need revisions in real world implementations.
Essentially, the method uses a 555 to quantify the relationship between the circuit components and the pulse width that is produced in a PPM system.
Applying a 555 to a PPM circuit, the period T is calculated as follows:
T = W + 0.693R2C
where,
- T is the PPM signals duration in seconds
- W is the pulse width, as previously determined
- R2 is the resistor attached to pin 7 the discharge pin of the 555
- C is the capacitor attached to pin 2 of the 555 IC trigger
The Formulas Operation:
According to the formula, the PPM signals period consists of two parts
The length of the actual pulse is known as the pulse width W.
0.693R2C: This word indicates how long it takes for the 555 IC output to get low enough for the capacitor C to discharge through resistor R2.
The interval of time that separates two successive pulses is this time.
The capacitor discharge equation, which states that the voltage across the capacitor decays to around 37% of its original value after a time equal to the time constant R2C, is the source of the constant 0.693.
Important Points
The total of the pulse width and the interval between pulses determines the period of a PPM signal.
In the 555 IC circuit, the discharge duration of the capacitor through R2 determines the interval between pulses.
The relationship between the circuits constituent parts and the PPM signals overall period is made simpler by the formula.
Essentially, the formula aids in determining how long it takes for a PPM signal to complete a cycle, taking into account both the length of the pulse and the time between pulses.
How to Build:
To build a Simple Pulse Position Modulation Circuit using IC 555 follow the below mentioned steps:
- Connect pin 4 and pin 8 Vcc of the IC 555 to the positive power supply.
- Connect the modulating signal Vmod to the control voltage pin 5 through a capacitor C2 10µF.
- Connect a series of resistor R1 10k and R2 4.7k from the pin 7 to the threshold pin 6.
- Connect a capacitor C1 1nF through pin 2 to ground.
- Connect pin 3 to output.
- Ensure the trigger pin2 is connected to the threshold pin 6.
- Connect the pin 1 to ground of 555 IC.
Note:
- Values of R1, R2 and Cin can be chosen based on the desired pulse width and modulation characteristics.
- Ensure proper power supply and signal levels are used to avoid damaging the IC or circuit.
Conclusion:
A Pulse Position Modulation PPM circuit using the IC 555 is a simple yet effective way to generate modulated pulses where the position of each pulse is determined by an input signal.
By varying the position of the pulses instead of their width or amplitude PPM offers advantages such as high noise immunity and efficient bandwidth utilization making it a valuable technique in communication systems and other applications requiring precise timing and modulation.
The IC 555, configured in monostable mode serves as the core component of this circuit, providing a reliable and versatile platform for generating PPM signals.
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