bridge-controlled multivibrators

Bridge-Controlled Multivibrators: A Novel Approach to Frequency Control

Multivibrators, ubiquitous in electronics, are oscillators generating periodic waveforms. While traditional multivibrators rely on fixed components for frequency determination, bridge-controlled multivibrators introduce a new level of flexibility by allowing frequency control via a resistive bridge. This article delves into the concept of bridge-controlled multivibrators, exploring its implementation using operational amplifiers and highlighting its potential applications in sensor design.

The Essence of Bridge-Controlled Multivibrators

The core of a bridge-controlled multivibrator lies in its ability to "rotate" the bridge configuration during each half of its oscillation period. This dynamic switching, typically achieved with transistors or comparators, allows the bridge to influence the timing of the oscillator. By detuning the bridge resistors, one can directly manipulate the frequency of the generated waveform.

Implementation: Two-Operational Amplifier Configuration

A simple bridge-controlled multivibrator can be implemented using two operational amplifiers (op-amps) in a classic astable configuration. The bridge, consisting of four resistors (R1, R2, R3, R4), is connected to the inverting inputs of the op-amps. Two switches (S1, S2), controlled by the output of each op-amp, effectively "rotate" the bridge during each half-cycle.

Operation:

Initially, op-amp 1 is in its active state, and S1 is closed, connecting R1 and R2 to the bridge.
The output of op-amp 1, due to positive feedback, is high. This triggers op-amp 2, causing S2 to close, connecting R3 and R4 to the bridge.
This connection change alters the voltage balance at the bridge, which in turn affects the feedback loop of op-amp 1.
Op-amp 1 is now driven towards its inactive state, causing S1 to open, and the bridge shifts back to the initial state.
Op-amp 2 is now active, initiating the next cycle.

Frequency Control:

By adjusting the values of the bridge resistors, one can manipulate the charging and discharging rates of the capacitors within the circuit, effectively controlling the frequency of oscillation. For example, increasing R1 and R2 will lengthen the charging time of the capacitor, resulting in a lower oscillation frequency.

Advantages & Applications:

Bridge-controlled multivibrators offer several advantages:

Flexibility: They provide a convenient method to adjust the frequency without physically changing components.
Compactness: The bridge can be integrated into the same circuit board as the multivibrator, simplifying the design.
Remote Control: By remotely controlling the bridge resistance, one can achieve remote frequency adjustment, ideal for sensor applications.

Sensor Applications:

Bridge-controlled multivibrators can be used in sensors with limited access wires:

Pressure sensors: By integrating the bridge with a pressure-sensitive element, changes in pressure can directly alter the bridge resistance, influencing the oscillator's frequency. The frequency can then be transmitted to a remote receiver using a single wire, simplifying the system.
Temperature sensors: A temperature-sensitive resistor (thermistor) can be included in the bridge. As the temperature changes, the thermistor resistance varies, altering the bridge balance and influencing the oscillator frequency, allowing remote temperature monitoring.

Conclusion:

Bridge-controlled multivibrators offer a unique and powerful approach to frequency control. Their adaptability, compactness, and remote control capabilities make them attractive for a variety of applications, particularly in sensor systems with limited access points. This technology opens doors for innovative and efficient sensor designs, contributing to advancements in various fields.

Test Your Knowledge

Quiz on Bridge-Controlled Multivibrators

Instructions: Choose the best answer for each question.

1. What is the primary advantage of a bridge-controlled multivibrator over traditional multivibrators?

a) Higher frequency range b) Lower power consumption c) Flexibility in frequency control d) Improved stability

Answer

c) Flexibility in frequency control

2. How is the frequency of a bridge-controlled multivibrator adjusted?

a) By changing the capacitor values b) By changing the op-amp gain c) By adjusting the bridge resistor values d) By varying the power supply voltage

Answer

c) By adjusting the bridge resistor values

3. What is the role of the switches (S1 and S2) in a bridge-controlled multivibrator?

a) To isolate the bridge from the op-amps b) To control the gain of the op-amps c) To dynamically switch the bridge configuration d) To provide a reference voltage for the op-amps

Answer

c) To dynamically switch the bridge configuration

4. Which of the following is NOT a potential application of bridge-controlled multivibrators in sensor design?

a) Pressure sensors b) Temperature sensors c) Light sensors d) Humidity sensors

Answer

c) Light sensors

5. What is the core principle behind the operation of a bridge-controlled multivibrator?

a) The bridge configuration rotates during each half-cycle of the oscillator. b) The bridge acts as a filter to shape the oscillator's output waveform. c) The bridge creates a feedback loop to stabilize the oscillator's frequency. d) The bridge provides a fixed reference voltage for the op-amp circuit.

Answer

a) The bridge configuration rotates during each half-cycle of the oscillator.

Exercise on Bridge-Controlled Multivibrators

Task:

Design a simple bridge-controlled multivibrator circuit using two op-amps (LM741) to generate a square wave with a frequency adjustable from 1 kHz to 10 kHz. You are free to choose appropriate resistor values for the bridge, but ensure that the frequency range is achievable. Provide a schematic diagram of your circuit with clearly labelled components.

Hint: Remember that the frequency is inversely proportional to the RC time constant of the charging and discharging capacitors.

Exercice Correction

Here is a possible solution for the bridge-controlled multivibrator circuit. It's important to note that this is just one example, and other component values and circuit configurations can also achieve the desired frequency range.

**Circuit Diagram:**

**Explanation:**

**Op-amps:** Two LM741 op-amps are used in the astable configuration for oscillation.
**Bridge:** R1, R2, R3, and R4 form the resistive bridge. The values chosen ensure the frequency range is achievable.
**Switches:** S1 and S2 are controlled by the output of each op-amp, dynamically switching the bridge configuration. (You can implement these with transistors for practical realization.)
**Capacitors:** C1 and C2 determine the oscillation time constants, in combination with the bridge resistors. Their value is chosen to accommodate the desired frequency range.
**Frequency Adjustment:** By changing the bridge resistors (R1, R2, R3, R4), you can adjust the charging and discharging time constants, thus controlling the frequency.

**Frequency Range:** The chosen components allow for a frequency range roughly between 1kHz and 10kHz. You can adjust the resistors in the bridge (R1, R2, R3, R4) to fine-tune the specific frequency range and obtain the desired square wave output.

bridge-controlled multivibrators