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:

1. Initially, op-amp 1 is in its active state, and S1 is closed, connecting R1 and R2 to the bridge.
2. 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.
3. This connection change alters the voltage balance at the bridge, which in turn affects the feedback loop of op-amp 1.
4. Op-amp 1 is now driven towards its inactive state, causing S1 to open, and the bridge shifts back to the initial state.
5. 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.