Electronic Keys & LocksFrequency multiplierPower Supplies

Frequency-operated switch

Innovative Frequency-Operated Switch Using 4046 PLL

While voltage-operated switches are commonly encountered, frequency-operated switches remain relatively rare. The one presented here is constructed around a 4046, a phase-locked loop (PLL) with primarily digital internal components. The choice of inductor L1 is directly linked to the switching frequency. Higher switching frequencies lead to smaller inductors but may raise switching losses. For low-input voltage applications, a favorable compromise is a switching frequency of fs = 150 kHz, as per the HV9910B datasheet, which necessitates a 150 k timing resistor between the RT pin and ground.

Comparing Two Frequencies for Control

To enable frequency comparison, the two frequencies in question are introduced to pins 3 and 14 of IC1. These frequencies must be in rectangular form, possessing amplitudes within the 3-5 V supply voltage range. Notably, their duty factor is unimportant since the IC is solely responsive to leading transitions or edges. When ƒ1 is lower than ƒ2, the output goes low. Conversely, when ƒ1 equals ƒ2, a rectangular voltage with a stable duty factor determined by the phase difference emerges at pin 13. This voltage is transformed into a direct voltage using R1 and C1, subsequently applied to the switching transistor T1 through a source follower. If the voltage level surpasses a certain threshold, T1 is triggered, initiating relay operation.

Frequency-operated switch Schematic diagram

Ensuring Accuracy with Practical Considerations

While the theoretical error rate of the switch stands at 0%, practical applications necessitate a margin for error, typically allowing 0.1% tolerance. Achieving accuracy demands a specific time constant for R1-C1, approximately 10 times the period of the input signal. However, excessively higher ratios lead to unnecessary delays in circuit operation.

Handling Close Frequencies and Circuit Activation

In scenarios where input frequencies are nearly identical, the circuit might require up to one period of the difference frequency to enable operation, especially in worst-case situations. This delay factor needs to be considered when dealing with closely matching frequencies to ensure timely circuit activation.

Versatility and Power Management

The circuit design accommodates a wide voltage range, operating efficiently within 3-15 V supplies. Crucially, it’s essential to match the supply and relay voltages for seamless functionality. Transistor T1 has a capacity to handle up to 100 mA, with a total current draw of approximately 0.5 mA plus the relay current, ensuring efficient power management.



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