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Phase-locked-loop tone decoder

LM567 Working Principle

Learn how LM567 works with an illustrated explanation and practical readings.

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LM567 at a glance

Follow the labeled connections and functional stages

LM567 working principle: illustrated functional stages, basic circuit and connections
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How it works

3 stages · Explore one step at a time

Step 1 / 3

Choose the center frequency

Use f₀ ≈ 1 / (1.1 × R1 × C1). With 10 kΩ and 100 nF, the nominal center is 909.1 Hz.

Step 2 / 3

Filter the detector signals

The 1 µF loop capacitor and 2.2 µF output capacitor reduce unwanted responses. Larger filter capacitance generally trades faster response for stronger filtering.

Step 3 / 3

Read active-LOW detection

A settled, sufficient tone near the center turns the output transistor on. The 10 kΩ pull-up limits sink current to less than 0.5 mA at 5 V. Out of band, it releases the output.

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