5-V-Only Circuit Generates Dial Tone

Aug. 19, 1993
This circuit uses inexpensive, common components to generate a precise dial tone for phone applications (see the figure). U1 (an Intel 82C54 timercounter)

This circuit uses inexpensive, common components to generate a precise dial tone for phone applications (see the figure). U1 (an Intel 82C54 timercounter) generates 350- and 440-Hz square waves that are filtered by R1/C1 and R3/C2 and mixed together by resistors R2 and R4.

An operational amplifier configured as a 395-Hz, Sallen-Key, second-order bandpass filter (halfway between 350 and 440 Hz) removes unwanted signal harmonics.

Almost any timer-counter can be used as the signal source, so long as it produces roughly square wave outputs.

If a counter-timer device that provides an overflow pulse is used, flip-flops configured as dividers should be connected between the counter and the filter-mixer to generate square waves.

The node at the noninverting (+) input of op amp U2A has a high impedance and should be well shielded to prevent noise from entering the circuit.

A 220-pF (C5) capacitor was added between the (+) and (-) inputs of the op amp to reduce the high-frequency impedance at the noninverting input and also to reduce its susceptibility to noise.

Sponsored Recommendations

Highly Integrated 20A Digital Power Module for High Current Applications

March 20, 2024
Renesas latest power module delivers the highest efficiency (up to 94% peak) and fast time-to-market solution in an extremely small footprint. The RRM12120 is ideal for space...

Empowering Innovation: Your Power Partner for Tomorrow's Challenges

March 20, 2024
Discover how innovation, quality, and reliability are embedded into every aspect of Renesas' power products.

Article: Meeting the challenges of power conversion in e-bikes

March 18, 2024
Managing electrical noise in a compact and lightweight vehicle is a perpetual obstacle

Power modules provide high-efficiency conversion between 400V and 800V systems for electric vehicles

March 18, 2024
Porsche, Hyundai and GMC all are converting 400 – 800V today in very different ways. Learn more about how power modules stack up to these discrete designs.

Comments

To join the conversation, and become an exclusive member of Electronic Design, create an account today!