DIAL TONE GENERATOR

This circuit provides a 1 volt peak-to-peak dial tone signal with equal proportions of 350Hz and 440Hz sine waves. The two frequencies are generated in multivibrator sections U1A and U1B. Triangle-like waveforms from the multivibrator sections are fed to summing amplifier U1D. A 470pF capacitor in the feedback path of U1D removes some of the unwanted harmonics from the composite waveform, which is then fed to section U1C, a Sallen-Key low-pass filter. U1C further reduces the harmonics, providing a high-quality summed-sine signal at its output. The two frequencies are close enough to permit a single-filter design, with the summing resistors at U1D's input selected to emphasize the 440Hz component, accordingly.

Calibration is performed by connecting a frequency counter to the "350 TP" and "440 TP" test points, and adjusting the respective trimmer potentiometers to obtain the correct frequencies. The frequencies are relatively independent of power supply voltage, and the single +7.5v power supply may be derived from a simple regulator circuit. The circuit consumes less than 3 milliamperes of current.

For convenience and economy, the circuit was designed with one CMOS quad op-amp integrated circuit.




PARTS FOR DIAL TONE GENERATOR

C1, C2, C5, C6 0.1uF, 5%, 50V
RADIAL POLYESTER CAPACITOR
PANASONIC TYPE ECQ-E2104JFB
MOUSER STOCK NUMBER 6667-ECQ-E2104JFB
C3, C8 1uF, 10%, 20V
RADIAL TANTALUM CAPACITOR
AVX TYPE TAP105K020SCS
DIGI-KEY STOCK NUMBER 478-1833-ND
C4 470pF, 5%, NPO, 100V
RADIAL CERAMIC CAPACITOR
MURATA TYPE 80-C320C471J1G
DIGI-KEY STOCK NUMBER 399-9780-ND
C7 4700pF, 5%, NPO, 100V
RADIAL CERAMIC CAPACITOR
MURATA TYPE  C320C472J1G5TA
DIGI-KEY STOCK NUMBER 399-4302-ND
J1 1/4" PHONE JACK SWITCHCRAFT TYPE 11
DIGI-KEY STOCK NUMBER SC1085-ND
R1, R2, R6, R7 30.1K OHM, 1%, 1/4W
METAL FILM RESISTOR
YAEGO TYPE MFR-25FBF-30K1
DIGI-KEY STOCK NUMBER 30.1KXBK-ND
R3, R8 178K OHM, 1%, 1/4W
METAL FILM RESISTOR
YAEGO TYPE MFR-25FBF-178K
DIGI-KEY STOCK NUMBER 178KXBK-ND
R4 78.7K OHM, 1%, 1/4W
METAL FILM RESISTOR
YAEGO TYPE MFR-25FBF-78K7
DIGI-KEY STOCK NUMBER 78.7KXBK-ND
R5 1M OHM, 5%, 1/4W
CARBON FILM RESISTOR
YAEGO TYPE CFR-25JB-1M0
DIGI-KEY STOCK NUMBER 1.0MQBK-ND
R9 61.9K OHM, 1%, 1/4W
METAL FILM RESISTOR
YAEGO TYPE MFR-25FBF-61K9
DIGI-KEY STOCK NUMBER 61.9KXBK-ND
R10, R12, R13, R17 470K OHM, 5%, 1/4W
CARBON FILM RESISTOR
YAEGO TYPE CFR-25JB-470K
DIGI-KEY STOCK NUMBER 470KQBK-ND
R14 56K OHM, 5%, 1/4W
CARBON FILM RESISTOR
YAEGO TYPE CFR-25JB-56K
DIGI-KEY STOCK NUMBER 56KQBK-ND
R15 10K OHM, 5%, 1/4W
CARBON FILM RESISTOR
YAEGO TYPE CFR-25JB-10K
DIGI-KEY STOCK NUMBER 10KQBK-ND
R16 470 OHM, 5%, 1/4W
CARBON FILM RESISTOR
YAEGO TYPE CFR-25JB-470R
DIGI-KEY STOCK NUMBER 470QBK-ND
RV1, RV2 10K OHM, 25 TURN
CERMET TRIMMER POTENTIOMETER
BOURNS TYPE 3299Y-1-103LF
DIGI-KEY STOCK NUMBER 3299Y-103LF-ND
U1 QUAD OPERATIONAL AMPLIFIER
INTEGRATED CIRCUIT
NATIONAL SEMICONDUCTOR TYPE LMC6484IN/NOPB
DIGI-KEY STOCK NUMBER LMC6484IN/NOPB-ND
MISCELLANEOUS
ITEMS
POWER SUPPLY,
CIRCUIT BOARD,
IC SOCKET,
COMPONENT TERMINALS,
WIRE,
CIRCUIT BOARD STANDOFFS,
FASTENING HARDWARE,
ENCLOSURE
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October 2, 2003
Updated August 5, 2004; April 8, 2009; November 25, 2011; October 5, 2012; January 17, 2017

Text and images 2003, 2004, 2011, 2012, 2017 by Arthur Harrison

Source documents dated November 21, 1987


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