Make your own DTMF decoding digital display

The circuit principle is shown in the attached figure. DTMF digital decoder IC1 uses MT8870. Pin 2 of the chip is the IN signal input terminal, and pins 7 and 8 are connected to external crystals as clock oscillators. The frequency of the clock oscillator circuit is 3.579545MHz. The generated dual audio signals are high-frequency groups of 1209Hz~1633Hz and low-frequency groups of 697Hz~941Hz, with a total of 16 combined frequencies. After digital operation, the dual audio signals input from pin 2 output four-bit binary code ABCD at the data output terminals 11, 12, 13, and 14, and a positive pulse signal is output from pin 15 at the end of decoding as the latch pulse of the subsequent circuit.
CD4511 is a seven-segment BCD latch decoder that drives a common cathode digital tube display. Pins 7, 1, 2, and 6 of CD4511 are ABCD code input terminals, pin 3 LT is a test light, pin 4 BI is a blanking pin, pin 5 LE is a latch control terminal, and pins 9 to 15 are used to drive the digital tube to display characters. The output terminal uses R5-R12 current limiting resistors to make the LED digital tube display 0-9 numbers. The digital tube uses a common cathode type. Pin 15 of IC1 outputs a positive pulse signal as a latch pulse for the subsequent circuit, which passes through VD1 and R4 to the base of VT1, so that the collector outputs a low-level signal. When there is a low-level signal on pin 8 of IC2, the digital tube can be driven to display characters. The displayed characters are numbers 0-9.
The author has been using this decoded digital display for more than half a year. With the help of a frequency meter and an oscilloscope, it has brought a lot of convenience to adjusting the dual-audio frequency and inspecting and testing the dual-audio signal when repairing the radio station.