Make your own DTMF decoding digital display

The author has made a DTMF digital display with reference to relevant materials and would like to share it with you.
　　The circuit principle is shown in the attached figure. The 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 oscillation terminals. The frequency of the clock oscillation circuit is 3.579545MHz. The dual audio signals generated 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.