Design and production of digital number answering machine

This article introduces the design and production of a multi-channel answering machine composed of digital circuits that displays priority answering numbers.

1. Circuit composition and working principle

The attached picture is the schematic diagram of the digital display number answering machine. Priority encoder 74LS147 and 9 group key switches 1-9 form a response input circuit; D flip-flop 74LS373 and AND gate 74LS21 form a latch circuit; NOT gate 74LS04, seven-segment BCD-decoder CC4511 and seven-segment digital display Compose a decoding display circuit.

This answering machine can provide up to 9 channels of answering. Each group is controlled by an answering key switch, which is active at low level.

The host controls the reset switch S, which is used to control the clearing of the system and the start of answering questions. Press button S to position a to clear the existing displayed numbers and display 0. At the beginning, the reset switch S bounces back to the b position, and the display is still 0, waiting for a response. If the group buttons 1-9 are not pressed, that is, the input terminals 1-9 of 74LS147 are all high level, then the output terminals A, B, C, and D are also high level, through the D flip-flop 74LS373 through the NOT gate 74LS04 The inverted signal is sent to the decoder CC4511 to make the seven-segment display display 0. At the same time, the high level after the AND gate is used to control the allowable terminal G of the D flip-flop and put it in a waiting state. If any one of 1-9 is pressed, the terminal inputs a low level, and the corresponding encoder output terminal is sent to the display circuit according to the encoding number to display the number (note that the encoder is the inverse code of the BCD8421 code ), at the same time, there must be a low-level output among the encoder output terminals A, B, C, and D, so that the D flip-flop enable terminal G=0 and the latch is latched, that is, the D flip-flop output terminals 1Q, 2Q, and 3Q , 4Q status remains unchanged, and the displayed number remains unchanged. If you want to answer again, you need to reset it through the reset switch S.

2. Component selection and circuit production

The reset switch S can be used as the AN8 series lathe control button. The power switch K is a toggle switch. Select 10kΩ/0.25W for R1-R9. R10~R14 select (100Ω, 200Ω)/0.25W. Choose a seven-segment digital display with a common cathode. If you choose a common anode, the decoder model should be changed. The circuit is powered by a single 5V power supply. Answer buttons No. 1-9 can be connected through the terminal block. from the circuit board. If the system is assembled and welded correctly, reliable operation can be guaranteed.

3. Design features

Using an encoder, the input terminals 1-9 are all at high level, that is, when the buttons 1-9 are not pressed, the output displays 0, which is a waiting state for answering. The control signal of the allowable terminal G of the D flip-flop is obtained directly from the output terminal of the latch. The circuit is simple and the response speed is fast. D flip-flops, AND gates, NOT gates and rich ports can be used to combine them with timing circuits and audio circuits to increase system functions.