This DIY multi-function electronic clock is designed with Alarm function, count down function, second chronograph function and counter function on the basis of the ordinary clock. This design makes full use of the SCM.

1.The circuit description

This circuit is made of SCM circuit, display circuit, Keyboard input circuit, signal ring circuit and power supply circuit.

① SCM circuit: it is based on the AT89C2051, including reset and clock circuit.

②Display circuit: The main components is 2 bit red LED tube, the driver is the PNP triode and each port is equipped with Current limiting resistor, the drive way is scan and it takes up the port P1.0 ~ P1.6. The colon part uses four Φ3 red light-emitting diode, and it uses independent port driver which takes up port P1.7.

③keyboard input: The key S1~S3 reuse with the P3.5，P3.4，P3.2 of the display part. The working principle is SCM read the value and input the corresponding value.

④Signal Ringing circuit: It is made of active buzzer and PNP triode. The working principle is that the buzzer will produce Fixed frequency noise when the PNP triode is working. It uses independent port driver which takes up port P3.7. The J1 is used to connect with external equipment and it will output low level when there is no signal ring, output high level when there is signal ring.

⑤Power supply circuit: It is made of three-terminal voltage regulator integrated circuit and provides stable voltage for the whole system.

2.Electrical parameters

• Working voltage: 7~15V DC
• PCB Size: 87.4*49.9mm
• Fixed hole: 3mm

3.Schematic diagram

4.Mechanical drawing Here