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Project Notes

#088 Electronic Clock Kit

Build and investigate a commercially available “electronic clock” kit

The Build


I picked up a AT89C2051 Electronic Clock for USD3.28, primarily to investigate how it works. There seems to be a flood of similar kits available from many sellers on aliexpress, ebay etc.

TL/DR - it’s an interesting parts kit, and relies on the programming of the AT89C2051 for all the clock functions. The device works fine, provided sufficient power is attached (a 9V battery is fine).

The microcontroller is supplied pre-flashed without source code - however this may be original design and source code.

How it works

The smarts of the clock are programmed in the Atmel AT89C2051 microcontroller. The speed of the processor is governed by the 12MHz external crystal that is stabilised by the two 30pF capacitors.

The code is not provided, but seems it would be relatively easy to re-engineer. The How to Make a Digital Clock Kit Based on Atmel instructable is possibly the original design and source code, although I haven’t tested the code yet.

Power is provided through the 78L05 5V regulator, with a standard set of high/low frequency filter capacitors.

The three pushbuttons are direct digital inputs to the microcontroller. When pressed, they pull each of the digital inputs low.

The LED segments displays are common anode, with multiplexed control:

  • digital pins are used to select the digit to display by turning on the associated S8550 transistor
  • 7 digital pins are used to enable/disable the associated segment with a 220Ω current-limiting resistor in series (the decimal point segments are unused)

The “colons” between digits are provided with 4 LEDs. These are enabled/disabled as a block with a single digital pin.

The LED components are configured to sink current through the microcontroller’s Port 1 pins. Port 1 is an 8-bit bi-directional I/O port that can sink 20 mA and drive LED displays directly. The direct driving of the LED displays appear within spec, but it is getting close. Maximum total current for all output pins is 80mA In practice the circuit is drawing 60-70mA, most of which passes through the LEDs.

The “alarm” is provided by a simple piezo. Two connectors are provided to bypass/enhance the alarm:

  • J2 can bypass the buzzer. It could be used to disable the onboard buzzer, or run an external alarm function in parallel (depending on how the bypass affected the current to the base of Q7). The external function could not draw much power, as the only current would come from the microcontroller pin
  • J3 can also bypass the buzzer in a similar fashion, except this acts acorss the piezo, so has the full Vcc power supply available.

The J2/J3 configuration appears a bit strange. Neither positively bypass or provide parallel operation of external circuits. And if they are used to directly “short” the buzzer to disable it, they will likely damage the circuit if sufficient current is available from the power supply. A J2 short will overload an AT89C2051 pin. A J3 short could burn out Q7.

Directions for Use

This is how it is meant to work… translated;-)

Function keys:

  • S1 - function selection button
  • S2 - function extension button
  • S3 - value increment button.

Functions and operation instructions:

Short press of S1 (less than 1 second) cycles through the selection of six functions in a continuous loop. A long press of S1 (more than 2 seconds) immediately returns to the clock function state.


(1) clock function: after power-up, the clock is reset to 10:10:00

(2) Time adjustment:

  • Short press S1 sets the current time
  • Press S2 increments the hours count
  • Press S3 increments the minutes count
  • Note that seconds are not adjustment

(3) Alarm clock function:

  • After selection, defaults to 22:10:00
  • Press S2 increments the hours count
  • Press S3 increments the minutes count
  • Note that seconds are not adjustment
  • When hours incremented past 23 it will display -- : -- : -- prior to cycling back to 0. This represents alarm off.
  • Short press S1 to set the alarm time
  • When the alarm goes off, the buzzer will ring for 3 seconds

(4) Countdown function:

  • After selection, defaults to 0
  • Press S2 increments the countown scale
  • Press S3 increments the countdown value
  • After S2 has been pressed 7 times, the countdown starts.

(5) Stopwatch function:

  • After selection, defaults to 00:00:00
  • Press S2 to start stop-watch
  • Press S2 to stop stop-watch
  • Press S3 to reset timing

(6) counter function:

  • After selection, defaults to 00:00:00
  • Press S2 to increment counter
  • Press S2 to reset


The Schematic


PCB front

PCB rear


Qty ID Description
9 R1,2,11-17 1kΩ resistor
7 R4-10 220Ω resistor
1 R3 10kΩ resistor
1 R18 5.1kΩ resistor
1   5Ω resistor
2 C5,6 30pF ceramic capacitor
2 C1,4 100nF 104 ceramic capacitor
2 C2,3 10μF electrolytic capacitor
1 T1 12MHz crystal
3 S1-3 pushbutton switch
1 U2 AT89C2051 microcontroller
1 U1 78L05 5V regulator
7 Q1-7 S8550 PNP transistor
1 U3 piezo speaker
4 D1-4 3mm red LED
3 DS1-3 CAI3261BS 2-digit/7-segment LED
1 J1 2-connector terminal block
2 J2,3 2-pin connector
1   PCB

Credits and References

Project Source on GitHub Project Gallery Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.