LED Travel Clock

Although I've built a lot of clocks over the years, I needed one more, one I could take with me on a trip. Since size and weight were a factor, I decided to look for a commercially built clock. I found a few candidates and even bought one, but the display was unsuitable. Then I found a clock kit that looked good, from a Chinese outfit called Banggood, for a very attractive price, which I bought. (Update: there are newer, less expensive models with more features available on ebay.) In case the link doesn't work anymore, the clock (in its optional clear plastic case) looks like this:

Banggood clock

The clock has a large bright (too bright for night use) LED display. It is easy to build and looks great. It has a STC15F204EA microprocessor and a DS1302 RTC, and two tiny pushbuttons on the side, which cannot be reached when the clock is inside the optional case! Power is supplied by a small 5V wall wart. Here is the spec sheet. All the clock programming is done by these two buttons, which is where the difficulties arise. Try as I might, I could not consistantly program the clock. After wasting an hour, and because of the unreachable buttons, I decided to build my own clock after all. But that display was so nice, how could I make use of it? The problem was, the displays are soldered on the bottom side of the PCB, so you lose access to the circuit board. No way I could unsolder them either. I decided to take the following approach:

the projectDaughterboard

I found a very nice aluminum project box in my collection, cut a large hole for the LED display, and glued the original unit in place. I cut a hole in the side panel for the power cord, which goes to the daughter board. The rotary encoder is mounted on the daughter board, mounted to the front panel which holds it all in place. Now the ATmega88 is in control, and I know how to program it.

The daughter board consists of the ATmega88, a power socket, a programming header, a rotary encoder and a power line decoupling capacitor. I used the original schematic, just assigning ATmega88 pins to the functions that the STC micro controlled.

Banggood clock schematic

STC Function AVR
1 P12 LED-c 4 PD2
2 P13 LED-d 5 PD3
3 P14 LED-e 6 PD4
4 P15 LED-f 11 PD5
5 P16 LED-g 12 PD6
6 P17 LED-dp 13 PD7
7 P00 DS1302 !RST 16 PB2
8 Vcc +5 7 Vcc
9 P01 DS1302 I/O 9 PB6
10 Gnd Gnd 8 Gnd
11 P30 S2 nc
12 P31 S1 nc
13 P32 DS1302 SCLK 10 PB7
14 P33 Beeper 15 PB1
15 P34 LED-A1 23 PC0
16 P35 LED-A2 24 PC1
17 P36 LED-A3 25 PC2
18 P37 LED-A4 26 PC3
19 P10 LED-a 2 PD0
20 P11 LED-b 3 PD1
Enc-A 27 PC4
Enc-B 28 PC5
Enc-SW 14 PB0
ISP 5 1 !Reset
A/D pins not used 20, 21, 22

Vcc is applied from the daughterboard to the main circuit board through a diode such as 1N4001. I found this to be necessary because without it, the LED displays do not turn on and off properly. The common anode LED displays are switched using four PNP transistors, which need their bases driven close to the Vcc rail to turn off completely. With a Vcc of 5 volts, the ATmega88 output ports turned on only deliver around 4.3V, insufficient to fully turn these transistors off. The diode drops the main board Vcc enough so that the transistors can be driven fully off.

The pushbutton rotary encoder is a model EC11-1B-18T made by Changzhou Xinze Electronic Co. It is very similar mechanically and electrically to the CTS series 290 rotary encoder I used before. As a bonus it is easy to panel mount, (as long as you can find a nut that fits it), and works quite well.

The decimal point on the last digit indicates PM if on, and AM if off. The decimals on digits 2 and 3 make up the colon between hours and minutes, and are always on. The decimal on the first digit indicates whether the alarm is on or off.

Programming the clock is simple.
Twisting the encoder knob turns the alarm on and off. Press the encoder button and programming mode is entered. If the alarm is on when the button is pressed, the alarm time is displayed and may be changed. Otherwise the clock time can be set. First hours blink, and twisting the knob changes the hours. Press the button and minutes blink and may be similarly changed. Press the button and you return to nornal clock display. Keep in mind that when clock time programming is complete, the time is updated in the DS1302 with seconds=zero.

When power is off the DS1302 is powered by its battery, a CR1220 coin cell. Alarm time and setting is stored in EEPROM so it will not be lost.

*Yes really. I tried 1K then 4.7K, both too bright in a dark room. Unless you like an alarm clock+nite light. I suggest that if you build the kit, use a 16 pin DIP socket in place of the 8 resistors, if it fits. Then you can use a DIP resistor, then it's easy to try different values. Update 2017-08: newer versions of this clock kit are available that have filters that go over the LED display and improve the contrast greatly. They also figured out how to make the programming buttons accessible. But programming it is still a PITA, and it forgets all its settings when the power fails.

Download C source code for the travel clock

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