--Oz--'s pages

tec test-1

I got this clock on AliExpress for ~$4, my 11yo boy put it together and soldered it up with my help.

It worked first try, nice clock.

What I didn't like was the 24h format and the blinking colon LEDs was too bright.

The mod was to cut 3 tracers , add 5 jumper wires, a diode and a cap (220pF), all marked in red in the schematic.
The way I did the 12h hack was to reset the hours when it rolls over to 13h. I needed a 3 input AND gate, and U12 had 2 spare 2 input AND gates, combined = 3 input AND gate. The cuts/jumpers decodes the hours and sends a reset via the diode to the hours counters. I found it was more stable adding a 220pF cap for the reset. You can see the SMD cap on the back of the board below the blue jumpers.

If you get a far off crystal, you can tune the 30pF caps to trim the speed of the crystal. I used a cap decade box and a freq counter, now its about 1 sec a month error.

The blinking colon LEDs was easy to dim by increasing the resistor values.

Here is the kit https://www.aliexpress.us/item/3256806133682830.html

Here is the description of how the clock works: (from the website)

This product is a 24-hour digital circuit clock, using CD4518, CD4511, CD4081, CD4013, CD4060 and other chips, the circuit does not contain a microcontroller, so there is no program, hours, minutes and seconds can be calibrated, without alarm function.

This kit mainly consists of a second signal generator, counter, decoding display and time calibration circuit. The second pulse is a 1HZ square wave signal obtained by precise frequency division of a high frequency signal generator, which is more accurate in timekeeping.

The second signal generator consists of CD4060 and CD4013, which generates a square wave signal with a frequency of 1HZ. CD4060 is a 14-level binary frequency divider/oscillator. It consists of a 32768HZ oscillator with external resistors R44, R43, C1, C2 and Y1. After 14 levels of binary frequency divider, the frequency of 2HZ square wave signal is obtained at pin 3. CD4013 contains two independent D flip-flops, take one of the flip-flops through the line configured as a binary counter, binary counting of the input 2HZ square wave signal can be obtained from the second signal.

CD4518 is a double-decimal addition counter. With 3 CD4518 for hours, minutes, seconds, hours configured for the 24-binary, minutes and seconds configured for 60-binary. CD4518 counting results in the form of BCD code from pin Q0-Q3 output to the BCD decoder CD4511, CD4511 will be converted to the BCD code corresponding to the display of the corresponding digital tube light segment code, the digital tube to form a digital can be recognized to visually display the current results.

Timing results.
Minutes and seconds are counted in 60 decimal, the seconds signal is introduced to the EN terminal of CD4518 through switch S1, and 1 is added to the falling edge of each seconds signal (if the seconds signal is connected to the CLK terminal of CD4518, 1 is added to the rising edge of each seconds signal), and when the counter counts up to 9, Q0-Q3 outputs 1001, i.e., Q3 is 1. As it is a decimal counter, when the next seconds signal arrives, the counter changes from 9 to 0, Q0-Q3 outputs 0000, thus forming a falling edge signal on Q3, this signal is introduced to the ten-digit counter of the second counter, and the ten-digit counter adds 1 to the ten-digit counter after each full count of 10 in the individual counter.

When the ten-digit counter value reaches 6, Q0-Q3 outputs 0110, i.e., Q1 and Q2 are both 1 at the same time, and Q1 and Q2 are connected to the counter through an and gate after outputting 1 to the Q1 and Q2 are connected to the reset terminal of the counter through an and gate and output 1, so that the counter counts from 6 to 0 in advance, completing the cycle of 1 hexadecimal counting. The reset signal is also used as the counting signal of the sub-counter, and the sub-counter will be increased by 1 for every full 60 seconds. The sub-counter...