"Clocks that don't tick" from China with a Japanese display

Continuing my reviews of all sorts of interesting things from Tao, I decided to write about something I ordered simply "for fun"—partly out of curiosity, and partly because I really like VFD displays. And let's not forget that every beginner electronics hobbyist is supposed to build a clock of their own :)
These aren't my first clocks, of course, but whenever I build something like this, I remember my first ones, based on the KR145IK1901. So today's review is more of a nostalgic look back at clocks.


During one of my usual wanderings around Taobao, I stumbled across a little shop selling various VFD displays and devices that use them. It looks like the seller deals in all sorts of salvaged electronics, since some of the items are in pretty sorry shape. Maybe that's why I decided to "save" at least a few of them.
One of the displays was part of a "build your own clock" kit, although the word "kit" is being used very loosely here, since the process was more like a quest.

The seller's items arrived in several packages because I'd ordered different things. The clock board and display were packed separately. When I was unpacking everything, I was a bit disappointed, since the display was supposed to be included in the kit.

It turned up later: they'd simply packed it in with some other displays I'd ordered from the same seller. I'll tell you about those another time.


So, we have a kit: a VFD display and a circuit board with a set of components.


We dump everything out on the table, and you've probably already guessed... there's no instructions in the box. Not even a basic schematic or assembly drawing—just a pile of components.


The circuit board looks pretty good on the outside; at least it looks nice.


The board is double-sided and dates from 2014.



Looking more closely, you can see that the quality is not quite up to scratch: the vias are nowhere near the centers of their pads. It doesn't really matter much in this case, though.


The components included in the kit.


1. The clock is built around an STC15W401AS microcontroller. It also uses an STF16360EN VFD display driver and a DS3231 RTC.
2. They also included six transistors and an SMD LED.
3. Small resistors, diodes, capacitors, and a Zener diode.
4. And a transformer, several electrolytic capacitors, a photoresistor, another LED, and a photodetector.


As I mentioned, there was no schematic in the box, so I had to assemble everything mostly by intuition. I decided to start by soldering the parts I was at least 99% sure about, if not 100%.
It was obvious where the ICs went. I also identified the bridge rectifier and the spot for the Zener diode. The pair of transistors marked Y1 clearly belonged in the inverter—that was obvious from the circuit design. There was nowhere else to put the second transistor in the same package, and there was only one large capacitor, with only one matching spot on the board. The small ceramic capacitors all had the same value, and there were just enough of them to fill the spots on the board.


Before installing the display driver, I noticed that it was probably used: some of its pins still had solder on them.


Here's how it turned out. Not all the decoupling capacitors are visible here; some are on the other side.


There were also three more transistors: two PNP and one NPN.
I didn't even bother looking up the marking to determine whether it was an NPN transistor, since it could only have been used in the control circuit of another transistor.
The remaining pair was trickier: same package, same conductivity type. Searching by the markings and package turned up these possibilities:
Y2 - SS8550 - PNP, 25 V/1.5 A
BA - 2SA1015 - PNP, 50 V/150 mA, or BCW61A - PNP, 32 V/100 mA


One transistor drives the buzzer, while the other is connected to the display. You can also see that the second transistor (BA) has a higher voltage rating, so it must be the one connected to the display (VFD displays usually need more than 20–25 V).

Pure deduction :)


The resistors were much worse. There are no markings on the board, there are fewer component positions than resistors, and two resistors are left over.


I found a photo of the assembled device on the seller's page. I could just about make out the values of some components, which helped a little. After that, I had to rely on logic and a bit of circuit-design know-how again.


With the larger components, the main challenge was figuring out what to solder and in what order, so one component wouldn't get in the way of another later on.
Actually, there was one real problem, and a pretty serious one: the transformer. Again, a bit of logic: this is a push-pull inverter, and the transformer is designed to step up 5 V to 20–30 V.
It has four windings:
Primary power winding — split into two sections, with lower resistance
Primary feedback winding — a single winding
High-voltage secondary — a single winding, with noticeably higher resistance than the primary
Heater secondary — split into two sections, with low resistance.

Once I knew which winding went where, it was easy to figure out how to install the transformer. Later, in some photos, I noticed that the bobbin's key was facing the same way in every case, which confirmed my guess.


The only problem I ran into was the buzzer's polarity. From what I could see, it should clearly be the other way around, and the conductivity type of the transistor driving it points to the same conclusion. But there was an easy solution: I don't need the buzzer at all, so I didn't even solder it in. I also left out the IR receiver, since I don't have a remote anyway.


The display is the whole reason I decided to build this thing.


It's marked LD8140, and it may well have been made in Japan. What's more, it's a universal display that supports both dynamic and static operation, though that was already clear from the number of traces running from the driver and from the driver itself. Dynamic drive is used most often because it requires fewer pins on both the display and the driver, but static drive can sometimes look a little easier on the eyes.


A closer look. I only noticed later that there are just two heater filaments. For some reason, I expected to see three at this size.


Even if I'd mixed up the resistor values, it probably wouldn't have been a big deal. It might just have worked a little differently than intended.
Anyway, I connected it to the power supply, and it looked like the clock was at least trying to work. The current draw wasn't very high, so there were no obvious assembly mistakes. But it was behaving strangely: sometimes all the segments lit up, sometimes only some of them; sometimes it ran, sometimes it didn't.
I inspected the soldering more closely and found a few cold joints: on the microcontroller, which explained why the clock worked intermittently, and between the driver and the display, which explained why a couple of segments weren't lighting up. The cause was either poor tinning or the board having been left exposed to the air for too long.


I soldered everything, cleaned the board, and had no more problems. Now I can show you how it turned out. By the way, I installed all the components correctly the first time; nothing needed resoldering.


There was no battery included, so I salvaged one from some motherboard.


Assembly is complete, the clock works, and so does the automatic brightness adjustment.
You may also have noticed that it connects via a USB cable. When I tried connecting it to a computer, the LEDs flashed several times and a message appeared saying the device wasn't recognized.
Both LEDs are connected to the data lines, which is why they flash when it's plugged in. I found information online about similar clocks saying they can synchronize with a computer to set the time automatically using a Unix timestamp, but I have no idea how to use it.


As I understand it, the main controls are on the remote, which isn't included. It should let you set the alarm, reset the seconds, and adjust the brightness manually. I only have two buttons: one switches the clock into hour or minute setting mode, and the other toggles between the 12- and 24-hour formats.
While preparing the photos for this review, I noticed that in some shots the seconds dots appear at an intermediate brightness, even though they seem to turn off instantly to the naked eye.
It may look here as if the display glows blue, but in reality it's the more familiar green.


While I was at it, I remembered that I had another couple of clocks lying around somewhere on the balcony, put there because I no longer needed them. I took the trouble to rummage through the boxes and found them. After connecting the power, I discovered they still work too :)
The one on the left is based on the KR145IK1901 and runs on 12 V. It was used as a timer for switching a load on and off.
The one on the right is based on the KA1016HL1 and is just a clock.


The second clock is mains-powered through a capacitive dropper, followed by an inverter that converts 15 V to the voltage the clock needs. It does have galvanic isolation, although the distance between the primary and secondary sides is so small that you could say there practically isn't any.
It was made back in the last century, and the circuit boards were even drawn by hand with a ruling pen. In short, it's a real mess.


I actually have another clock too, from the Start kit, but in an enclosure version. I built it about 30 years ago, and it still works reliably and keeps pretty good time. I calibrated it back when I was studying at technical college; luckily, they had a decent frequency counter.

As for this clock, I can say that it's an interesting device in its own way, mainly thanks to its branded display, properly powered filament, backup battery, 5 V supply, automatic brightness control, static display, and generally good build quality. But the seller gets a thumbs-down for providing absolutely no assembly instructions—or any documentation at all. How can you sell kits like this?
It would be nice to make an enclosure for it next, but I haven't decided what kind yet. Acrylic is an option, but I don't like it much; wood would be a bit too ordinary, and I probably couldn't make a decent one out of metal. So I'm still thinking about it.