Monday, December 19, 2016

A simple and cheap alarm clock with the old TMS3450NL / LM8560 (putting scrap components back to work)

Introduction

Last week I found this strange chip (TMS3450NL) in my junk component box while searching for something for a project. After doing some research, it seems to have been produced by Texas Instruments  and it is equivalent to Sanyo's LM8560. And it's old, really old. From the time stamp, it looks to be made in the third week of 1993. Yes, that old.

According to the datasheet, it it a 4-digit clock IC that uses a duplex LED display to show the time. The chip doesn't use too many external components, it has a 12/24h display mode, supports an alarm (900 Hz tone) and it can use a backup battery to keep the time in case of power loss. Pretty neat.

If you have one of those clock radios from back in the day, it most likely uses a variant of this chip (or maybe a Chinese knock-off).


The schematic

Since I was in need of a easy to read clock, I decided to make one with this chip. The only difficult part was finding a suitable display (duplex LED, model 6052-S). I was lucky to find one in one of the scrap boxes. In case you want to build the project and you can't source a suitable common cathode duplex display, here's the electrical schematic of one that's tested to work:

6052-S LED Duplex display

Common cathode duplex displays are different from the single common cathode ones because instead of a single cathode, they use 2 of them which are lit one by one. So when half of the display is on, the other half is off. This process is repeated fast enough that our eyes see the complete image. The advantage of this design is that it allows to use less wires to connect the display to the board.

It shouldn't be too hard to use normal leds instead of a display. And as a plus, you can make it as big as you wish as long as the the power consumption is held within the chip limits. Before going further, there are a few things that need to be kept in mind when working with this chip:

1. It's quite sensitive to ESD, so make sure to avoid touching the pins by hand (or better, use an ESD wrist strap)

2. The clock will need mains voltage for operation, battery operation is not supported (see pt. 3 for the reason)

3. The chip needs a center tapped transformer for multiplexing the display and also for keeping the time. The 50 Hz or 60 Hz pulses from the mains are used instead of an external oscillator, probably to keep things simple and thus cheap. One disadvantage of this design is that the clock will only be as accurate as these oscillations are close to the rated value. This is not much of a problem today, but back in the day (especially in eastern Europe), the mains had probably around 46-47 Hz so electronics engineers either avoided using this IC or made separate oscillator blocks with quartz crystals (search for 50 Hz timebase circuit).

4. The chip can drive the LED display directly. The power supplied for each of the display segments is 18 mA. The chip's absolute maximum power dissipation is 700 mW. It's best to keep the power consumption to a minimum, especially if discrete LEDs are used for the display (for details consult the datasheet available here: http://www.unisonic.com.tw/datasheet/LM8560.pdf).

5. The maximum input voltage for the chip is -14V so make sure to use a suitable transformer. I used a 2 x 6V one, rated at 800 mA so it doesn't get too hot. You can also go with a 2 x 7.5V one, but nothing higher.

That being said, here are the schematics and a PCB for the clock:

TMS3450NL / LM8560 clock schematic

PCB:
https://drive.google.com/open?id=0B7qGmYL2UHFNdDVieE1ta1dhWFE

Silkscreen:
https://drive.google.com/open?id=0B7qGmYL2UHFNby1LTkxORkJaczA

The PCB image is already mirrored. Arrange it over the photo-sensitive PCB in such way that you can read the text normally through the transparency.


Some other details

The chip can work with both 50 Hz (this is what I used) or 60 Hz mains frequency. You can leave pin 26 disconnected for 60 Hz, or leave it grounded to use it at 50 Hz (with care, as ground is VSS here. (+) terminal since this is a PMOS chip). The brightness of the display can be adjusted by changing R1 and R2 (increase their value to make it dimmer). Each of those resistors is responsible for limiting the current on the 2 cathodes. Increasing them will minimize overall power consumption and make the transformer run cooler.

When the power is out, the chip will activate an internal 900 Hz oscillator that uses C3 and R4. While the power is out, screen output will also be off. Without the backup battery, the time will simply reset.

Regarding the display mode, the current build shows the time in military format (24H). To make it use 12H mode (AM / PM), leave pin 28 of the IC disconnected.


Putting it all together

First the PCB was built using the photo-transfer method then etched.

Etching the board

After thorough cleaning, I used a small piece of expended desoldering braid to cover the traces with solder. This will prevent oxidation of the copper.

Ready board

The next step was to solder all the parts and find a cable to connect the screen. The one in the picture was cut from a broken floppy disk ribbon cable.

Parts soldered on the board


Upon testing, this is what I got:

Display problem. Fortunately there's a simple fix available


If  the screen shows 15:66, you need to reverse the 2 cathode wires (display wires 1 and 2 on the PCB).

Last step was to build a case for the clock. I wanted something vintage, so the material of choice was plywood. The pieces of plywood were glued together using epoxy (all of them except for the bottom, which was fixed using 4 screws).




As a last step, I have varnished the case, added a border around the screen and some rubber feet. This is the result:


Not the best looking project out there, but it works :).

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