www.ledwatch.net   For Buyers and Collectors of '70s LED Watches.
		Almost every day I get emails from people asking how to repair, adjust and understand their LED watch and so as a resource for fellow collectors I aim to take you through the operation of a LED module, in this case the Pulsar P3 module. These models are quite easy to understand and repair, and although I have seen various sites giving very technical information such as ''the crystal oscillator works on 32khz'', this doesn't really tell people with little or no Electronic knowledge how these watches work. So without the technical jargon, here is the Pulsar P3 module explained. On the right is a picture of an early Pulsar module showing the relevent components, let us look in turn at what each one does, and why when it stops working it will cause the fault that it does. If you look at the circuit diagram on the right and refer to the picture above, lets start, now dont panic - theres no meaningless techno-babble here. The Crystal oscillates at a steady frequency of 32khz, and this signal is fed into the 'front end' of the main IC (Integrated Circuit). Should this part of the circuit fail then the module will simply stop, freezing on a particular number because the IC is not getting its initial feed. This is the most common fault on these modules, ever seen them showing a '8' or a '0'?? ofcourse you have!! The main IC is a collection of over 1400 transistors which form a clever logic circuit. This is the brains of the operation and it divides the oscillator frequency down and also provides an output to the display. Should this part of the circuit fail then you will usually have no display whatsoever and if the IC does fail then this is really terminal :-((( Although a dead module doesnt necessarily mean that the main IC is dead. The main IC then drives the output transistors or buffers. These were required on the early modules since presumably the main IC wasn't up to driving the display directly and so these devices take the strain. On all the later Pulsar modules the main IC was adapted and drove the display directly, so you wont see any buffers which means that the module shrank in size, and so did Pulsars! If the display drivers or buffers fail then you will usally see a module working perfectly except that some of each number on the display segments has failed. The display itself is a clever device. It is actually a 'matrix' and further down the page I will tell you what I mean by this and why it is used. Right now lets get a liitle technical but DONT PANIC, I'm going to be gentle with you... What you can see on the right is an Oscilloscope waveform of 'test point one' on my diagram and also the module picture. This is the 32khz oscillator working and it is telling us a few simple things.... This is a 'graph' of voltage against time. The voltage is up and down, time left to right, and if you look at it you can see that the voltage on the crystal is varying from around 0.5 volts to around 1.5 volts and it is doing this 32,000 times per second. Now dont get frightened, thats all this is telling us, simply that the voltage is going up and down, very quickly. If you put a lightbulb on this point and you had the power of superman you would be able to see the lightbulb pulsing bright and dim 32,000 times every second. 32,000 can also be written as 32khz and that is why you will read people going on about the 32khz crystal. As you can see by the previous diagram, this varying voltage is simply a pulse which is fed into the main IC which reduces it down and decodes it. Wasn't too scary was it?? OK now you've been really good and concentrated so now we'll see if we can blow up a thirty year old display by messing around with it!! ;-)) I told you before that the display is a matrix, but what is that and why?? Lets start with the why: The display consists of this:[ l8:88 ] and each number has segments, the first number only ever reads '1' and so it has two segments. The next number needs to have seven segments, the colon needs two segments followed by the last two digits which need seven segments each. So the total number of segments to display the time would be twenty five, which means that to drive them all individually would need twenty five connections and one common, so twenty six in all. Too many!! So the display only has eleven connections, and by using a different combination of these connections and by flashing the display very quickly to fool the eye into thinking they are all lit at once, the number of connections is cut down by over half and because the display is not on permanantly, the batteries dont fry.... wonderful! And if you dont believe me that your display is flashing on your Pulsar or any LED watch - turn your house lights off and shake your watch from side to side whilst pressing the button and keeping your eyes fixed. Believe me now?? Well this is somewhat technical and you may want to skip this bit!! If you look to the left, this is the circuit diagram of a Pulsar Display matrix, if you have missing segments then use this to tell if you have a faulty display, or if the fault lies in the main IC. The blue dots and lines show the negative connections and the red dots and lines show positive. If you connect a two volt source between the relevent pins, your segments will light. DONT try this on a working module...haha... If you study this you can see that there is a pattern to the display and this will tell you if your display is faulty, or the driver.