Can we some how connect 7 segment LED display to readily available and very cheap LCD wrist watchs. We can supply additional voltage required to drive the LED display and the wrist watch can continue functioning on its own button cell supply.

Any feed back.

-Mahesh

 

It will be a very messy job.

First of all you'll require to know precise SMD soldering.
Second thing is you require current booster (like transistor) to be connected on each segment line to drive the LED.
Plus a lot of fine wiring stuff will make the job tedious.

Its better you start building one from scratch. Or get a kit.

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well, i think that basicly you need to know the connections of the LCD and to create a display with the same connections from 7 segment displays. there are some led displays with 4 digits wich are designed for clocks, but i am not sure about the connections, if they match.
the biggest problem is to make the connections on the pcb. you could use a buffer and some transistors to drive the display.

 

That sounds simply, but not sure. First make some reverse engineering for LCD. The watch-chip is an ASIC, and most of case the LCD is a topology-multiplex type. Better case if the LCD "only" multiplexed. (each digit have own backplane and segments have common pin.In this case the four digit display have 7segment pin,4 backplane pin, an some pins for bell symbol, comma, e.t.c.)

 

Kinjalgp, you have got the point. If some how I can extend the end point connectors which connect the LCD with a conductive rubber strip, can I put some circuit on every connector (as you said current booster transistor on every connector). What will be the circuit? Please reply.

-Mahesh Joshi

 

You actually don't require conductive rubber strip.
Identify the segment connections and solder fine wire to each of the segments.

The schematic of simple current booster is shown below:

Attached Images

sch.gif (1.9 KB, 1525 views)

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Thanks for circuit guidance. What will be the value of the transistor?

-Mahesh JOshi

 

Transistor can be any general purpose NPN i.e BC547 or BC548

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you think that the IC cand drive the transistor? maybe a better solution will be to use a darlighton like BC516 and a larger resistor value, like 50K
just a suggestion

 

Although I am not an electronics guru, I am surprised that the circuits shown (thanks for it) does not connect to any of the power source of the clock itself. What i understood from that circuit is additional 5V supply will be required with registers and transistor whose base is the input of the wrist watch's any segment. Is it right?

-Mahesh Joshi

 

you need to have a common ground. the ground from the +5V suply needs to be connected to the ground(negative) of the clock battery, if you intend to power it from the battery and power the LEDs from another suply.

 

Yes ofcourse, the IC can drive a transistor. Even if source sink capacity of IC is 1mA, it is sufficient to drive the transistor into saturation.

4.7k resistor is used for harder base drive and thus obtaininhg harder saturation. If you don't want to make the transistorhighly staurated or if you face recovery problems (ghosting effect on displays) increase the value of base resistor. But at low multiplexing frequency (few KHz), I don't think this problem is going to occur.

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ok, i believe you.
but i just didnt think that the ic from a watch can provide enough current.
well, a test should be made before trying the circuit in its final form.

 

I think, the IC can supply atleast 200uA current which means, for BC547 (Hfe ~= 100) the collector current can be 20mA which is good enough to drive an LED. This is just an assumption, but I guess the current will be more than 200uA.

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