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Topic # 180978 28-Sep-2015 15:10
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Where to post this? Not sure which forum fits, hopefully a mod will fix it.

I bought a little automotive digital clock with illuminated display (red segments). It has + and - power leads, a little setting button on separate leads, and a compartment for a 3-volt button battery, but no installation instructions at all. I am fairly handy with this sort of thing, but not sure how to proceed in this case and the clock was sold (Ebay) with no specs of any kind. 

I figured there would be some kind of dual power connection as with radios to keep the clock chip going when the car is not running with a circuit through the ignition switch to light up the display. This does not seem to be the case. Although the clock is advertised as being suitable for cars and motorcycles, there is only the one power connection with the unexplained battery compartment. 

If I hook the power through the switch, the clock loses its setting when the switch is off (unless the button battery has something to do with this). The only other option is to connect it directly to the car battery, meaning the display is always on. It is LED but will this not drain the battery over time? The display is pretty bright and there are several segments.

I am not sure how to proceed. Does anyone know the correct way to do this?
 




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  Reply # 1395839 28-Sep-2015 15:16
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Fit a 3v battery in the compartment and you should be sweet

gzt

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  Reply # 1395848 28-Sep-2015 15:26
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3v battery holds the time when ignition power is off.

 
 
 
 


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  Reply # 1395884 28-Sep-2015 16:23
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That makes sense. Thanks.





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  Reply # 1395887 28-Sep-2015 16:27
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-ve lead to Earth (Black), +ve lead to Ignition (usually Red). You should be able to hook into the stereo loom?

Be interesting to see how long the battery lasts as it will spend more time on battery than it will on car power.



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  Reply # 1396064 28-Sep-2015 20:26
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I'm wondering about that. Makes it difficult to permanently install the clock where I wanted to since I don't want to have to disassemble the dash every time the battery needs replacing. What if I make a voltage divider (3 volts) and solder a connection to the terminals of the button battery holder and run that directly to the car battery?





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gzt

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  Reply # 1396112 28-Sep-2015 21:12
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Does it support daylight saving? ; )



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  Reply # 1396126 28-Sep-2015 21:33
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I doubt it but the external button makes the time easy to change. The same can't be said for the battery.





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  Reply # 1396200 28-Sep-2015 23:03
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Just get one of these batteries http://www.jaycar.co.nz/Power-Products-Electrical/Storage-Batteries/Cordless-Phone-%26-Specialty/Lithium-AXIAL-AA-3-6v/p/SB1775 It will probably outlast the clock. Or if you really want to get rid of the battery. Get a 3.3V voltage regulator. http://www.jaycar.co.nz/PRODUCTS/Active-Components/Regulators/Low-Dropout/LM3940-Low-Dropout-Regulator/p/ZV1565 just wire up 2x 0.1uF capacitors between the input pin and ground and the output pin and ground. And wire an LED in series with a 1Kohm resistor on the output as well. just to provide a little more load on the regulator. As the clock mem backup load will be too small.







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  Reply # 1396318 29-Sep-2015 08:55
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OK thanks for that. It is really helpful.





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