powering clock from 12 volt battery?

Jim Shoe wrote:
Fight the urge to make this way more complicated and expensive than it needs to be. I have this Westclox. It runs forever on a battery. I've replaced it once in 10 years. You don't have to reset it every day after you stop driving, you can push a button for a back light at night and it has an alarm. $12.00.

Mine too has a button that you can press for a backlight but I cant use it because the clock is located on my dash , the dash location is nice because I can see the clock when I'm driving and I can also see the clock when I'm in the rear of the van , The only problem is when I wake in the morning its dark and so I cant see it.

I think wiring it to the vehicle is much simpler then you think , Also in regards to having to reset it every day when you stopped driving , Thats only if you chose to wire it to the ignition , Mine wouldn't be wired to the ignition , It'd be wired hot .I would be powering the clock from the hot all the time wire

Or just set your phone display to "always on". Plug it in to a 12v charger if necessary.

Fight the urge to make this way more complicated and expensive than it needs to be. I have this Westclox. It runs forever on a battery. I've replaced it once in 10 years. You don't have to reset it every day after you stop driving, you can push a button for a back light at night and it has an alarm. $12.00.

Yes, if you happen to have one laying around, then using a variable resistor as a voltage dropping resistor to establish a +1.5 Vdc clock voltage will work quite well.

Just a word of caution, though. Be absolutely certain that you have the variable resistance set towards the top of the range before you connect it. If the resistor is near-zero resistance when you hook it up, you will surely fry the clock innards because you will be applying the full 12 volts to the circuit.

Furthermore, unless you have an ongoing need for that variable resistor elsewhere, you could actually leave it in place after you have set the voltage.

Dale

@ Dale ,,, Do you think I could just install a varible resister and then adjust it until I had 1.5 volts ,, after adjusting to 1.5 volts I could take the reading of the varible resister with a ohm meter and then substitute the varible resister with a fixed resister..
The internet is amazing sometimes , thanks Dale

There are dozens of trucker alarm clocks out there with all kinds of displays and features. Much easier and don't cost much.

I had to do this for an aircraft - the clock was designed to operate on +5 Vdc and the aircraft battery was +24 Vdc. The way I solved it was simply to add an inline series resistor; total cost of 29 cents.

The clock consumes a very small but fairly consistent amount of power, probably something around 1 to 2 milliamps (that's one-thousandth of an amp) - to make the math easy, let's assume 1.5 mA. With a battery voltage of roughly +1.5 Vdc, we're looking at an equivalent resistance for the clock at 1,000 ohms using Ohm's law (i.e., R=V/I, or Resistance equals Voltage divided by Current).

So.... if you want to drop the +12 Vdc of your vehicle electrical system down to +1.5 Vdc, you need to install a series resistor in between the battery voltage and the clock of roughly 10 times the clock resistance, or 10,000 ohms (10 Kohms).

If you don't have the ability to accurately measure the actual current draw, then you won't be able to calculate the exact value of resistor needed, but 10 Kohms will probably get you into the ballpark.

A cheap DVM would allow you to nail it precisely, even if it won't accurately measure current way down in the milliamp range. Simply put the 10 Kohm resistor in series with the clock (i.e., connect the resistor between the +12V terminal and the red or plus lead of the clock, then connect the black or negative lead of clock to GND, or ground).

Next measure the voltage between the positive and negative leads for the clock - if it's less than +1.5 Vdc, then you will need to DECREASE the value of the dropping resistor, if it's more than +1.5 Vdc, then you will need to INCREASE the value of the dropping resistor.

Please note that the clock is probably fairly tolerant towards the voltage input, so anything between 1 to 2 volts would likely work OK.

Dale

sch911 wrote:
Why?? AA battery powered clocks run for many months for us...

I cant see it in the dark , it has a button on the top that turns a light on so you can see the display momentarily until you let off the button , I was going to hold the button down with duct tape but then I started thinking that the light will run the battery down , so I plan to wire it to my house battery and then duct tape the button

2oldman wrote:
Something like this

I happen to have one of them but I forgot all about it , I'll be digging it out , Thanks so much!