Converting a battery powered clock to run off phone charger.

Thread Starter

narkeleptk

Joined Mar 11, 2019
540
My wife has this clock she loves but it chews through its battery a bit too fast. She has tasked me with converting it to a plug in unit instead. Its normally powered from 1 AA alkaline battery. At first I was just going to ditch the battery all together and use a old phone charger with a lm317 but figured I could maybe keep the battery around in case of power cuts. I drew this out to get started with but I don't know much about this type of thing. Would the following be effective enough for what I need? Is D1 even needed, I feel it may be unnecessary since the battery supply is juts 1.5V.
Clipboard01.jpg
 

wayneh

Joined Sep 9, 2010
17,153
I’d consider replacing the mechanism. Is it one of those plastic square things about 2” wide? They’re everywhere and all you’d have to do is swap the hands (carefully!). Most I’ve had last for months on even a wimpy battery that’s no good for anything else.
 

Thread Starter

narkeleptk

Joined Mar 11, 2019
540
There is nothing wrong with the mechanism I don't think. Its a big clock with big hands so I imagine any similar boxes would have the same outcome. I do appreciate the reply but that is not what I am looking to do.
 

wayneh

Joined Sep 9, 2010
17,153
Ok, plan B. The mechanism needs very little power and takes it in surges, one each second. I think you could run a low current through a zener diode (to establish the voltage) and put a capacitor in parallel with the diode. The capacitor would charge up during the quiet time and could then deliver the repetitive surges. Very cheap and easy solution, one resistor, one zener and one capacitor.
 
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AnalogKid

Joined Aug 1, 2013
9,353
Yes, you need D1. It prevents a current path through the 470 and 100 ohm resistors from draining the battery even faster.

Assuming your phone charger is new enough to be a small switching power supply with a regulated output, you can ditch the LM317 and replace it with a small signal NPN transistor - 2N4401, 2N3904, 2N2222, etc. Smaller, cheaper, lighter, fewer parts, etc. Schematic later.

ak
 

Thread Starter

narkeleptk

Joined Mar 11, 2019
540
Yes, you need D1. It prevents a current path through the 470 and 100 ohm resistors from draining the battery even faster.
Right, don't know why I didn't see that. Blinders on I guess only focusing on the 317 and the reverse voltage.

Assuming your phone charger is new enough to be a small switching power supply with a regulated output, you can ditch the LM317 and replace it with a small signal NPN transistor - 2N4401, 2N3904, 2N2222, etc. Smaller, cheaper, lighter, fewer parts, etc. Schematic later.
I'll look into that method with the npn's thanks! I have a slew of the stuff in the schematics already so not a big deal long as it works.
 
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AnalogKid

Joined Aug 1, 2013
9,353
First pass at a non-317 solution. The Q1 base-emitter junction blocks a current sneak path when on battery power.

UPDATE: Includes Wayne's diode circuit.

ak
Clock-Power-Switch-1-c.gif
 
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crutschow

Joined Mar 14, 2008
27,718
How about converting to a C or D alkaline battery?
They have about 2.5 and 5.4 times the capacity respectively of an AA alkaline.
Alternately you could put more than one AA in parallel.
 

Thread Starter

narkeleptk

Joined Mar 11, 2019
540
First pass at a non-317 solution. The Q1 base-emitter junction blocks a current sneak path when on battery power.
Or you could just put 5 diodes in series to drop about 3.5V off the 5V supply. Or an LED and maybe one diode.
Thank you sir's, I think I will build this one and the 317 one for fun.

Alternately you could put more than one AA in parallel.
This is a good thought too thank you. Very useful even after adding the plug in option she wants. I'm planning to install a barrel jack on the bottom for the plug just in case the wife wants to move it in a spot where plugging is not convenient. This would be good for those times. She likes to move stuff around often.
 

Alec_t

Joined Sep 17, 2013
12,201
If the clock would be happy with 1.2V (many are) you could just replace the alkaline cell with a NiMH cell on trickle charge from the 5V.
 

Thread Starter

narkeleptk

Joined Mar 11, 2019
540
If the clock would be happy with 1.2V (many are) you could just replace the alkaline cell with a NiMH cell on trickle charge from the 5V.
Good idea. Dont think I have any nimh bats tho. Just trying to use stuff I have laying around and dont have much use for in my normal work.
 

wayneh

Joined Sep 9, 2010
17,153
Good idea. Dont think I have any nimh bats tho. Just trying to use stuff I have laying around and dont have much use for in my normal work.
Got any old solar landscape lights laying around? They typically contain a Ni-cad battery because that chemistry can tolerate a continuous trickle.
 
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