modifying 12v to 3v circuit to give 4.5 volt output

I just bought a sailing boat and it had all these LED ceiling lights mounted that run on 3x AAA batteries.
instead of wasting batteries and the environment i figured i would hard-wire them into the 12v battery system that's on the boat and after digging around a pile of electronics i found an little circuitry that was used to power and 3volt radio from a 12 volt powerpack.

The voltage on the boat is between 10.5 and 13.8 volt (charging/discharge batteries)
when i feed 13.4 volts on it. it gives about 3.7 volt output.

Now i like to adjust it so it will give a steady 4.5 volt output.

the circuitry on the board is pretty basic but it uses some nice components so i just want to replace one or two of them to get the 4.5 volt output.

i drew a diagram of it and it looks like this:

sorry for my bad drawing (i only had ms paint available)

is it just as easy as replacing the Z1 (zener diode) for one that is 4.5 volts?
or do i need to change the Resistor as well?

and how can you tell what kind the current Z1 is?
i had all of this in school but that's a long time ago (before LED's were easily available)

ooh i didn't measure how much current the LED's draw but they are 5 bright white LED's so i guess around 100mA total.
i have 3 of these so i like to make more of these circuits. can i make one simpler/cheaper if the LED's only draw 100mA?

Thanks in advance.

Every added diode series to zener gives about 0.7...0.8V higher output voltage.
( diode eg. 1N4001....1N4009)

As kak111 said, you can add signal diodes (like the 1N4148) to increase the voltage, or you could change the zener diode.

You don't need to change the resistor.

You can use a voltmeter to measure the voltage directly across the zener diode in the circuit when it is operating - that is its value. (I bet it's about 4.5V - see below)

The two transistors are wired as a 'Darlington', so the output voltage from them will be the zener voltage minus two diode drops of about 0.6V. In other words, you want a higher voltage zener, by about 1.2V, than the final output that you need.

A 5.6V zener would give about 4.3V output. Remember that alkaline cells drop in voltage throughout their life - they aren't at 1.5V for very long so 4.3V is good. Also, it allows for some variation in actual voltage because they are usually 5% tolerance.

Hi FoxyRick and kak111,

Thanks for the reply, i finally got around to do this (to busy stopping my boat from leaking and i found out that the zener diode was a 4.0V one.
when i wired one signal diode into it, it raised the output voltage to 3.6V so i wired another in there in series and got to 4.2V which is good enough for the light.

When the light is connected the voltage drops from 4.2 to 3.5V and it draws about 100mA.
The light also has a dimmed setting and when i switch to that the voltage goes to 3.8V and it draws about 25mA.
i guess this is normal but when i feel the TIP41CL transistor it feels kinda warm. not too hot yet but i didn't leave the light on for very long.
it does have a heatsink attached to it but this is quite small.
The light is a ceiling light to lit up the cabin and will run for 4 to 5 hours every evening. Do you guys think it will dissipate the heat well enough or should i attach a bigger heatsink to it. there's not much space left to fit the circuit board in the housing so it can't be too big.

Then another question:

I like to make one circuitry similair to this but then for 5Volt and if possible something that can draw a few amps (2 or 3) because we have a lot of USB charging devices and other stuff that charges off 5V (camera/phone/torches etc)

I have googled for such a diagram put most DC to DC converters are either very simple and can only give a few hundred miliamps or they are very complicated and use a lot of high end components.

what would be an easy put pretty good diagram that could handle at least a 1 amp load?

Thanks again for all the help.
you guys are amazing and helped me to enjoy these electronics projects again since high school.

Seatz