Unless you are a tinkerer and willing to risk failure it’s probably best to start by just stacking or bridging the resistors to see what that gives you. However, if like me, you can’t leave well enough alone I’m willing to lead you astray.
This is essentially an adaptation of the master/slave mod developed by Techjunkie and posted here by Oldlumens.
First a bit of identification.
On that FET the big tab at the top is called the drain and it’s connected to Led-.
The smaller pin bottom left is called the gate and is how the mcu turns the FET rapidly on and off(what’s called pwm). The amount of (on) time is called the duty cycle and gives you your modes.
The other small pin bottom right is called the source and is connected through those resistors to V-.
7135’s have the same 3 connections but they have different names. Don’t ask me why, maybe an EE could tell you.
The large tab is instead called ground and is equivalent to the source pin.
The bottom right pin is called Vdd and has the same function as the FET gate pin.
The bottom left pin is called Led out or Led -.
The middle pin is the same as ground and is often left unconnected without consequences.
All the 7135’s on a typical driver are connected in parallel so if you make those 3 connections between this board and any of the 7135’s on the slave board then all the chips are used.
What you would need to do is remove the FET and the two resistors and solder in 3 wires to connect the slave board there instead.
If you’re not comfortable soldering and desoldering smallish stuff then you shouldn’t try it but here’s why you should consider it. The difference between the two is in regulation and resistance. The FET is either fully on or fully off and will always pass as much current as the voltage allows during each (on) cycle with the maximum controlled by those resistors. As the battery voltage falls since the resistors are fixed value the current falls as well and the LEDs get dimmer in every mode. With the 7135’s, when they are on each one passes a fixed amount of current with the maximum controlled by the number of 7135’s. As the battery voltage drops the resistance of the 7135’s drops as well to maintain current level. The lights won’t begin to dim until the battery voltage drops down close to the forward voltage of the led and stays constant until then in all modes.
The FET/resistor combo would be more similar to the 7135’s if instead of fixed value resistors a pot were used instead that automatically lowered it’s resistance as the batteries drained.
If you’re interested I can post a sketch later showing the connections but you would need a slave board or a stack of 7135’s to do this.
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