I seriously suggest NOT bypassing the transistor yet. My concern is that the transistor might be for linear power control, rather than on/off control. It might be serving as a voltage or current regulator, in other words.
There are two switch installation locations that I don't think have been adequately investigated.

 

I seriously suggest NOT bypassing the transistor yet.
There are two switch installation locations that I don't think have been adequately investigated.

Actually they have. The transistor just drives the LEDs, it's all good.

 

And I can just solder the wires off the adaptor to the + and - terminals top left and bottom right on the board?

No! That would apply 5V when 4.5V is required. You need the diode suggested in post #2 and illustrated in post #5. That brings the voltage down to just below 4.5V..

 

i doubt you need that diode... but it cannot hurt.... specially in the long run.
the thing is that we do not know for sure what the circuit will tolerate so a conservative suggestion is to not exceed supply voltage that batteries will provide. and diode will drop the supply voltage by some 0.5V (roughly). so if using 5V supply, you will end up with roughly 4.5V.

on the other hand, 0.5V is only 10% difference... pretty sure IC and whatever is on it is rated for 5V operations. also brand new batteries are never 1.5V, there are more like 1.65V. so in this case, three of them are going to add up to 4.95V.

 

i doubt you need that diode... but it cannot hurt.... specially in the long run.

It would likely shorten the life of the LEDs to be run at 5V continuously, compared to batteries that would soon be below 4V.

I am going to guess the circuit now consists of the battery and two LEDs, each with an 18Ω resistor in series. Choosing a typical Fv of 3.3V we can calculate the current.

(4.5 - 3.3) / 18 = 67 mA

with 5V we get:

(5 - 3.3) / 18 = 94mA

That is enough of an increase to be worrisome.

 

Yup, that would make the LEDs run warm if continuous....

 

It would likely shorten the life of the LEDs to be run at 5V continuously, compared to batteries that would soon be below 4V.

I am going to guess the circuit now consists of the battery and two LEDs, each with an 18Ω resistor in series. Choosing a typical Fv of 3.3V we can calculate the current.

(4.5 - 3.3) / 18 = 67 mA

with 5V we get:

(5 - 3.3) / 18 = 94mA

That is enough of an increase to be worrisome.

What if I just get a 4.5 V power supply then? Instead of 5 V.

Also, just for my understanding, what does the term Fv stand for? The potential required to actuate the diodes? Is that value a combination of all 4 diodes? I'm following your math beyond that.

 

How much will it matter if the light output drops 50% after 17,000 hours instead or 20,000 hours????

 

How much will it matter if the light output drops 50% after 17,000 hours instead or 20,000 hours????

If that was sure to be the result I would agree. How did you determine that?

What if I just get a 4.5 V power supply then? Instead of 5 V.

It would be fine, but they are much less common than 5V ones. You probably have a 5V one around the house you are not using.

 

Also, just for my understanding, what does the term Fv stand for? The potential required to actuate the diodes? Is that value a combination of all 4 diodes? I'm following your math beyond that.

Fv is the voltage across the LED at its rated current. It stands for forward voltage. It is not a well controlled parameter and is typically listed as a range, like 3.3 to 3.6V.

 

If that was sure to be the result I would agree. How did you determine that?

It would be fine, but they are much less common than 5V ones. You probably have a 5V one around the house you are not using.

The LED lifetime is usually described as the point at which the light output has dropped 50%. And the hours of standard operation are typically stated in the full product data sheets. 20,000 hours is a fairly common lifetime, although some are only 10,000 hours.

and about the voltage of the plug-in supplies: for at least most of 50 years they were mostly all transformer/rectifier types, and mostly the claimed voltage was only correct at the specified current. That was because they were simple diode rectifiers plus one capacitor systems. Most of the "5 volt" supplies are some sort of switcher devices with some level of regulation. The really BAD news is that some fools are now using the exact same connector for higher voltage supplies. SOME, BUT NOT ALL, of the higher voltage supplies communicate with the load to decide the output voltage. SOME DO NOT COMMUNICATE, but simply supply 12 or more volts to the same USB connector.

 

The LED lifetime is usually described as the point at which the light output has dropped 50%. And the hours of standard operation are typically stated in the full product data sheets. 20,000 hours is a fairly common lifetime, although some are only 10,000 hours.

Yes, at the rated current, which we do not know for those LEDs. Did you see my calculation that the current would be 42% higher at 5V than at 4.5V?

 

True enough. So the added diode trick will be worth the extra bit for the diode.