Two IR LEDs in series, using direct 18650 power

A while back I built an IR remote for my Roku box. It used a single LED and a Darlington transistor, with everything powered directly (no regulator) by an 18650. The Darlington was probably not a good idea, and I’ve thought about modifying the circuit to replace it with a mosfet. But I also wondered about using a second LED in series with the first one, so as much battery power as possible would be converted into signal power instead of just wasting it as heat in current-limiting resistors. The attached circuit is where I am at the moment. The LEDs would probably be Vishay TSAL6100, and the mosfet would be one of those SOT-23 jobs with vanishingly small RDSon.

But I wonder if this is a practical design. It seems the current is going to vary a lot depending on the battery’s charge state. Has anyone here done something like this? Does it seem to work ok?

Also, I’m assuming the Arduino could drive the mosfet gate at 38K without issues. Does that seem right? I guess it would depend on gate capacitance?

Anyway, before I start ordering parts, any comments or suggestions would be appreciated.

Hello Sherman,
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Have you checked the datasheet for the TSAL6100? Vf is variously shown as 1.35V, 1.6V, 2.2V and 3V, depending on the test conditions. The Vf of 2 in series is therefore possibly 6V depending on how you operate it, so you need to consider that when operating from lithium ion battery.

Yes, the current will vary as the battery discharges, whether that matters in your application is something you will have to experiment with.

ShermanP:
Also, I'm assuming the Arduino could drive the mosfet gate at 38K without issues. Does that seem right? I guess it would depend on gate capacitance?

Well, the 100 Ohm resistor sounds about right.