Powering a Teensy LC from 2.2V source for IRremote

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Trying to embed a Teensy LC in a Logitech K400r wireless keyboard that we use to control a media PC running Ubuntu. The keyboard controls the computer just fine, but I thought it would be nice to be able to control the Samsung TV as well. I prototyped this with a portable battery and the Teensy with the IRremote library works beautifully.

The keyboard takes two AA batteries. I didn't notice before I started carving things up that the batteries are wired in parallel. The highest voltage available from the keyboard controller is 2.2V. The Kinetis KL26 has a minimum Vdd of 1.71V, so maybe? On the Welcome to Teensy-LC card, Vin is spec'd from 3.7 to 5.5 volts. I don't need USB Serial during normal operation.

Is there a simple way to run the Teensy LC on 2.2V?

Adding a boost converter would be challenging due to space limitations inside the keyboard case, but might be possible, especially if there are any boost converters that don't require too many external components.

Adding a 3V coin cell could work, though I'd need to find a way to change the battery without disassembling the keyboard.

Would it make sense to use diodes to have the batteries both in series and in parallel simultaneously?

Would it help to power the Teensy only when a momentary contact button was pressed? Maybe some supercapitor circuit could power the Teensy just long enough to turn on/off the TV.

If all else fails, I could remove the Teensy from the keyboard and attach it to the PC. I'd arrange a hot key on the PC to trigger the Teensy to send a command to the TV. The keyboard would have a few embarrassing holes, but that's what I get for failing do my homework before cutting.
 
3V lithium coin cells are rated for a few mA at most, IRremotes often use high current emitters at upto 100mA pulsed.

I think you need to figure out the actual current requirements first as this may be a limiting factor.

Something like a LiFePO4 cell at 3.2V might be a possibility capable of some current. These come in several sizes IIRC.

Would it make sense to use diodes to have the batteries both in series and in parallel simultaneously?
Erm, as in a quantum superposition? I don't think it makes any sense, no.
 
MarkT, thanks for the insights.

The IR LED I'm using didn't come with a datasheet. Not sure how to measure my working prototype. I've got a cheap hand held multimeter, an inline USB tester, a bunch of Teensys, and various 'jellybean' components. I could measure the steady-state current, but to measure the brief pulses, I'm guessing I'd either need a new measuring device or some clever circuit. I'm not an electrical engineer, so I'm learning as I go.

I was wrong to write 'simultaneously'. I don't really need both the keyboard and the Teensy powered at the same time. The main use case is to push a new button on the keyboard to turn on/off the TV. Except when the button is pressed, I don't need the Teensy to be powered.
 
The boost converter gets the Teensy LC to work and it can flash its builtin LED. Alas, the 3mm IR LED doesn't seem to illuminate. With a big USB battery, it worked fine.

IR remote controls flash many brief pulses to encode commands. A capacitor across Vin and Ground could charge up between flashes. Would this be a solution? Anyone hazard a guess about a reasonable capacitor value? For example, I've got a 4.7MFD electrolytic and some 0.47UF ceramics.

Or is this just hopeless without a proper LED driver circuit or better power source?
 
Think 100's of µF or above.

Not sure what you mean by "4.7MFD". "M" means 1000000, "m" means 0.001, "µ" (or "u") means 0.000001, the symbol for farad is just "F".
I suspect you mean either 4.7mF or 4.7µF - the former would be suitable I think.
 
4.7MFD is literally what's printed on the cardboard the capacitor came in. Looking closer at the capacitor itself, it's 4.7µF. Thanks again, MarkT.
 
Depends on the current requirements though - which depends on the range you want and the particular emitter and receiver involved. Doubling the range involves quadrupling the current (inverse square law), so its usually better to do some investigation than just guess+hope.
 
Wondering how to do the investigation/measurement with what I have...

I've got an external USB battery that powers the Teensy + LED well enough to work at the intended range and a multimeter. I could use the multimeter to measure the current output of the USB battery as compared to the boost converter with the pair of AA batteries in parallel, but this doesn't work so well for time varying signals like the LED pulses.

A professional would have some sort of light meter that they'd set up at the required distance. What I have is the TV itself. It either turns on or it doesn't. I can observe the IR LED through the camera on my cell phone. Maybe I could compare this visually to the remote control that came with the TV. Maybe I could measure the current draw of the LED when it's hooked up to the USB battery and the Teensy is configured to leave the light on continuously. Maybe this project is my justification for buying an oscilloscope.
 
Summing up, I thought it would be easy to add an IR remote to a wireless keyboard. A Teensy LC, an IR LED, and the IRRemote library work beautifully, but with only two AA batteries in parallel, the low power design challenge is beyond me.


  • Added a boost converter module and the Teensy LC runs fine and blinks its built-in LED.
  • Added a 470μF capacitor in case the IR LED was causing the power to sag.
  • Added a transistor circuit to avoid relying on the drive strength of pin 16.

Not bright enough. Trying to work on this without breaking the keyboard's flex cable was a bit nerve wracking.

Time to put this aside while I figure out what knowledge and equipment I need to take out the guess work. There are several time-varying current measurements I'd need to make and I don't know how with a basic multimeter.
 
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