Lighting a while LED from two NiMH batteries

Thread Starter

seanspotatobusiness

Joined Sep 17, 2016
206
I have an old incandescent flashlight which uses two AA cells. I prefer to use NiMH rechargeables. I would like to replace the incandescent bulb with an LED. It does not need to be bright at all; there is no heatsinking and the flashlight is rubberised so it mustn't be so bright that heat is a concern. My problem is that all the drop-in incandesent-to-LED upgrades I've found assume alkaline batteries and don't even seem to function below 2.8 V which isn't even good for alkalines IMO. They're basically just an LED and no circuitry to regulate the current or increase the voltage. Does anyone know what I can do or get to replace the incandescent with an LED? I don't mind making it myself but it needs to stop at 1.8 V so I don't risk reversing the voltage on one of my rechargeables which a joule-thiefy thing won't do. Again, I don't want anything flashy - I just need it to not walk into things. I'm thinking a current draw of 30 mA. Thanks!
 

Alec_t

Joined Sep 17, 2013
12,148
Perhaps hack a solar garden light? Those have a single rechargeable cell plus a DC-DC boost converter blob to drive the LED. Not sure if the blob can also prevent over-discharge of the cell.
 

Thread Starter

seanspotatobusiness

Joined Sep 17, 2016
206
Unfortunately those ICs will overdischarge cells in series - I want to use two NiMH cells and those ICs go down to 0.9 V which would mean one cell could get reverse polarised.
 

LesJones

Joined Jan 8, 2017
3,252
Why do you have to use 2 cells ? Why not just use one cell with the parts from a solar garden light a and use a dummy cell to make up the length ? Is it really worth the effort when you can get LED flashlight that uses a single AA cell for a very low price on ebay or other sources.

Les.
 

Audioguru

Joined Dec 20, 2007
11,249
Most of my solar garden lights use a single Ni-MH battery cell. The LED current and therefore its brightness depends on the value of the inductor used in the blob-on-board voltage boosting circuit. I used RGB colors changing LEDs in most of my solar garden lights and used a fairly low inductor value tor a high current and brightness probably bright enough to be used in a white LED flashlight.
 

Thread Starter

seanspotatobusiness

Joined Sep 17, 2016
206
A dummy AA cell costs about the same as the LT1932 so I'd like to try using that. The datasheet talks about an inrush current of 600 mA and a soft-start circuit which would reduce this to 300 mA (for a shorter time period). Would 20 mA LEDs be toasted by an inrush current of 600 mA lasting around 150 uS?
 

Bernard

Joined Aug 7, 2008
5,788
For a dummy cell I use Cu tape wrapped around a dead cell.
If you like your 2AAs & complications try a C555 + driver as a V doubbler with V reg. feed back.
 

SamR

Joined Mar 19, 2019
3,448
I just wrap aluminum foil around it, same thing and the wife has plenty of it available. Works the same for fuses but you didn't hear me say that.
 
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