Hi.
I hope this is ok, but I tried to get some help on another forum regarding this problem, but came up to a road block. Maybe some fresh eyes on this forum would be good.

I'm running a bare bone Arduino mini on 3V (2xAA) batteries. When the voltage is good, I'd like to single flash a green LED on a button press. When it reaches 1.8V however, I'd like to flash a red LED (ie an RG LED). I'm aiming at a current of 1mA for all presses. This needs some sort of doubler or inverter circuit - preferably not a bought unit, but made from scratch if possible. Any suggestions would be appreciated.

 

When what reaches 1.8V, the battery supply?

You could just use the boost circuit from a solar light to get enough voltage for the LED. Use a 1.8V zener as a voltage reference and a comparator to switch from one state to another at the reference voltage. The two color LEDs will straddle the output such that only one is on at a time.

That may not all be clear but I can draw it out when I get back to my laptop, if someone doesn't beat me to it.

 

To get the higher voltage you could pulse the arduino pin and use a standard voltage doubler rectifier to drive the LED. If you put a fixed reference voltage on one of the ADC pins, the arduino can measure the supply voltage by checking the ADC reading on that pin.

 

This needs some sort of doubler or inverter circuit - preferably not a bought unit, but made from scratch if possible.

The MCP1640D is perfect for this, IMHO. I use it quite a bit for 2 cell applications.

It has an "ideal diode" regulator bypass when the regulator is disabled. If your Arduino can start-up at 1.8V, it can manually activate the 1640 to boost the Vdd to 3V (or whatever you want -- up to 5V). This assumes that Arduino doesn't mind its Vdd transitioning while it is running. PICs don't seem to care.

 

...You could just use the boost circuit from a solar light to get enough voltage for the LED

The unit I'm making is a hand held remote with an NRF24L01 and keypad. So solar wouldn't be appropriate in this application.

The MCP1640D is perfect for this, IMHO

This is probably the best solution, but given my board making skills and soldering skills, I would have liked if possible to make this from none "SO" size components. But your suggestion still is an excellent (and probably the best solution)

To get the higher voltage you could pulse the arduino pin and use a standard voltage doubler rectifier to drive the LED

Yes, this is what I was attempting, but couldn't get it working. I have written the code to measure the fixed reference voltage already. I am also able to turn on the appropriate LED depending on the voltage. It's just that at about 2.1V, the red LED is too dim. At 1.8V, I can't see the LED light up at all (forward voltage / amps issue) - hence the voltage doubler. I did also try a voltage inverter. The issue I was finding was getting the range of voltages to end up supplying 1mA (with the correct components and code).

 

It's just that at about 2.1V, the red LED is too dim.

But at 2.1V it should be the green LED?
What circuit did you use for the doubler?
You should be able to vary the current by varying the mark:space of the doubler input.

 

I'm aiming at a current of 1mA for all presses.

drive both of them from a 1ma ccs.

But it is hard to envision the benefits of such a complex setup.

 

But at 2.1V it should be the green LED? What circuit did you use for the doubler? You should be able to vary the current by varying the mark:space of the doubler input.

Yes, I wasn't clear. At 2.1V it should be and is the green LED. It was more the point that at the lower voltage, the appropriate LED would no longer light up (especially at 1.8V). This is the reason I need to double the voltage (and more importantly have sufficient mA). Here is the circuit I tried:

I also tried a voltage inverter, but with no luck. What do you mean my "mark:space"?
DannyF - what's a 1m ccs?

 

The unit I'm making is a hand held remote with an NRF24L01 and keypad. So solar wouldn't be appropriate in this application.

I didn't mean the solar cell, I meant the boost circuit they all use to light an LED from a single AAA cell. There's a 4-pin IC with a small external inductor that provides boosted voltage to the LED. One pin is used to turn the light off in sunlight, but you could just ignore that.

A simple voltage doubler should have worked but there is an issue with the diode drops. It won't really double a low voltage. And the power you can pump through depends on the frequency. Maybe it was too slow?

 

Have a look at this one. Basically a doubler, a shunt regulator made of 5 standard diodes and a switch.

 

Thanks Ronv, but I think I resolved the issue.
I was playing around with a 2 stage inverting charge pump instead. This way I could drive the LED from a negative to positive value. I had to fiddle a bit, but in the end did get consistent 1mA supply to drive the LED. Circuit I used is attached. The only difference is that I removed C3 in order to get 1mA across all voltage supply ranges. Why? Don't know - but it worked

 

The MCP1640D is perfect for this, IMHO.

Suh-weeeet.

ak