Hi! Engr. Jalil here
Need help to solve power issue for hearing Aid.
My Grandpa is 70 year old and rely on Hearing aid. Device grandpa uses is 1 Zinc Air cell of 100mAh capacity. These coin cell last for about 6 days. These cell are expensive and supply is running dry due to pandemic.
I am thinking to replace Cell with li-Po battery. Device itself is small and I can't fit battery inside, but, as a string is already attached to device which keep device captive so that grandpa don't lose it. So I can use a thin wire with battery and Circuit in pocket.
Problem:
Device need voltage of 1.45 V and current of about 0.5mA. li_Po supply voltage between 3 to 4.2 V. I need to use a buck IC to Step down voltage. I know there are BUCK ICs for Li-Po on LCSC.com and DIGIKEY.com but shipping will take several month. I can get generic Buck and Boost ICs and breakout Board from local market.
My Approach is to use Attiny 85 to step down voltage. as I only need 0.5mA so I think Mosfet isn't needed because Attiny can provide 40mA on pins. My question is "is it feasible to do this and what will be efficiency of BUCK Converter?"
Second approach is to use boost converter followed by Buck.
third is to use Two Lipo cells in series and use Mini 360 Buck or similar. but problem is finding charging Solution for battery pack with 5v mobile phone charger.
I assume you mean using a pwm output to generate the “1.4v “. It might work but you’ll need a filter to get the average voltage smooth to stop the peak voltages hitting the hearing aid . You may well get a lot of interference
Efficiency you can work out and consumption will be dominated by the Arduino . ( look at its spec)
Have you looked for alternative batteries of higher capacity ?
yes I will use LC filter to smooth voltage and feedback to keep voltage constant. Attiny 85 consume about 2mA at 1Mhz. and presume Sleep function can't be used during working.
Ni-MH batteries provide 1.2v but I am unable to find protection and charge circuit from local market
I wonder if a 3.7 volt LiPo cell in serie with 2 silicon diodes and a resistor wold produce some 1.2 volt across the diodes. 3.7 - 1.2 volt == 2.5 volt. Selecting a resistor of 2k7 ohm.... Small size, simple components...
The hearing aid doesn't draw a constant current, so you need to correct for that: the filter output voltage depends on the current draw.
In this case you may be able to make a basic linear regulator using an OpAmp. Using diode drops as reference sounds like a good idea (or of course a standard 431 reference diode in combination with resistive voltage divider to get to the desired 1.45V), just a regular resistor voltage divider won't work as the input voltage is not constant but depends on the charge level of the LiPo. For sure it'll be easier than trying to deploy an ATtiny85 for this, and OpAmps likely react far faster than the ATtiny can.
jalilurrehmaan9:
Ni-MH batteries provide 1.2v but I am unable to find protection and charge circuit from local market
Hard to believe that the shops selling rechargeable batteries don't also sell the chargers.
Protection circuit against low discharge may be harder to find, unless it's built into the batteries (like is common for LiPo).
No danger, the battery pack/etc. will drop in a shirt pocket, just a pair of flexible wires going to the ear pieces. Maybe just to 1 ear, with the 2nd connected by a wire around the pack of the head.
You have low input voltage and very low current requirements: Why not just go with an LDO? (please verify just 500uA of current is required?)
LDOs are available that produce exactly 1.45V or you could choose an adjustable one and use precision resistors in the feedback path to generate the 1.45V.
hmeijdam:
Would a 1,5 volt Alkaline AAA cell (12 grams) be an interim solution? They are dirt cheap and should last a month or so.
The obvious answer.
Panasonic suggests the voltage of the Zinc-Air battery technically could be as high as 1.65 V, so it is reasonable to presume the hearing aid will accept the voltage of a common AAA battery, alkaline or otherwise.
Railroader:
Anybody having inside knowledge about the circuitry in hearing aids?
I know about the trouble faced when a hearing aid by some reason fails....
I know they use a single IC that has a programmable gain/frequency curve.
Paul
Paul__B:
The only "power supply circuit" would presumably be a capacitor.
"Presumably be".....
As my 95 year old mum use the hearing aid that did, and do, call for my attention often I ask You helpers for knowledge, not assumptions. I know what it takes to get damaged hearing aids repaired, and what it costs until it's done.
It is battery powered. It will operate at the battery voltage. A voltage converter would be dreadfully inefficient though it almost certainly uses a bridge configuration to drive the earpiece (as this actually saves an output capacitor) and it has no switch. So what other "power supply circuitry" might you anticipate?
