Thermoelectric Hearing Aid
A basic hearing aid whose battery is extended with energy harvested from the human head. Again, this process was inspired by my elderly grandmother who struggled with handling the small zinc-air batteries that powered her hearing aids - the eventual goal of this project was to create a battery-less hearing aid.
The schematic is as follows: a peltier tile is wedged behind the ear, the hot side touching the bare skin behind the ear with the other side attached to a small heat sink. The small voltage produced by the seebeck effect is fed to an LTC3107 energy harvesting IC designed to extend the life of a primary battery. The IC utilizes an “ultra-low voltage step-up DC/DC converter and power manager” in order to manage the harvested energy for the primary battery and has the ability to switch between the primary batter and harvested energy when it is available. In order to take full advantage of the peltier tiles, I made several molds of the inner ear and behind the ear were taken using Instamorph, a plastic that is moldable in hot water and hardens after a couple minutes in ambient air, in order to find the flattest and greatest real estate.Below you can also see a bronze casting of one of the instamorph molds in an attempt to incorporate the heat sink with the hearing aid housing. Despite being able to string several micro (3mm x 3mm) peltier tiles in parallel, there ended up not being enough flat real estate inside the ear. It was clear from these molds that behind the ear was the best choice to rest a peltier tile on. I ended up choosing a 15mm x 7mm tile from CustomThermolectric (part# 05001-9330-35RU7F) which, after affixing it to the area behind my ear with a small heat sink, produced 30mv after settling from 55mv after 10 minutes of temperature stabilization. The audio gain portion was a bare-bones single transistor circuit with a gain of 100. This circuit was chosen for its simplicity and its ultra low power design. So, the apparatus is as follows: peltier tile(s) behind ear → energy harvesting IC → rechargeable battery The control test consisted of turning on the gain device for a period of 1 hour and measuring the power consumption as a function of voltage drop from a recheargable 1.2 volt NiMH (nickel metal hydryde) 200mhA battery. The peltier assisted gain circuit saw a 3% less voltage drop after the 1 hour period.