Are you sure you want ~40Hz? My device is more like 0.5Hz. If I recall correctly, a TENS machine can hit 40Hz. You can get a pretty nice one for ~$30. It does NOT supply a micro current though, and uses voltages over 100V. Definitely not for tongue use.
My circuit should work fine at 40Hz, and limit the current to 600µA or less, but you'll need different timing components on the 555 (or 556). If you want to set the current limit higher, that shouldn't be hard either but it would be a good idea to work it out in simulation to be sure. A 9V battery across a human head my not be able to drive more than a few mA anyway.
My circuit should work fine at 40Hz, and limit the current to 600µA or less, but you'll need different timing components on the 555 (or 556). If you want to set the current limit higher, that shouldn't be hard either but it would be a good idea to work it out in simulation to be sure. A 9V battery across a human head my not be able to drive more than a few mA anyway.