The fertility awareness method Sensiplan is proven to be as effective as the contraceptive pill in avoiding pregnancies. The android- and mobile phone app LadyCycle is based on the rules provided by this method of birth control. In addition, the app also allows to determine the ideal time for conception. The method requires regular measurements of ones basal body temperature in the morning before rising. Even though the effectiveness as birth control is not reduced by irregular waking times, the number of "safe" days (days where no additional contraceptives are needed) can be reduced. Regular measurements can also be a challenge when small children require instant attention on awakening. This challenge can be met by using internal worn temperature sensors that autonomously measure the body temperature.

Commercial products available include Endotherm® Basal, OvuSense Realtime Fertility Monitor and OvulaRing. OvuSense is only for women trying to conceive and can't be manipulated to work otherwise. OvulaRing can only be readout by turning on a PC and is fairly expensive (219€ for the first three months and 21€ for each additional month).

Endotherm Basal consists of an Maxim iButton ds1922L as temperature logger (49€), a vaginal capsule (2 pieces for 18€) out of medical grade material, an usb adapter for sensor setup and readout, and a PC fertility software. The sensor is calibrated, featuring a higher accuracy than the industrial version that can be purchased from the iButton manufacturer (for 39€). The iButton's battery will last for about 5 years (typical use). The set is certified as medical product in Switzerland (149€). The components of the set can be purchased independently, however. Out of the three mentioned products, Endotherm Basal is (by far) the least expensive one. It is also the most flexible, allowing the user to build own soft- and hardware applications. Nonetheless, one should be aware of the fact that the iButton's temperature resolution (0.0625°C) shows a small deviation from the resolution recommended by the Sensiplan rules (0.05°C, which is the resolution of the analogue thermometers available when the Sensiplan rules were evaluated). Some rules can be adapted to guarantee at least the same effectiveness as the measurement with the recommended resolution. For others, a definite statement can't be made.

In this instructable, I will show you how to build a low cost iButton ds1922L-to-Bluetooth adapter using an Attiny85. The provided firmware for the microcontroller will (soon) enable communication between ds1922L-iButtons and the android fertility app Ladycycle (until the readout is fully integrated in LadyCycle, you can use this app to program and read the iButton's data).

If you are interested in a prepared set with all necessary materials, including a preprogrammed ATtiny85, shortened cables, prepared bluetooth module, prepared dental box and a professional pcb made by fritzing fab, you can buy it on tindie.

With this set, only soldering will be necessary.

Ich habe mir zwei Förstnerbohrer angeschafft, damit lassen die Löcher sich jetzt direkt auf passende Größe bohren