Fireman device
This device is used for digital indication of air pressure inside a balloon, estimating time left under current breath tempo, issuing sound alarm (siren) when air pressure goes low. When equipped with acceleration sensor, it also takes into account fireman's liveliness and turns siren on in case of more than 30 seconds of immobility to help finding the fireman.
Initially, this device didn't have any control buttons: it stayed in sleep mode, woke up periodically to measure air pressure and, if air expenses exceeded some limit (when fireman started to breath), it switched to work mode. Later on, on/off button was added for convenience. In work mode the device measures instant air usage and, after estimating time left, displays it using LED indicator.
When a pressure threshold is reached, sound alarm is turned on (it starts to "peep"). The less air is left in the balloon, the more frequently the siren "peeps".
Device settings and functions can be adjusted and re-programmed.
Schematic diagram of fireman device (old version)