Bruel & Kjaer 2209 USB power and Li-ion battery mod
Description
Li-ion USB power and battery mod compatible with Bruel & Kjaer 2209 Impulse Precision Sound Level Meter. This is not an original Brüel and Kjær product and is not endorsed by Brüel and Kjær. Use this mod to give new life to a unit corroded by leaked alkaline batteries, like I did. Or modernize a working unit. CAUTION: BK2209 needs negative supply voltage from the battery, and this was achieved by using battery + as BK2209 ground and battery - as the negative supply voltage. Battery - is also connected to USB ground, so there will be a potential difference of up to 4.2 volts between BK2209 ground and USB ground. This means that you must NOT connect BK2209 to equipment grounded to the same ground with the USB. The safest is to use a non-grounded USB charger dedicated to BK2209.
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