Main Breath Circuit

Main Breath This diagram shows a one-chip circuit that amplifies the signal from a solid-state pressure sensor to the 0-5 V range and provides a gate signal when the breath force is above a small threshold value. The pressure transducer used is a SenSym (invensys.com) unit with a 0-1 psi range. Amplifier A1 has a gain of ~50 and converts the differential transducer voltage to a single polarity. Amplifier A2 further amplifies the signal to the 0-5 V level. Two offset adjustments are required, one to compensate for the zero-pressure offset of the transducer and a second to compensate for the amplified offset of A1. (The LM 324's output will only go down to ~4 mV.)

The circuitry around A3 provides slew limiting (or low-pass filtering) of the breath signal. This limiting is required to prevent popping of the audio signal being controlled. For rapid increases in breath force, D1 switches on and increases the slew rate somewhat. The gate signal is produced by the comparator A4, which switches at around 0.15 V. This circuit includes some hysteresis to reduce chattering near the threshold.

Back   Home