I'm building a system which is intended to supply a microcontroller with a smoke-detection alarm. I'm currently intending to base the circuit on a Microchip RE46C190, which I can source fairly easily.
The datasheet has a typical-application circuit on page 21 which looks suitable for adaptation, but I need advice on how to remove the piezo transducer from the design (as I don't require an audible output; I intend to link the IO pin to the microcontroller). I gather that FEED will need to be tied to one of the rails (when not in test mode, anyway), but what should I do with pins HS and HB which are intended to drive the transducer? Can I simply leave then unconnected, and omit R3, R4 and C5?
Best Answer
A piezo with three connections shown in the circuit on page 21 is used in a self-ocsillating piezo circuit. The smallest electrode (the one connected to R3) is the sense electrode, which provides feedback to the input (that's what the "FEED" means: feedback).
The image is from the block diagram on page 2. The horn driver signal from the level shifter will just be an enable signal, so that's DC. When high the AND gates work as inverters, and you have a typical 2-inverter CMOS oscillator. When the driver signal is low the outputs of the AND gates are permanently high, and therefore HS and HB both low, and the circuit won't oscillate.
To disable the buzzer you simply can cut the connection to FEED, and neither HB nor HS will be driven. You won't need to remove any components.