January 15, 1998

Controller provides multiple alarm-driver formats

William Grill, Riverhead Systems, Littleton, CO

Using a piezoelectric element for alarm applications offers low cost, low power, and flexibility. By coupling this element with a 12C508 programmable controller (Microchip Technology, Chandler, AZ), you can implement an eight-pin alarm generator. This approach provides multiple driver formats with a minimum of additional cost and footprint.

The controller in Figure 1 drives the piezoelectric element directly from pins 2 and 7 with complementary square waves. A siren, chirp, warble, or constant-alarm output is available by setting the corresponding mode on pins 5 and 6 (Table 1). The design codes each mode as separate processes, which you can consider as variations of frequency, frequency step, and dwell.

The design also codes the positive and negative true alarm enables, pins 3 and 4, into the device. The controller retests the mode and these alarm enables at periodic intervals in the currently selected mode's cycle. This retesting permits dynamic selection of the output formats using the mode pins without a power reset. Applications can then use any or all of the output formats to indicate application alarm or status conditions.

The 12C508's internal RC oscillator provides the timing control used in each of the modes. Using an average of 3 mA, the controller operates from 2 to approximately 5.25V. The frequency-stepped formats are in constant "timebased" increments with constant frequency dwell times. Based on a 2.2-kHz piezo-element resonant peak, each of the mode's characteristics uses code-settable, dedicated registers to establish the output format.

The coded sequences use 127 bytes of code space. You can port the sequences into one of several code-compatible Microchip controllers or use a stand-alone peripheral controller, as in Figure 1, for any number of alarm applications. You can download applicable code by clicking here: DI-SIG, #2147. (DI #2147)

| EDN Access | Feedback | Table of Contents |