A very interesting horn driver
project presented by Microchip technology is presented in this circuit diagram . This horn driver project is based on PIC16F886 microcontroller , from Microchip . This microcontroller horn driver circuit
diagram is very simple and require few external components . The PIC MCU
has peripheral resources within the device to provide horn driver services in a
very simple manner.
The PIC MCU peripherals include the
Enhanced CCP (ECCP) module in Pulse-Width Modulation (PWM) Half-Bridge mode to
drive the 2 horn drive leads and a single ADC input to monitor the horn
feedback after it has been conditioned.
Horn characteristics are required to
determine the defined parameters for the range of the PWM module.
For example , a horn with a resonant
frequency of 3.5 kHz ± 0.5 kHz; the PWM module generates a PWM frequency output
from 3 kHz to 4 kHz with 50% duty cycle.
With a device that is running off of
the internal oscillator at 8 MHz, the clock source to Timer2 that drives the
PWM period generates 2M clocks per second. For a 3 kHz period, this is 667
clocks per cycle, and for a 4 kHz period, this is 500 clocks per cycle. Because
Timer2 is an 8-bit timer, accepting only a maximum value of 255, these clocks
per cycle must be divided (a prescaler of divide-by-4, yielding 166 clocks per
cycle for 3 kHz, and 125 clocks per cycle for 4 kHz) .
The PWM output driven by the ECCP
module in Half-Bridge mode, with both the P1A and P1B outputs active-high, will
step through the 125 through 166 clocks per cycle periods at a period rate, and
as the Period register is loaded, the value will be dividedby- 2 and loaded
into the Duty Cycle register for a 50% duty cycle. This will become the new PWM
period for measuring the feedback from the horn driver, and drives the
transistor that raises the level that the horn lead sees, to 9V.
You can download the software for
the PIC16F886 horn driver following this link
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