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.
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