PIC16F876 with PWM applications can be an example of a project 9 fans can be controlled 3 BUZ72 Mosfet with the PIC outputs reinforced LM60 temperature sensor information from the relative speed is changing the R and G LEDs with temperature condition can be observed CC5X prepared with the C source and hex code, circuit diagrams, there

Note: before programming (erasing) a 16F676 device, first read its CONFIG register and its last (0x3ff) program word! These contain factory-programmed calibration values, that become lost by (re-)programming! 1st sample: read last program word=343C config:’bandgap midhigh’

Operation:
TMR0 is used to count up, driven by the internal oscillator.
In ‘set_fans()’ the counter value is compared with target values
to create PWM output signals.
Four analog inputs are sampled: three LM60 temperature sensors
and one ‘minimum fan speed’ setting.
At TMR0 wrap-around all PWM outputs are activated and a limited
amount of computation is done. During the TMR0 loop, the PWM
outputs are reset one-by-one.
The PWM frequency can be adjusted with the TMR0 prescaler,
typically between 64 and 512 Hz.
A global ‘time’ is maintained for a slow reduction of fan speed,
avoiding control loop instability. Note that this notion of time
floats with the TMR0 prescaler.

The PIC16F676 performs A-to-D conversion on the 4 analog inputs, and creates a PWM output signal to drive the fans. Besides 3 temperature values, one for each group of fans, it samples a value for a shared minimum fan speed setting. On rising temperatures, the fan speed rises linearly and immediatly. On decreasing temperatures, the fan speed decreases only very slowly, to avoid control instability: from 100% to 0 speed in about 8 minutes. Lowest temp setting (turn left): 20 to 30 degrees celcius results in 0 to 100% fan speed. Highest temp setting (turn right): 65 to 80 degrees celcius results in 0 to 100% fan speed.

Source: vaneijndhoven.net

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