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KrisBlueNZ submitted a new Showcase Item:
Fanrunon - delayed turn-off for bathroom extractor fan
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Fanrunon - delayed turn-off for bathroom extractor fan
My 'Fanrunon' project is a turn-off delay timer module for use with an AC mains load such as a fan or a light. The rated maximum current is 2.5A RMS. It uses a triac for switching, and a Microchip PIC12F675 microcontroller.
At this time, I have not built up the hardware, and I have only partly tested the firmware using the Microchip MPLAB X emulator. The design and/or the code may change once I have built and tested the circuit and the code.
The module fits inside the cavity behind the wall switch, and receives its power from the fan current itself. Because of this design feature, and unlike many commercial turn-off delay timers, this design doesn’t require a Neutral wire in the switch cavity.
Fanrunon uses the mains frequency as its timing source, and supports 50 and 60 Hz mains frequencies. The run-on time can be selected by switching the wall switch ON and OFF a certain number of times. The wall switch must be a changeover type (SPDT aka SPCO) so the circuit can detect its position.
Since the Fanrunon module doesn’t connect across the mains (it doesn’t use a Neutral connection), it gets its operating power by slightly dropping the voltage available to the load.
IMG
Circuit description:
Turning ON the wall switch completes the electrical circuit from mains Phase, through the Fanrunon circuit (CN1 pins 1 and 2), through the wall switch, through the fan, and back to mains Neutral, and this powers up the Fanrunon circuit. D1~4 form a bridge rectifier that ensures that the voltage across the series diode string D5~9 always has the right polarity — positive at the top. The diode string conducts the load current and limits the voltage across it to around 5V (depending on load current). On every half-cycle of the mains waveform, current flows through D1~4 and produces this voltage drop across the diode string.
This 5V supply rail passes through fusible resistor R2 to D10, a 5.1V zener diode. This arrangement ensures that if excess current flows through the D5~9 string (due to a short-circuited fan or a wiring error), D5~9 may be damaged but R2 will very quickly go open-circuit, while D10 protects the microcontroller from losing its smoke. C2 smooths the DC rail so it remains active around zero-crossings of the mains voltage, when there is insufficient voltage available at the bridge rectifier input. This provides a 5V DC (approximately) supply to the microcontroller.
U1 is a Microchip PIC12F675-I/P microcontroller in an 8-pin DIP package. CN2...
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