AN2839 Freescale Semiconductor / Motorola, AN2839 Datasheet - Page 8

• Manufacturer Part Number: AN2839
• Description: Implementing a Lamp Dimmer with an HC908Q Family MCU
• Manufacturer: Freescale Semiconductor / Motorola

Design Customization
Design Customization
This design works for many applications without modification. However, some customers may want to
customize its functionality. A few variations for this circuit include:
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Modifying the circuit to use a single button. For this modification, pressing the button would turn the
lamp on and off and if held would gradually brighten the lamp to full bright, then gradually dim to
full dim. The brightness would stay at whatever level it was at when the button was released.
Enhance the timer feature to allow the user to choose the period (“on” time).
Add a sensor to automatically switch the lamp on and off based on the room occupancy.
Use the two available pins to add a serial bus for control from a remote computer.
Add a photo sensor to adjust a given brightness level in a room according to the ambient light.
Isolate the load by an opto-isolator IC. This provides isolation between the load and the control
circuit, especially when high isolation voltage is required. Applications involving industrial controls,
vending machines, or motor controls would benefit from this technology.
implement this type of modification. T1 must have a secondary of 9 V or 12 V so that the zener
operates adequately.
Another practical application is to keep the load turned on for a certain amount of time. In this case,
the load is supplied at full power and the triac is triggered at a desired time. Refer to
schematic diagram of the timer.
When the start button is pressed, the MCU is reset and enters into run mode. The software counts
the desired time (set previously) and enters into stop mode, which turns off the lamp. When the
MCU is in stop mode, all internal modules are disabled and the power consumption is negligible.
The circuit is kept in this state until the start button is pressed again.
The software was set to a one minute delay time. This is the value used in most applications where
it is required that a lamp must be turned on for a short time. However, the software can be easily
be changed to set the desired delay time and lamp brightness.
For higher loads (greater than 100 watts), the triac must be changed to accommodate the
maximum current and a heat sink might be required. A high-current triac requires a non-negligible
gate current and it might not be possible to drive the triac gate directly from an MCU port. In this
case, a driver is needed.
The circuits above control resistive loads only. For inductive loads, it is
recommended that the MCU be isolated from the load with opto couplers
and that a triac snubber network be adopted as shown in
Implementing a Lamp Dimmer with an HC908Q Family MCU, Rev. 0
NOTE
Figure
Figure 6
8.
Freescale Semiconductor
illustrates how to
Figure 7
for a