Background and Problem Statement
In the author’s living room, the television set boasts built-in Ambilight lighting, enhancing the viewing experience. However, the bedroom television lacks this feature. To compensate, the author attached a lamp to the wall, providing some background illumination. Although this setup improved the viewing experience, there was an inconvenience. While the TV could be turned off with the remote, manually switching off the lamp meant getting out of bed. To address this, the author developed an automatic lighting switch that synchronizes with the TV.
The circuit is integrated into the TV’s AC power cable, eliminating the need for any modifications to the TV set. R1 monitors the current drawn by the TV, which ranges from 50 mA in standby mode to approximately 500 mA during operation. D5 and D1–D4 limit the voltage across R1 during negative and positive half-cycles, respectively. During positive half-cycles, these diodes charge capacitor C1 through D6. This voltage powers the internal LED of solid-state switch TRI1 via R2, enabling the internal triac to conduct and supply mains voltage to the lamp.
Additional Component and Safety Measures
Diode D7, though not mandatory, is advisable to prevent potential damage. It protects the LED in the solid-state switch, which is sensitive to reverse polarization. To safeguard the circuit, fuse F1 is included, ensuring protection against overloads and enhancing the reliability of the solid-state switch. These components collectively enhance the longevity and stability of the automatic lighting switch.
Resistor Value Selection
For a 32-inch LCD screen measuring 82 cm, a 10 Ω resistor (R1) is suitable. However, with smaller TVs consuming less power, R1’s value can be increased to 22 or 33 Ω, in which case a 3-watt resistor should be used. Care must be taken not to select an excessively high resistance, as it might cause TRI1 to activate when the TV is in standby mode.
Consideration for Power Supply Variations
Certain TV sets incorporate a half-wave rectifier in the power supply, leading to an imbalanced load on the AC outlet. If the TV draws current only on negative half-cycles, the circuit may malfunction. In regions where AC power plugs are reversible, this issue can be resolved by reversing the plug orientation.
Caution with Optically Coupled Solid-State Relays
Optically coupled solid-state relays have limited resistance to high switch-on currents (inrush currents), especially older-model TVs with picture tubes having demagnetization circuits. If the relay fails, it often fails shorted, causing the TV background light to remain on continuously. To mitigate risks, careful consideration is necessary, especially concerning older TV models.
Construction and Safety Measures
When constructing the circuit on perfboard, it is imperative to remove all copper traces adjacent to conductors and components carrying mains voltage. Additionally, PCB terminal blocks with a 7.5 mm spacing should be used, ensuring a 3 mm separation between connections on the solder side. When configuring the entire setup as a Class II device, parts of the circuit at AC line potential must maintain a minimum separation of 6 mm from any metal enclosure or electrically conductive external components, adhering to local and national electrical safety regulations.