Enhancing Nighttime Visibility
When you step out of your well-lit home into the darkness, your eyes need time to adjust. A practical solution to this challenge is an outdoor light equipped with an automatic switch-off feature. This not only aids your vision but also simplifies the task of locating the keyhole when returning home late at night.
Overcoming the Lack of AC Neutral Connection
In scenarios where there’s no available AC neutral connection for the installation of a switch-off timer, many circuit arrangements become impractical. However, the design presented here is tailored to function in such situations. This setup eliminates the need for bulky components like transformers, allowing the entire unit to be incorporated into a flush-mounted fitting. Furthermore, the circuit exhibits minimal quiescent current consumption.
Initializing the Circuit and Power Supply
The circuit is initiated by closing the switch or pushbutton S1. Subsequently, the lamp promptly receives power through the bridge rectifier. The voltage drop across diodes D5 to D10 amounts to 4.2 V, serving as the power source for the delay circuit itself, constructed around the CD4060 binary counter.
Continuous Operation and Reset Mechanism
When the switch is opened, the lighting supply current continues to flow through Tri1. The NPN optocoupler in the triac drive circuit detects the triac’s activation, aided by the antiparallel LED D1 maintaining drive symmetry. The NPN phototransistor within the coupler generates a reset pulse through T1, driving pin 12 of the counter. Consequently, the entire time period progresses even if the circuit is retriggered.
Counting and Light Shutdown
The CD4060 counts according to the AC grid frequency. After 213 clock cycles, which equates to roughly 2.5 minutes, pin 3 goes high. If this duration is insufficient, additional cascaded CD4060 counters can be employed. T2 is then activated, short-circuiting the internal LED of opto-triac IC2. This action deprives Tri1 of its triggering current, causing the light to extinguish. The circuit remains without power until the next triggering event.
Load Compatibility and Power Limitations
This circuit is exclusively suitable for resistive loads. With the provided components, especially in the bridge rectifier and diodes D5 to D10, the maximum total power for connected bulb(s) is 200 watts. It’s a well-known fact that a bulb’s filament is most susceptible to failure when power is applied. Tri1 faces minimal risk during this stage, as it is bridged by the switch. Overload scenarios are more likely to result in the failure of one or more diodes (D5 to D10). In the prototype, no fuse was incorporated due to accessibility concerns. However, it’s important to note that this practice isn’t necessarily recommended.