When you step out of your brightly-lit house into the darkness, it takes a while for your vision to adjust. A solution to this problem is this outdoor light with automatic switch off. As a bonus, it will also make it a little bit easier to find the keyhole when returning late at night.
Often no AC neutral connection is available at the point where the switch-off timer is to be installed, which makes many circuit arrangements impractical. However, the circuit here is designed to work in this situation. The design eschews bulky components such as transformers and the whole unit can be built into a flush-mounted fitting. The circuit also features low quiescent current consumption.
The circuit is started by a closing switch (or pushbutton) S1. The lamp then immediately receives power via the bridge rectifier. The drop across diodes D5 to D10 is 4.2 V, which provides the power supply for the delay circuit itself, built around the CD4060 binary counter.
When the switch is opened the lighting supply current continues to flow through Tri1. The NPN optocoupler in the triac drive circuit detects when the triac is active, with antiparallel LED D1 keeping the drive symmetrical. The NPN phototransistor inside the coupler creates a reset pulse via T1, driving pin 12 of the counter. This means that the full-time period will run even if the circuit is retriggered.
The CD4060 counts at the AC grid frequency. Pin 3 goes high after 213 clocks, which corresponds to about 2.5 minutes. If this is not long enough, a further CD4060 counter can be cascaded. T2 then turns on and shorts the internal LED of opto-triac IC2; this causes Tri1 to be deprived of its trigger current and the light goes out. The circuit remains without power until next triggered.
The circuit is only suitable for use with resistive loads. With the components shown (in particular in the bridge rectifier and D5 to D10) the maximum total power of the connected bulb(s) is 200 watts. As is well known, the filament of the bulb is most likely to fail at the moment power is applied. There is little risk to Tri1 at this point as it is bridged by the switch. The most likely consequence of overload is that one or more of diodes D5 to D10 will fail. In the prototype, no fuse was used, as it would not, in any case, have been easy to change. However, that is not necessarily recommended practice!
