The Nightlight – evolution of a key stage 3 project
If the monostable is constructed from an NE555 then a driver is not required because the timer can source up to 200mA. But the NE555 draws a quiescent current of over 3mA. This means that at the end of the timing period after the light automatically switches off, the circuit will continue to draw a current of at least 3mA. Modern CMOS 555 timers have a much lower quiescent current and some of them can drive a 60mA load although this is rather close to their absolute maximum rating.
To provide a simple and inexpensive night light circuit, I settled on using the then new 8 pin PIC instead of a 555 timer (fig 3). It was ready programmed for pupils to switch the light on for 20 minutes and then go to sleep. A latching touch switch was used to start the timing period (turn the light on). The light could also be turned off (before it timed out) by pressing the touch switch again. The PIC only draws a few microamps in sleep mode, but can be woken up by a signal on one of its inputs (by the touch switch). To reduce costs, the touch switch was built into the PCB. We successfully used this night light circuit for several years. Of course, with Logicator 2 and PIXACE, we ought to upgrade the project so that pupils can programme their own . . .
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