The Nightlight – evolution of a key stage 3 project
This nearly completes the circuit (fig 7); all that is left to do is to ensure that the timing capacitor is fully charged when the power switch turns on. This is the purpose of D1. It tops up C1 through the LDR when it is light. This happens quickly because the resistance of the LDR is low in strong light. In the dark, the LDR resistance is high so the output from the light sensor is low, reverse biasing D1 preventing further charge from entering C1. As it stands, there is no provision for adjusting the light threshold needed to switch the circuit on an off (to minimise costs). But R2 could be replaced with a preset potentiometer to provide this. R2 needs to be a high as possible because this is where the quiescent current flows in strong daylight. (R2 was set at 10K for ease of use in the classroom but it can be increased to 100K to bring the quiescent current down to 50 uA).
Arrival at this final circuit which is inexpensive (and with no PIC in sight) has been quite a journey. Although I am a keen advocate of the systems approach (and of PICs), I confess that thinking input/process/output blinkered my view. Its synthesis is the result of my knowledge of the behavior of individual components rather than the systematic assembly of standard ‘building’ blocks.
When you place the nightlight by the bedside and switch off the room lights it instantly lights up. This ‘remote’ control is rather engaging and adds to the mystery of the project. Perhaps this is just what parents need to encourage children to sleep with the lights off!
This article first appeared in the spring 2006 edition of Electronics Educaton published by the IET.
A kit of parts, including PCB, for the the nightlight is available from Rapid Electronics (order code: 70-2600) and from Electron Electronics (order code: 525-1560)
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