The Nightlight – evolution of a key stage 3 project

. . . On a recent course for teachers, I wanted a simple non PIC project for practising PCB design. (We had already produced a simple PIC circuit). My thoughts turned to the RC timing circuit. I could use this to teach about potential dividers and we could use this to control a lamp – a simple and quick circuit to design with lots of important electronic concepts. Fig 4 shows my initial ideas – an RC timing network to control a transducer driver.

We used a high intensity white led instead of a light bulb so that less transistor base current was required and longer time delays could be achieved. It then struck me. . . if a MOSFET was used instead of a bipolar transistor, the transducer driver would not take charge from the timing resistor and longer time delays could achieved (fig 5). In fact, very long time delays could be achieved without the possibility of the timing period extending indefinitely (see long time delays).

Fig 4 Night light based on a bipolar transistor

Fig 4 Night light based on a bipolar transistor

This is because the timing period is set by the capacitor discharging not by charging. (There is no competition between the leakage current and the charging current because the leakage resistance appears in parallel with the timing resistance not in series with it). Unlike conventional 555 timer circuits, R and C can be as large as you like. R can be made as large as possible by using the leakage resistance instead of an external resistor. If the external resistor is omitted, the only way to make R larger still is to reduce the leakage resistance. Selecting a capacitor with a higher working voltage can achieve this, but interestingly a more effective way is to use a smaller capacitor e.g. 10nF ceramic. Non electrolytic capacitors have very much lower leakage (of the order of 100G for high voltage ceramic disc types) which can more than compensate for the reduction in capacitance. In fact, C can be left out of the circuit altogether due to the MOSFET gate capacitance!

The teachers duly designed and constructed their night light circuits. They were impressed by the white LEDS and the significant time delays we were getting but they thought it would be better if it was automatic – could I not think of a way of using an LDR to control it? I could not. The trouble is that a light sensor needs a process block to monitor it. Such a process block, perhaps a PIC or a comparator would add significant complexity and increase quiescent current drain perhaps requiring an on/off switch and defeating the whole point but I agreed to sleep on it.

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