Circuit Labs - Electronic Circuits Collection

Author: Friedrich Weigand

It has been known since Einstein that time is relative. This seems to apply particularly to children, who can make seconds and minutes seem like days and years when faced with unpopular chores like cleaning their teeth. To get the little dears to spend the three minutes recommended by experts on their teeth, parents concerned for the dental health of their children can resort to electronic means. And so much the better if you can get your children to help out with building the device!

The circuit of the toothbrush timer (Figure1) uses the familiar Atmel RISC AT90S2313 microcontroller, together with an oscillator formed by X1, C1 and C2. The microcontroller is available ready-programmed (order code 050146-41). It drives a row of LEDs. Green LED D1 flashes every second. The green ten-second LEDs and the red one minute LEDs light in sequence and remain on until three minutes have elapsed. Then all the LEDs go out and the blue ‘finished’ LED D9 starts flashing every second, indicating the end of brushing time. The port currents are limited to 2 mA to 3 mA by the array of 1 kΩ resistors: this is enough for the low-current LEDs, which are used in order to ensure a long battery life. A 5 V fixed voltage regulator (IC2) and decoupling and smoothing capacitors C3 to C5 round off the circuit. Diode D10 prevents damage should the 9 V PP3 (6F22) type battery be accidentally connected the wrong way around.

The whole circuit can be built on the printed circuit board shown in Figure 2. There are a few wire links near to the resistor array. The voltage regulator, crystal, C4 and C5 are mounted flat on the board to allow the LEDs to stand above the other components and poke through holes in the enclosure. A socket should be used for the microcontroller. Almost all the components are polarity-sensitive: check carefully before you solder!

Figure 2.	PCB layout

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 (Elektor Electronics Magazine – 07/2006)

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