from DOCTRONICS:

Everything you need to know!

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Kits available . . .

Safety lights	Too hot : too cold
Biscuit tin alarm	Games timer
Chuckles	Metronome
Electronic dice	Combination lock
Light Chaser	Quiz referee

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Cyclists and pedestrians need to be seen. The safety lights project flashes superbright red LEDs 10 times each second giving a high visibility display. Students taking Design and Technology courses will want to design their own acrylic or vacuum formed case.

This project is developed as a Design Example in the book 'Discovering Electronics with crocodile clips ©', also available from DOCTRONICS.

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Someone is stealing the biscuits! This project will detect the biscuit thief the moment the tin is opened. The circuit provides a delay of around 10 seconds when the power supply is first connected during which the circuit can be placed in the tin. After this, if the LDR is exposed to light the alarm is triggered and remains ON, producing a penetrating high pitched sound until the power is disconnected.

The development of this project is explained with detailed instructions for practical work in Design Electronics.

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'Chuckles' is an electronic circuit which makes a silly noise. When you hear the noise, you will understand the name. The circuit includes a SLOW astable and a FAST astable. The frequency of each astable is controlled by an LDR. The astables interact so that when either LDR is covered, the frequency changes . . .

In Design and Technology, you can use 'Chuckles' for other projects, for example as part of a child's toy. Find out more about the circuit by clicking this Chuckles link.

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The development of this project is described in detail in the DOCTRONICS book Discovering Electronics Projects, publication September 2000.

The construction kit contains the printed circuit board and all the electronic components needed to build the dot matrix die. Everything fits on the printed circuit board and, with the exception of the battery leads, there are no trailing wires.

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The DOCTRONICS light chaser illuminates six LEDs in a pattern which chases from left to right and back again. Adjust the rate to give a lazy side to side motion or a frenetic bouncing pattern. Outputs from the main chaser printed circuit board are used to control letter boards. Design your own letters or symbols on a 5x5 matrix to create your own eye-catching display.

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DOCTRONICS Light Chaser construction kit

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How can Goldilocks tell if the porridge is too hot, too cold, or just right? This circuit measures temperature with a thermistor and gives one of three different results. If the temperature is below the lower threshold, a yellow LED illuminates. If the temperature is above the upper threshold, a red LED illuminates. However, if the temperature is somewhere between the two threshold values, a green LED illuminates to show that everything is just right.

By choosing appropriate resistor values, you can change the temperature thresholds to suit a particular application. The action of thermistors is explained in Chapter 3 of Design Electronics.

The circuit uses a 20 kW thermistor as part of a voltage divider. As the temperature increases, the output voltage from the voltage divider increases. The threshold levels are set by the 18 kW, 4.7 kW and 18 kW divider chain. Both 741 op-amps are connected as voltage comparators. The output signals from the voltage comparators are converted to illuminate the appropriate LEDs using diode and transistor logic gates.

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How often have you fallen asleep waiting for Granny to take her turn at Scrabble? (To be fair, most grannies are pretty smart, so it may be she who falls asleep waiting for you.) The brief for designing the Games Timer was to develop an electronic replacement for the sand timer which could provide timed intervals up to three minutes, a visible indication of how much time was left, an audible indication at the end of the timed period, and which could be instantly reset. The Games Timer solves all of these problems. The development of the circuit is described in detail as part of Design Electronics and is also included in the DOCTRONICS book Discovering Electronics Projects.

The Games Timer is an extended project suitable as a GCSE Electronics Products or GCSE Electronics project. If you follow through the practical work, you will develop a good understanding of the process of developing an electronic circuit. The reasons for choosing particular components and subsytems are clearly explained.

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DOCTRONICS Educational Publishing