Colour Changing Night Light - How It Works

Colour changing LEDs allow students to make excellent mood lights. The ‘dark activated colour changing LED kit’ uses a circuit with an input, process & output. It also adds some soldering to the project. The circuit is designed so that the LED is only on when it is dark.

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As with all lighting projects there is the option to illuminate Perspex by pointing the LED into the edge of the plastic. Laser cutters allow the plastic to be cut and can also ‘engrave’ the sheet. These engraved parts ‘shine’ when the plastic is lit by the LED. Alternatively a frosted PVC sheet can be used to diffuse the light. This can be cut and decorated to add more interesting features to the product (see details, left).

With this project the LED only turns on at night. This is achieved by using a light dependent resistor or LDR for short (R2 in the circuit diagram). When the light level falls the resistance of the LDR increases. The potentiometer allows the light level that the LED turns on at to be set. This potentiometer (R1) and the LDR (R2) make a potential divider, the mid point of which is feed in to the transistor. In this configuration as the light decreases, the resistance increases and so does the voltage going in to the transistor Q1. The two transistors Q1 & Q2 are connected as a Darlington pair, so when the voltage on the base reaches 1.4V the transistor switches on. When the transistor is turned on, current can flow through it and the LED is illuminated.

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