Activity 9 - Automatically Switching an LED using a Light Sensor

Activity 9 - Automatically Switching an LED using a Light Sensor

The students build a first-iteration prototype of a night-light circuit.

Activity 9 is the first in a series of activities in which the students work towards building their own small flashlight/night-light. The objective is for the students to put their new soldering skills to use to build something they can keep at the end of the activities. So far, each activity has been self-contained and the students have to disassemble their work at the end of each session. With soldering, the circuits built are permanent, so it’s a great opportunity to build something useful! The circuit will function both as a flashlight and as a night-light that turns on when the room becomes dark. This past weekend, the students used a breadboard to build the first iteration of the night-light circuit and explore some of its weaknesses.

A student demonstrates their breadboard prototype of the night-light by covering the light sensor with his finger.

The circuit uses an inexpensive sensor that changes resistance depending on the ambient light. Putting this sensor in a voltage divider configuration allows it to be used to turn off an LED when the surroundings are bright, and switch it on in the dark. However, the students noticed that the LED is much dimmer than usual. They were able to recognize that the brightness is reduced because the current must flow through extra resistance in the voltage divider. If only there were some kind of electronic switch that we could control with the voltage divider, instead of connecting the LED to the divider directly! In the next activity, we will be improving the circuit by adding exactly that, a transistor.

Five boys and seven girls were absent from the activity, leaving 15 boys and 30 girls (33%:67%)1. The cost of one parts-set was $7.74 CAD, with 10 sets shared across 45 participants, bringing the price per student to $1.72 CAD per student2. The majority of this cost is the rechargeable battery, and only $0.11 CAD of parts per student is expected to be broken/lost in the course of the activity.

Activity 9 was created as a Free Cultural Work and is available for use and adaption under the CC BY-SA 4.0 license:

  1. SMD has a nearly equal gender distribution among boarding students, with 175 boys and 174 girls. The gender distribution among the Himalayan Makers Guild members is 20 boys and 37 girls (35%:65%). 

  2. Part details and suppliers are given in the lesson plan.