What it shows
An LED is a diode that makes light when current passes, but in strong light it also makes a small voltage itself. A transistor can use this voltage to switch another LED on, so one LED acts as a light sensor.
The science
Photons striking the semiconductor of an LED free a few charges, giving a small voltage of about 0.5 to 1.5 V in bright light (the photovoltaic effect). If this voltage exceeds about 0.7 V it can drive the base of a BC547 through the LED itself, so the transistor turns on the indicator LED. A red LED responds to light of its own colour and to every colour of higher energy, so it responds to most visible light.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A clear 5 mm red LED (the sensor)
- A second LED of any colour with a 470 Ω resistor
- One BC547 transistor
- A 9 V battery with snap connector
- A multimeter, a bright torch or phone flash, and a breadboard
Safety
For this project: Battery power only. Do not look straight into a bright torch.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Set the multimeter to the 2 V DC range and connect it across the sensor LED, long leg to the red lead.
Hold the LED in room light, near a window and under a torch; record the three voltages.
Place the BC547 on the breadboard with its emitter on the negative rail.
Connect the sensor LED long leg (anode) to the base and its short leg (cathode) to the negative rail.
Connect the 470 Ω resistor from the 9 V rail to the second LED long leg, and its short leg to the collector.
Snap on the battery and bring the torch very close to the sensor LED.
The second LED should glow when the sensor voltage passes about 0.7 V; take the light away and it goes off.
Try LEDs of other colours as sensors and record which one needs the least light.
What you should see
The indicator LED lights when a bright torch shines on the sensor LED and goes off in dim light, and the meter shows the voltage generated.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1How can an LED produce a voltage?
Light frees charges in the junction, like a very small solar cell.
2What voltage does the transistor need?
About 0.7 V between base and emitter.
3Why does the sensor need very bright light?
An LED is a weak light detector, so it generates only a small voltage.
4Which colour LED would be a poor sensor of red light?
An infrared LED, which responds poorly to visible light.
Take it further
Use two BC547s as a Darlington pair so that room light is enough to switch the indicator.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Robotics
Electronic Components Handbook
Resistors, capacitors, LEDs, diodes, transistors and more, with how to use each