What it shows
A light dependent resistor has a low resistance in bright light and a very high resistance in the dark. In a voltage divider with a fixed resistor, this change swings the base voltage of a transistor, which switches a buzzer on when it gets dark.
The science
The 10 kΩ resistor goes from 9 V to the transistor base, and the LDR goes from the base to ground. In light the LDR is only a few hundred ohms, so it pulls the base almost to ground and the transistor stays off. In the dark the LDR rises to hundreds of kilohms, the base is pulled above 0.7 V through the 10 kΩ resistor, the transistor conducts and current flows through the buzzer in its collector. The 1 kΩ resistor in series with the base protects the transistor.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- One LDR (5 mm or 10 mm)
- One BC547 transistor
- A 10 kΩ resistor and a 1 kΩ resistor
- An active buzzer rated 3 to 12 V
- A 9 V battery with a snap connector
- A breadboard and jumper wires
Safety
For this project: Battery power only. Do not short the 9 V battery terminals; it gets hot.
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.
Push the BC547 into the breadboard with its three legs in different rows: collector, base, emitter with the flat face towards you.
Connect the emitter to the breadboard ground rail and the battery negative to the same rail.
Connect the buzzer: its red (positive) lead to the 9 V rail and its black lead to the collector.
Place the 10 kΩ resistor from the 9 V rail to a spare row, then the 1 kΩ resistor from that row to the base.
Place the LDR from the same spare row to the ground rail; it has no polarity.
Snap on the battery. In room light the buzzer should be silent. If it sounds, the room is too dim: test near a window.
Cup your hand over the LDR: the buzzer sounds. Uncover it and it stops.
If it never sounds, swap the 10 kΩ for 47 kΩ; if it never stops, use 4.7 kΩ. Record the value that worked.
What you should see
The buzzer is silent in light and sounds within a second of the LDR being covered, showing the transistor switching on a sensor signal.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What happens to the resistance of an LDR in the dark?
It rises sharply, from a few hundred ohms to hundreds of kilohms.
2Why is the buzzer in the collector and not the base circuit?
The collector carries the large current; the base needs only a tiny current to control it.
3What base voltage turns a silicon transistor on?
About 0.7 V between base and emitter.
4How would you make it sound in brighter conditions?
Use a larger fixed resistor so the base reaches 0.7 V with less darkness.
Take it further
Replace the 10 kΩ resistor with a 100 kΩ potentiometer so you can set the darkness level with a knob.
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.