Electronic component: Light Dependent Resistor (LDR)

Part of the project is to find out what a LDR is and does. As the name suggests it has something to do with resistance and with light. Depending on the type the resistance will be lower when more (or less) light falls on the LDR. But you can  build the circuit either way.

Today's shopping list contains:
1 x LDR
1 x 10 kOhm Potentiometer
2 x 330 Ohm resistor
1 transistor BC547B
1 x green LED
1 x 9V battery

The following circuit will light a green LED when there is little light. By adjusting the potentiometer you can control the amount of light required.

This works basically because when the LDR recieves a lot of light, the resistance will drop. The current will not go through the transistor. As the amount of light drops, the resistance of the LDR will decrease. After a certain point, the current will prefer the transistor instead of the LDR, enabling the green light.

Project: Building a light meter (1)

This is the first project I have started. Nothing to fancy, but something fun to do. Goal is to build a light meter, which measures the amount of light in the environment. It outputs the amount of light by lighting LEDs. The more light, the more LEDs will be lit. We will measure the amount of light with a LDR component.

The key component will be the PICAXE. This is a programmable IC (PIC) for low-end purposes. During the project we will be learning:

- How to use a LDR component
- How to program a PICAXE
- How to build the light meter

Circuit basics: Resistance (2)

Last time I blogged about resistance. Today we are going to look at resistance within a circuit. Everything has basically to do with Ohm's Law, which states states:

the current through a conductor between two points is directly proportional to the potential difference or voltage across the two points, and inversely proportional to the resistance between them. (http://en.wikipedia.org/wiki/Ohm%27s_law)

This means that you can calculate the current if you know the voltage and the resistance. Also true is that you can calculate the voltage if you know the current and the resistance, and finally the resistance if you know the current and the voltage.

The formula for Ohm's law is: U = I x R, where U is voltage (in Volts), I is current (in Ampere) and R is resistance (in Ohm). You can also write this as I = U / R or R = U / I.

Given the following circuit:

Here we have the three variables:  U = 9 Volts, R= 450 Ohm and I = 0.020 Ampere (0.020 A = 20 mA). Let's check:

I x R = U
0.020 x 450 = 9 Volts


Ok, here are some circuits, which value is the missing variable? Rhe answers are at the bottom.

1. Given: U = 9, I = 0.0257. What is R?

2. Given: R= 350, I = 0.0143. What is U?

3. Given: U = 9, R=650. What is I?

4. How much resistance does the LED generate?

5. This circuit has a current of 23.6 mA. How much Ohm is the resistance of the resistor?






1. 9 / 0.0257 = 350 Ohm
2. 350 x 0.0143 = 5 Volts
3. 9 / 650 = 13.8  mA = 0.014 A
4. 1.9 / 0.0103 =   184 Ohm
5. As we have learned, the sum of the components in series is the total voltage. So  9 - 1.92 = 7.08 V.  7.08 / 0.0236 = 300 Ohm