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

Circuit basics: Resistance

Last time we discussed the resistor. I promised to do a post on resistance as the resistor is quite useless if you don't know about resistance. So what is resistance?

If you imagine electrical current as a water flow, you can imagine the resistor as narrowing of the pipe as can been seen in the following image. We already learned that voltage can be seen as water pressure. At point A the water pressure could be, for example, 8 Volts. Because of the resistor, the water can not flow freely to point B. The pressure at point B would be lower, let's say 6 Volt. This means that the resistor resists 2 Volts.

Basically that's it. It's not very hard to understand resistance, but using it in your circuit is much harder. Next time we investigate resistance using resistors in a circuit.