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February 25, 2007

LC IV: Checking components

I wanted to introduce the kids to being able to use the multi-meter to test the components before they made any boards with them. We used the resistor measuring facility to measure the resistance. However, since we had only three kinds of resistors they already knew what the value was and it wasn’t much of a challenge. I then started making networks of resistors and asked them to measure it.

When the resistor setting is too high the value of the resistance appears very close to zero and when the setting is too low the meter gets stuck to a single digit one. They understood this quite well. I then pushed it a bit further and started asking them to guess the resistance between different corners of a square made out of a 33 ohm and three 10 kohm resistors. Senthil got the answer I asked from him and now the other two were curious how to “guess”. I finally got my opening to squeeze in my water analogy and there were able to understand series parallel resistors and an idea of what would be a limiting resistance if two of them were way off.

We then went over checking if an LED was working using a battery and a resistor. Earlier in the morning we had sort the LEDs and found some bright, dull and busted. I gave them three each and asked them to sort it out that they did.

Having done something that they were comfortable I introduced the transistor as a three leg device a little different from the two leg elements they were used to. Luckly, the multi-meter had a way of measuring the npn bipolar transistors and I didn’t need to come up with a circuit to identify it. I drew the symbol and explained the names of the terminals as simply as I could and asked them to identify the terminals and crosscheck with the multi-meter.

Here are some notes that Ani took in the class:
Transistor: Collector – base – Emitter
When flat side faces you, the right most leg is the emitter. You can test it in the multi-meter (NPN transistor – all transistors now are NPN). It gives you a parameter of the transistor (HFE). Mine had a value of 292. If you do not put it in the correct setting, the multi-meter shows a much smaller number (around 1/10th).

How do you check if an LED is working?
Test it using a battery. If a 9V, then add a 210 ohm resistor in series to test.

We then built the circuit of the torch (with the constant current) on the breadboard including the transistors. They seemed comfortable to throw in a three terminal element they could test.

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