Tulley Family Tinkering Lab Website

Robbie has the “Magic Touch”! — Science fair project progress

by on Apr.04, 2010, under science fair

We have neglected to post what was up with the science fair project on a daily basis. Sorry!

I’ll try a quick summary in several posts, of some of the key things we did and learned after that last post about the 555 timer chip:

For some reason, the relay didn’t always fire. We were really confused by this, and thought there was something majorly wrong. One thing unusual that we had done was use ten 0.1uf capacitors instead of one 0.01uf capacitor. After buying some assorted capacitors at the RadioShack, we tried to consolidate down the whole circuit, and replaced the ten caps with only the one. Could this be the problem? It sure didn’t seem like it.

Here is the odd thing — whenever I was measuring voltages in the circuit while Robbie triggered it (by pulling a wire out from Vcc), it seemed to work. I wondered if my testing was influencing the circuit.

After a while, I just concluded that Robbie had some sort of “Magic Touch”. He could even get it to work sometimes when I wasn’t measuring things in the circuit with my voltmeter. Hmmm…

Suddenly when it was my turn again to try to pull the trigger wire, I accidentally bumped this wire with my hand. The circuit triggered and the relay ran. Woah. I suddenly realized that what the circuit needed was a high resistance path to ground so that that input to the circuit would drain all of its voltage when not directly connected to Vcc.

In other words, Robbie really did have the “Magic Touch”. When he pulled the wire, he was not as careful as me and was constantly bumping the wire and creating a path to ground, whereas I was very carefully pulling the wire straight up, never contacting the metal.

It’s a good thing we are talking about low current and low voltage here, eh?

One large resistor later, the circuit was triggering every time we’d pull the wire!

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