Archive for March, 2010
click this magic button and it will lead you to a mystical land of electrical tape!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Robbie and I went to bed last night thinking that his first 555 timing chip circuit did not work. The relay was either not firing at all, or firing in rapid succession. I realized that we probably needed a diode in the circuit, and researching it showed this to be the case. At that point, nothing worked at all — the voltage coming into the relay’s coil was too small to trigger the relay.
Overnight, one of my online friends pointed out that the diode needed to be connected in parallel to the relay’s coil (we had it in serial), and I realized that a transistor was needed to overcome the too-small voltage problem.
We needed this circuit: http://www.kpsec.freeuk.com/components/relay.htm#protect
After adding in the transistor and putting the diode in correctly this time, it worked! After the trigger came in we heard, “click”….. (2 second pause)… “click”!
Do you notice in these last two pictures the capacitance problem we had? I thought i had the correct capacitors, but we didn’t. Luckily Robbie’s Digital recording lab had one correct capacitor, and 10(!) other capacitors in series gave us the value we needed for the “control voltage” capacitance on the 555(really 556) chip.
Robbie has completed the first part of his project — wiring up a K’NEX motor so that there are wires sticking out that will work as a switch. Then that is mounted to a light switch face-plate. (with holes drilled at a 45 degree angle in the bottom of the faceplate). We verified that the motor was strong enough to turn on the light switch like this, and that it took about 2 seconds to rotate one full rotation.
Next we found a circuit on the web (listed in the first entry about this), that showed how to use a 555 chip to create a 2 second signal whenever it gets a small signal as an input. ( http://www.ecelab.com/circuit-monostable-555.htm ) Robbie’s is very similar but with one more 555 chip circuit added in to make sure that the signal only comes out one time for every 20 or so beeps of his alarm. I had to teach him about logic gates to design this. Luckily we had an AND and a NAND gate chip, since RadioShack does not carry any of those.
Hopefully tomorrow he can put together this part of the circuit. Then the only thing left is the sound detection circuit. We have a few different examples of those on the Internet. One with 3 transistors, and the other with an op-amp. The op-amp uses the lowest amount of battery power, but I’m not sure how well you can adjust the noise sensitivity. He will probably just have to build it and see.
Robbie has a science / invention fair on the 17th and 18th of March (2010). He has been coming up with a whole bunch of ideas, and finally settled on one — a gadget that turns on his light in his room when his alarm goes off. With this, he will be more likely to get up and not just go back to bed.
Now we are poring over schematics for a whole bunch of circuits — sound detection — signal latching — servo or motor control. We’re pondering a whole bunch of electronics questions — is a 2N3904 transistor close enough to a BC548B to work in his circuit? If not, how does one modify the circuit to make it fit? (Radio Shack only had one type of amplifying transistor — NPN or PNP, I do not recall which). How do you interpret the markings on a capacitor?
Putting it all together will be fun too.