Hi , my 8 year old son has a school project due . It involves triggering or switching a 9volt battery anywhere from fifteen to twenty times per second . The end function is not important it just has to perform that many times a second w/ 9volt output . I have limited to NO circuit experience and really want a better understanding of how this project is supposed to work .

Thx
Matt

 

With the little information that you provide, I'll hazard a wild guess on what might be appropriate. The rate at which the output has to switch is just fast enough that it precludes the use of a relay, so a semiconductor switch powered by the battery itself is best. You also need an oscillating circuit that provides the 15 to 20 times-per-second control to the switch. The oscillator is probably easiest using the old favorite, the 555 timer IC, configured as an astable multivibrator (an oscillator that keeps going). The on/off switch can be done quite simply by an appropriately biased PNP transistor, like a 2N3906 or virtually any PNP transistor.

Start by looking up the data sheet for the 555 timer (http://www.fairchildsemi.com/ds/NE/NE555.pdf). The data sheet will have example circuit schematics, one of which is an astable multivibrator. Copy that, and power it from the 9V battery. The data sheet will give you an idea of what parts are needed to get 20 times per second switching. Wire the output of the 555 through a resistor (value anywhere from 1000 ohms to 10K ohms, whatever is handy) to the base of the 2N3906 transistor. Connect the emitter of the 2N3906 to the 9V battery supply and wire the collector through another 1000 ohm resistor to the negative terminal of the 9V battery. This latter resistor's wires are your output terminals.

In fact, you would probably satisfy the basic criteria using the 555 IC alone, without the additional 2N3906 transistor if it is acceptable for the output voltage to be only higher than 7.3 volts rather than exactly 9 volts. If this is the case, then just take the output from the 555 directly as it should be switching between 0 volts and about 7.3 volts (approximate, as this depends on what kind of meter or load is placed on the output) when it is operating. The addition of the 2N3906 helps pull the output voltage up to within about 0.2 volts of the battery voltage, or about 8.8 volts or higher depending on the battery condition.

One serious difficulty that you may face is that when you hook the circuit up and get it going, you won't know if it is working or not unless you have an oscilloscope to look at its output.
But you are not likely to have such a thing available. When a voltage pulses up to 9 volts at 20 times a second, you won't be able to read those pulses on a voltmeter, since the voltmeter will tend to average the voltage to a single number, which is ambiguous. You might be able to attach an LED (light emitting diode) to the output through a 330 ohm resistor and see if the LED is turning on and off, but honestly, at 20 times a second it may just appear to be constantly on. Your eye can't really distinguish that speed. I'm not sure what to suggest. One other way might be to couple the output voltage through a capacitor (10 microfarards or higher would be good) to the input of a hifi (audio) amplifier (using the AUX or CD input) and use your ear to see if you can hear a very raspy or buzzy low frequency sound on the speaker. It should be very loud and obvious if the thing is working at all and might sound a bit like a buzzy hum. You can distinguish this sound from the usual AC hum that you might hear due to the 60 Hz 120V in your house by changing the switching speed of the 555 (by, for example, changing the critical resistor value that affects the frequency of the 555) and noting that the buzz in the speaker changes tone.

__________________
RadioRon

 

Thank you very much I've always felt that it's easier to learn from people than to get it from a set of instructions . We had already puchased a 555 timer and the sheet that came w/ it just confused me . The gentlemen at the store also sold us a bundle of switching diodes . This is greatly appreciated , the way you explain it helps me understand the instructions that came w/ the 555 a little better . Anymore information is also appreciated .

Thnx
Matt

 

Have you done any soldering before?

Did they teach your son how to solder?

I think this is probably a bit too advanced for an eight year old if they've not picked up a soldering iron before.

You need to assemple the circuit on a veroboard and cut the tracks in the appropriate places.

Here's a good site which describes this very well:
http://www.kpsec.freeuk.com/constr.htm

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There are 2 free programs that calculate 555 components and give you a simple schematic. ss55511.exe and ic_555.exe you can find them online.

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I found some good stuff while verifying that old info. ss55511.exe doesn’t seem to be available any more but ic_555.exe is on this page http://pages.prodigy.net/rich_demartile/files/ and others here
http://freespace.virgin.net/matt.wai.../555/index.htm
http://www.electronicsaustralia.com....oads.pl?area=7

The people that made ss55511.exe have this free lite version of 555_Timer_PRO at http://www.schematica.com/555_Timer_..._Timer_PRO.htm. I don’t know if it is the same.

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Is this a project that the 8 year old is supposed to do? (where is this school, I want to send my kids.)

No offense but who's doing the project - you or the child? Unless the 8 year-old is a total genius, if I were the teacher I'd flunk the kid, give the parent an A on the project and an F on parenting skills. I see too many projects at my kids' school where it's plainly obvious the kid had nothing to do with the project. I make my kids do it on their own so they actually learn something and get a sense of accomplishment.

I'd have the kid make a wheel with a crank and put contacts on the wheel. The the kid can turn the crank fast enough to get the required pulses.

 

None taken , It's perfectly fine . He's in advanced math & science at private school . I would rather not name it, this being the Internet and all . As for my posts they are mostly for my understanding so I don't feel as much of a dummy . Plus he's doing one and I'm doing one , only his will work and mine probably won't . They also don't recieve a grade for this project . Thanks again for all of the information and links it has been very helpfull .


Matt

 

Hero you stole my kpsec.freeuk site!

Lol

I personally like kpsec.freeuk site A TON. Very handy and helpful!!!

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MechTronics

World of Warcraft is a disease

 

For an 8 year old? I might help him modify a low voltage mechanical doorbell buzzer. Just add some weight to the hammer end to slow it down to 20 Hz.... or some other simple solenoid or a couple of relays?

The kid should have some time to play tag and socially mingle instead of learning how to be a geek.

 

The kid isn't learning anything. The dad might be.

__________________
Uncle $crooge

 

Matt,

Do you know if this assignment has been given to any other kids? If so, I would be interested in knowing what they come up with.

Also, I would probably ask the instructor what he/she wants your kid to get out of this exercise. That'll help you decide how much assistance you should give.

Finally, if he were my kid, I would steer him towards the doorbell circuit. It's a circuit that he can understand, and it's a good example of a relaxation oscillator. Even though it probably cannot achieve the required frequency, there are parameters he can experiment with (voltage, knocker mass, strength of the electro-magnets) which will give him some valuable insight into the general characteristics of oscillators. One challenge is just going to be measuring the oscillation frequency - that's another thing you might ask the instructor about.

Let us know how it goes!

 

Is the school a summer electronics camp or similar? No children in this country go to school in August.

 

They don't here either, except for some boarding schools (where you pay a lot of money to dump your kids and never see them again!).

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And get buggered in the dorms? Sod that!

If I do have any childeren there's no way I'd send them to boarding school even if I did have the money!

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