Hi,

I am pretty much an electronics newbie. Some soldering experience now and I did take an E&M class in college (physics intense version, highly theoretical, like you know derive special relativity from Maxwell's laws and vice versa not something that would help here).

I am trying to make this:
http://www.roombareview.com/chat/viewtopic.php?p=6029#6029

From what I understand, it is a relaxation oscillator (?) that makes a 500Hz pulse modulated on a 35Khz carrier pulse using a 4093 quad schmitt-trigger chip. I am trying to figure out how to calculate/change frequencies based on resistance and capacitance values for something like this. I found something for a much simpler circuit, but nothing with a 4093. Any suggestions/help/leads?

Thanks a bunch.

Misha

 

Hello Misha,
Have you downloaded any datasheets from the manufacturers of the IC's that you are trying to use?

Basically, it's an RC time constant. Feedback to the input is R, with the C on the input being charged by the feedback R.

Have a look at a few datasheets for details.

ONsemi, Fairchild, etc.
MC14093B, CD4093B, etc.

 

In case you are unfamiliar with where to locate the manufacturer's datasheet for the part in question, here is a link to the CD4093B datasheet on the National Semiconductor website.

hgmjr

 

Thank you both for the datasheet idea. I will check out it and let you know if I have any more questions.

Thanks again
Misha

 

The CD4093 cannot properly drive the 220 ohm resistor when its supply voltage is only 5V. 10k is the recommended minimum value.

Change the 220 ohm resistor to 10k, and change the the 22k resistor to 1M.

Then the capacitors can be changed as well. Change the capacitors to 2.2nf.

 

Thanks for the replies guys. Btw, what wattage resistors do I need to use for the circuit (1/4, 1/2, 1, 2) and how do I figure this out?

Thanks
Misha

 

1/4 watt will give enough dissipation. Power dissipated in a resistor is the product of the voltage drop across it times the current through it.

Rule of thumb: if the resistor gets dark and stinks, then you need a larger one.

 

Btw is there an instrument I can use to test the frequency of the signal I am producing? I believe I can use an oscilloscope but I mean one that would be fairly easily available to me without spending a lot of money. Or some trick I could use? Perhaps another circuit?

Thank you
Misha

 

A frequency counter is used to accurately measure frequency. My Fluke multimeter measures frequencies accurately up to 200kHz.

 

A frequency counter is used to accurately measure frequency. My Fluke multimeter measures frequencies accurately up to 200kHz.

So it looks like I have to finally make the multimeter plunge (I've avoided owning one at least for more than a few days - buy/return from Radio Shack). Fluke's look pretty pricey, I know they are the industry standard but still I don't really think I would like to spend that much.

It seems like there are some decent prices on Amprobe's, something like this:
http://www.tooldiscounter.com/ItemDisplay.cfm?lookup=AMPPM51A&source=froogle&kw=AMPPM51A

Also futurelec has really low prices, not sure of brand:
http://www.futurlec.com/Multimeters.shtml

And finally I found this guide, but not sure how recent/dated it is (esp w/regards to Amprobe):
http://www.tangentsoft.net/elec/meters.html

Any advice? I'd like something that's ideally not too expensive (I am really thinking the less the better but I mean $30-40 range at least is "ok", $50 is pushing it if there is _really_ something I can't live without). I'd like something that will be good for occasional hobby use (I do my hobby electronics stuff only on weekends basically and then only a few hours a weekend usually if that) but that will have the most capabilities so I won't have to look for something else (frequency obviously, capacitance and diodes I guess are nice, not sure what else)... Any advice would be greatly appreciated.

Thank you
Misha

 

I am thinking of this one Amprobe AM-240:
http://www.mouser.com/Search/ProductDetail.aspx?R=AM-240virtualkey56910000virtualkey569-AM-240

For $40 seems to have all the features one could ask for. Not sure about brand reliability etc but then again that may not make a big difference in this price range (Flukes are all much more). Any comments?

Thanks
Misha

 

The Amprobe multimeter might be a good one.

 

The Amprobe multimeter might be a good one.

Thanks. Just got it this morning.

Misha

 

Rule of thumb: if the resistor gets dark and stinks, then you need a larger one.

That could also mean something is shorted! LoL

 

Personally, I would go the longer dollar and get yourself a nice Fluke. I have tried different meters over the years, but Fluke meters are accurate, dependable, and take a pounding without crapping out on you after an accidental fall. You can get them in all kinds of various configurations. I found it best to make a list of all the things I wanted my meter to be able to do (now and in the future) and get one that fits that ticket. Better to spend a little more once, than to have to keep re-buying until you get what you want. And if you are going to keep experimenting with electronics, you will need a good meter.

 

Thanks guys for all the help. The fluke's were 100's of $$$ and I am a student... The Amprobe is nice though and much better than nothing or Radio Shack.

I am assuming the 500 Hz being modulated should be the frequency on output 3 of CD4093BCN compared to 0V anywhere from power supply, and the carrier frequency 35 KHz should be the frequency measured on output 4 of CD4093BCN.

With the original circuit I get 421 Hz on output 3 and 3.36 KHz on output 4. If I make the replacements Audioguru suggested (thank you) I get 450 Hz on 3 and 27.4 KHz on output 4. Apparently the 37.5 is quite critical though. Any suggestions? I might try something like the first post at top of:

http://www.roombareview.com/chat/viewtopic.php?t=1178&postdays=0&postorder=asc&start=20

to see if that helps tomorrow if I don't hear any other suggestions then. Don't know, can't hurt, play with potentiometer see what I end up adjusting. I'm pretty new at this still, but having the multimeter definitely helps a lot.

Thank you
Misha

p.s. I included a picture of the circuit I built so far (original spec version). I believe it is correctly assembled per spec. Also, I put dots where I am putting leads to measure frequencies above, I believe it is correct places let me know otherwise. The black dot is where the black lead is for both measurements (I tried flipping leads too but it doesn't make difference), the two red dots are the two frequencies I report above. Thanks for all your comments guys.

http://www.google.com/search?q=cd40...=com.ubuntu:en-US:unofficial&client=firefox-a

 

Bump. Umm, help? I did some calculations based on spec sheet, and my measured values don't seem to make sense, even if I make no assumptions about the V+/V- levels and just assume they are the same from the first input to the second and try plugging in the RC values. Am I measuring in the wrong places or could something else be going on? Thanks

 

Try measuring pin 11 to ground.

 

Try measuring pin 11 to ground.

You mean for frequency or voltage?

Thanks
Misha

 

You mean for frequency or voltage?

Thanks
Misha

Frequency.