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Everything posted by darrins

  1. Thanks to all who responded. The power supply I'm using is one I built using the following schematic. http://www.uoguelph.ca/~antoon/circ/vps.htm I do have a small cap (0.1 uF) b/n pin 5 and ground of both 555's, although the schematic I posted doesn't show them. I will try the suggestion of putting some filter caps between my power supply and my circuit. The problem usually arises when I flip the main switch of my power supply. I wonder if I'm getting some "bounce" from the switch? If so, will the filter caps take care of this issue, or do I need to create a debouncing circuit? Thanks again. Darrin
  2. I have a circuit that uses a monostable 555 timer to control an astabable 555 timer; something like the first circuit in the following image My circuit works ok, but occasionally, I'm getting some strage behavior when I first apply power. Sometimes, when I turn on my power supply, the astable 555 starts sending output pulses, even though I haven't pressed the trigger button for my monostable 555. Is this some stray capacitance or possibly a noisy DC signal? Does anyone know of a way to make sure pin 4 of the 2nd timer is completely grounded when I first apply power? Thanks. Darrin Smith
  3. Thanks Yevgeni. Know of any good IC's for this purpose? If I can't figure out how to use the counter, I can always just use multiples of my timing resistor. If my timing resistor is R, then I can use 2R to run twice, 3R to run it three times, etc. I know this isn't the most elegant solution, but then again, I'm no electronics guru.
  4. I have a simple circuit that uses a 555 timer to turn on a stepper motor for a certain amount of time. I'd like to connect the circuit to a rotary switch to make the circuit run 1, 2, 3....N times. What's the simplest way to do this? Thanks. Darrin
  5. Thanks for the advice. I don't like to do anything that's dangerous, which is why I posted the original question. I had not heard of a variable transformer or a variac. They are a little pricey, but I could probably get a deal at my local discount electronics shop. Thanks again. Darrin
  6. I hope this isn't a stupid question. I want to use voltage from AC to power a circuit, but at a reduced voltage level. I'm not sure exactly how much voltage I need, so I want to build a variable voltage divider with a resistor and potentiometer in series. Is this a reasonable approach and, if so, what rating of resistors should I get? I estimate that I need anywhere from 25% to 50% of the AC to power my project. If I get a pot with a metal knob, is there any danger of the knob becoming HOT? :o Thanks for any help. Darrin
  7. bitsdoo, I actually built this power supply and use it all the time. You shouldn't have to change too much. The most expensive parts are probably the transformer, the volt meter, and the box. You could probably use almost anything for the box, but if it is metal or has a metal bottom, make sure you ground it. Some of the resistors are just to limit the current going a) the led and b) the volt meter. You should be able to calculate the resistance you need for both. You need the pot on the voltmeter for calibration. Most likely you don't need a fan for the LM317 if you have a good heatsink and drill some holes in your box. If you have an old power strip, you can recycle the cord and the switch from it. Don't forget to use a fuse. With regard to your first question, its a good rule-of-thumb to always use components that are rated HIGHER than what you'll actually need. This was my first project too. I'll give you one more piece of advice. Make sure you give yourself some slack when it comes to the wiring, especially those parts that will be attached to the box. It makes putting it all together a lilttle easier. :) If you decide to use the voltmeter, they usually have the cutout pattern on the back of the package. Don't throw it away. It will help you cut out the proper size opening on the box. I used a Dremel tool to do this. Good luck. Darrin
  8. Thanks audioguru. I will use one of my spare amps as a voltage follower between my sensor and my diff amp. I'll post my results when I get a chance. One more question. I've noticed that when the differential input goes negative, i.e. when my sensor voltage is greater than my reference voltage, I'm still measuring some small voltage at the output of my diff amp. I'd like the output to go to zero when this occurs. Is there a way to do this? ??? Thanks again. Darrin
  9. audioguru, I tried a couple of different things. Removing the resistors from the non-inverting input, as you suggested, appears to work fine. :) You've mentioned in a couple of your posts that the op-amp will affect the accuracy of my temperature sensor. I'm no theory expert (I'm a mechanical engineer), so could you explain, in very simple terms, how the diff op-amp will degrade the accuracy of my temp sensor? Thanks Darrin
  10. Thanks audioguru. I think I found something similar to what you are talking about. High Input Z, DC Differential Amplifier (p. 15 on this datasheet) http://www.national.com/ds/LM/LM124.pdf Thanks again. Darrin
  11. I've heard that the Eagle software is good. If all you want is a good PCB design software, I like ExpressPCB (http://www.expresspcb.com/). And, it's free. ;D Cheers. Darrin
  12. I'm pretty sure they have switch boxes to do this. You turn a knob on the switch box, say to computer "B", and suddently your monitor is showing computer "B" and your keyboard/mouse are controlling computer "B". Better yet. Just use Windows Terminal Services.
  13. Thanks audioguru. Actually, there's a typo in my original post. The output from the heat sensor (http://www.national.com/ds/LM/LM34.pdf) is 10 mV/deg F. When you said that I don't need the "common-mode balancing resistors", are you referring to BOTH resistors connected to the non-inverting input? I'll try your recommendation of wiring the unused op-amps as voltage followers with non-invert inputs grounded. I'm still somewhat confused why a circuit that is in several of my textbooks doesn't function "as advertised". Thanks again for your help. Darrin
  14. I'm trying to amplify the voltage DIFFERENCE between two inputs. One input (V2) is a reference voltage of about 1 volt. The other input (V1) is from a heat sensor that is fairly linear at 0.1 volt per degree Fahrenheit. So, for my application, the input from the heat sensor is about 0.7 volt. I'm using 1/4 of a quad op-amp (LM324). I've wired the op-amp as a differential amplifier, like this: I'd like the gain to be 22, so I'm using 2, 220K resistors -- one between the output and the inverting input (feeback) and one between the non-inverting input and ground. I'm using 10K resistors in between my inputs (V1 and V2) and the op-amp inputs. If my math is correct, I should have a gain of 22. For my example of V1=0.7V and V2 = 1V, the output should measure 6.6V, but when I use my voltmeter, I'm only reading about 1 Volt. ??? Any suggestions? Is it possible that my quad op-amp is damaged? If so, is there a way to test it? By the way, I grounded all the pins on the quad op-amp that I wasn't using. Thanks for any advice. :) Darrin
  15. Anyone know where to find low-cost LCD or LED numeric displays? I have a project that will display temperature in Fahrenheit, so I need a display capable of 4 digits. Thanks. Darrin
  16. audioguru, Sounds good to me. If it doesn't work on the mossies, you can crank it up and scare the hell out of the neighbors, especially that part right after the heartbeat ends. 8) Darrin
  17. I don't know that these ultrasonic devices are effective at repelling mosquitoes. An alternative that I've heard about uses low frequency sound to ATTRACT mosquitoes. The theory is that the low frequency sound mimics a heartbeat. http://www.sonicweb.com/
  18. I want to try my hand at making homemade PCB's. I read an article describing how this can be done by printing the layout on glossy photo paper (w/ a laser printer) and then using an iron to transfer the toner to the copper clad PCB. The board is then etched w/ Ferric Chloride. Here's the article: http://www.fullnet.com/u/tomg/gooteepc.htm Is this a good technique for a beginner? Can someone recommend a good (and cheap) PCB layout software? Also, how should one dispose of used Ferric Chloride? Is this stuff harmful to the environment? Thanks. Darrin
  19. blacksheep, There are a couple of amp projects on this site that use more "modern" technology. ;D http://www.discovercircuits.com/A/a-audioamp.htm Darrin
  20. Sorry about the links to tube amps. :-[ I just did got these links from a google search and didn't read all the content. You may be able to find a good amp on eBay. Sometimes it's cheaper to buy than build. I certainly got a good deal on my oscilloscope. Not that I could build an oscilloscope. ;) Hope you're not building one of those giant wall amps from "Back to the Future". Your neighbors might get upset. Cheers. Darrin
  21. Try these sites: http://www.ax84.com/home.html http://freespace.virgin.net/vyse.co/build_your_own_amp.htm http://www.duncanamps.com/brewlink_amps.html Good luck. :) Darrin
  22. Ante, Funny you should post that. I just downloaded the same paper yesterday, but the one you posted looks more recent. I guess I need to stop being lazy and just read it. :-[ Looks like a good paper from what I've read so far. Thanks. Darrin
  23. I have a couple of basic questions related to a differential amplifiers. If these types of questions are better suited to the Theory Articles board, please let me know. :) I want to amplify the voltage difference between a sensor (Vs) and a reference voltage (Vref). I only want to amplify this signal when there is a positive difference, i.e. Vref - Vs > 0. Otherwise, I want the output to be zero (ground). If I just connect a standard diode to the output of the amp, will this do the trick? Or, is there a better technique. Another question. I've seen differential amplifiers called "Instrumentation Amplifiers" where the two imput signals are fed through voltage followers (or buffer amplifiers) before being feed to the differential amp. I have no common mode carrier signal to remove (they are both DC voltages). Is there any advantage to using this so-called instrumentation amplifier vs. a standard differential amp for my application? I have plenty of quad op-amps, so that's not a problem. Last question. I want to calculate the average voltage coming from 3 or 4 different sensors. I know this can be done usign what is called a "passive averager" circuit (see below). This can also be done using a summing op-amp by selecting appropriate values of resistors. Is there any advantage to the op-amp approach? Thanks. Darrin
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