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  1. darrins

    Ground a astable 555 at startup

    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...
  2. darrins

    Ground a astable 555 at startup

    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...
  3. darrins

    timing circuit to run N-times

    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...
  4. darrins

    timing circuit to run N-times

    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. darrins

    voltage divider for AC

    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. darrins

    voltage divider for AC

    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...
  7. darrins

    LM317 Power Supply questions?

    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...
  8. darrins

    Stupid op-amp question

    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...
  9. darrins

    Stupid op-amp question

    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...
  10. darrins

    Stupid op-amp question

    Kevin, Thanks. I'll try that too. Darrin
  11. darrins

    Stupid op-amp question

    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
  12. darrins

    Circuit design and simulator software

    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
  13. darrins

    Connecting multiple PC's to one monitor, keyboard

    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.
  14. darrins

    Stupid op-amp question

    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...
  15. darrins

    Stupid op-amp question

    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...
  16. darrins

    Where to find LCD/LED displays

    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
  17. darrins

    ultrasonic mosquitoes repeller

    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
  18. darrins

    ultrasonic mosquitoes repeller

    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/
  19. darrins

    Iron-on PCB's w/ photo paper

    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...
  20. darrins

    Guitar Amplifier

    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
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