Search results

Search Forums New Posts
  1. J

    741 differential amplifier question

    to clarify point (1). If the voltages on the negative and positive inputs of the 741 are equal then the output to me would have to be zero. But you said in your first post But anything multiplied by zero should be zero. http://en.wikipedia.org/wiki/Differential_amplifier The wiki tells me...
  2. J

    741 differential amplifier question

    Well lets start off with a couple things first. 1. I would expect the output voltage to be the gain of the amplifier only when the wheatstone is balanced. I would assume that is when the variable resistor was set to equal 10k ohms. The graph doesn't show that at all. 2. I also expected to...
  3. J

    741 differential amplifier question

    Yeah, I sort of assumed I needed a negative and positive supply voltage. Maybe this attachment will make things clearer. I used 9V for the wheatstone and input voltage and used 5 +/- for the 741 itself. As you can see I added a graph to show what the circuit is doing when the variable...
  4. J

    741 differential amplifier question

    Ok. so taking away the negative feedback resistor gives me the desired output of 0V. So then I have to ask myself, why is the negative feedback resistor network even needed? From the looks of it, I would say the 2.5 volts I am seeing on the output is coming directly from the voltage dividing...
  5. J

    741 differential amplifier question

    I was under the assumption that when hooked up as a differential amplifier the 741 op-amp will output the difference between the inputs. If that assumption is correct then what am I doing wrong in my simulation to cause a -2.5V on the 741 output?
  6. J

    Looking for an electronic component - no idea what I need.

    Oh, I wanted to ask another question about the base voltage once the transistor conducts and the relay latches. In my simulation it's telling me that I am getting about ~650mV to the base of the transistor (photocell = high resistance). This is down from the 750mV I used as the number to figure...
  7. J

    Looking for an electronic component - no idea what I need.

    Steve, is this what you meant? :D Took me a long time to see what you meant. I feel sort of embarrassed.
  8. J

    Looking for an electronic component - no idea what I need.

    How does this look? The only thing I don't fully understand about this circuit is why I can't use a spst relay and hook the alarm/cap in parallel with the lead that comes off the NO contact of the relay that controls the base voltage after the relay has been tripped. When I put that in the...
  9. J

    transistor switch?

    I have been working on a very similar project. NPN transistors pretty much regulate the C-E current based on the B-E current. This ratio is known as 'beta'. IE. if your base current was 2.5mA the transistor had a beta value of 10 its C-E current would be 25mA. The transistor, I think, has a...
  10. J

    Looking for an electronic component - no idea what I need.

    Is this what you had in mind? is there anything significantly wrong with this schematic? edit: dumb question removed. :)
  11. J

    Looking for an electronic component - no idea what I need.

    Not to be so naive, but how exactly are you wiring this,? Secondary power source? The SCR will work perfectly though and is a really good suggestion. Had I of only known about them before asking --shrugs edit: NVM, after googling a little more I was able to see the error of my way. I needed to...
  12. J

    Looking for an electronic component - no idea what I need.

    I am looking for a way to switch a circuit on with the same power source as the original circuit or a separate power source. The issue that I am having is I can not find a way to keep power to that second circuit using a relay once the original circuit switches back over. my original project...
  13. J

    WiFi for house in North woods.

    If he wants cable tv he has no choice but to run the cable himself. A dingo with a ditch witch rented at your local rental station will deal with the digging. I have used these on many occasions for countless jobs. It'll still cost a couple hundred, maybe even a thousand, but certainly not 4...
  14. J

    capacitor pos. / neg. ?

    resistors no diodes yes Also on capacitors and diodes one of the leads is shorter, That also tells polarity, but shouldn't be relied upon unless the components are brand new.
  15. J

    + or - on a capacitor

    Well, if precedence matters, the stripe on a electrolytic cap is the neg '-' Better wait till someone with some more experience chimes in though. A search of the site produced this thread https://www.electronicspoint.com/which-side-tantalum-capacitor-do-attach-ground-t19772.html
  16. J

    Help in understanding the math behind the schematic.

    I really appreciate the time you took to help me Laplace. I have a working model sitting on my kitchen table now. Works exactly as it should!
  17. J

    Help in understanding the math behind the schematic.

    with my handy calculator and this webpage http://www.opamp-electronics.com/tutorials/thevenins_theorem_1_10_07.htm I came up with .43 volts, that is with the 10k Ohm resistor and the 500 ohm photocell/resistor. Does that make sense?
  18. J

    Help in understanding the math behind the schematic.

    You're exactly right, I was interpreting the simulation wrong. When the alarm is 'on' the voltage should drop across the load, as such, there should be less then 3 volts potential on the collector of the transistor since the load works from 6 - 14 volts. How does this look now? Couple more...
  19. J

    Help in understanding the math behind the schematic.

    Ok, well i took your advice and re-arranged the circuit or so I think I did. What I got now is a voltage divider across the base of the transistor. I tested my photocell out and I get fluctuating numbers, but the most consistent numbers I got was 500 Ohms when the PC was illuminated and 33k...
  20. J

    Help in understanding the math behind the schematic.

    Laplace, well I appreciate your answer. But, you haven't really left me with anything to understand or even attempt to learn. Is there another way that would be better? Can you explain how the transistor would blow? That is all part of what I am trying to figure out. I am really glad that you...
Top