Search results

Search Forums New Posts
  1. BobK

    Op-amp help please!!!

    Think of two batteries. The + of one is connected to the - of the other. That junction is then the ground. The unused + terminal is the positive supply, and the unused negative terminal is the negative supply. Two 9V batteries make a fine dual supply for op amps. Bob
  2. BobK

    Need Transistor to relay help ASAP

    Let's say you are shooting for 2V as the turn on point. Then you want to divide the 24V by 12. Which means you need and 11:1 ratio. You could put your 2.2K on the botton then put a 22K + 2.2K on top. Or you could use a potentiometer like I originally proposed and then you could adjust it for...
  3. BobK

    Need Transistor to relay help ASAP

    Please re-explain what you are seeing as the signal. Your last explanation seemed to say the neutral position was 2.5V and the forward position was 1.25V which reverses the direction of the voltage change. In any case you are going to have to redo the divider chain, and you may need to swap...
  4. BobK

    Need Transistor to relay help ASAP

    But don't be too discouraged, at least you didn't let the smoke out of anything yet, (though there could be damage anyway). Bob
  5. BobK

    Need Transistor to relay help ASAP

    These connections bear little resemblance to the schematic I posted. The emitter is supposed to be connect to ground and only to ground. You have it connected to the relay and to the minus input of the comparator. The base is supposed to connect to the 3.3K resistor as well as the output...
  6. BobK

    Need Transistor to relay help ASAP

    Thanks, it is hard to miss those dots! Bob
  7. BobK

    Need Transistor to relay help ASAP

    I looked at the schematic I posted and the dots vs crossing lines are really hard to see. There are very obvious dots where components connect, but when lines cross and connect there is just a tiny grey square. Anyway, let's make sure you are reading the schematic correctly. The ONLY non...
  8. BobK

    Will this simple circuit work?

    The circuit you are trying to design is called a relaxation oscillator. To make it work you need a device that stays off until a threshold voltage is reached (as the capacitor is charging) then turns on and stays on even after the voltage drops, and finally turns off again when a lower...
  9. BobK

    Need Transistor to relay help ASAP

    I missed the word input. The throttle input should turn the transistor on when connected to > 6V and off when connected to < 6V. The middle of the 4 resistor chain should be at 12V so that would be a way to test for it going on. Actually, a better test would be to just connect it to the...
  10. BobK

    Need Transistor to relay help ASAP

    If you haven't powered it up yet, and you have a multimeter, here is what I would recommend: 1. Check the resistance between the power leads. It should read a few K ohms. If less do not apply power. 2. Put a resistor of 1K or more across the relay leads. 3. Connect the throttle...
  11. BobK

    Cheap Lithium Ion Chargers - Safe?

    I believe the batteries sold to consumers do typically have this protection. Bob
  12. BobK

    Total noob looking for getting started advice

    This is just a wee bit advanced for a newbie. If you want to learn electronics, try first to do something a little more modest, like lighting and LED with the correct current and voltage. Bob
  13. BobK

    Need Transistor to relay help ASAP

    Well, my wife is with her father, who suddenly weant deaf this morning, so I had time on my hands and no distractions. Edited to add: And of course, I have NEVER edited one of my posts after reading it and seeing the glaring errors :) Bob
  14. BobK

    Using a transistor as a relay

    Yeah, but it still could be replaced by and Intel Core i7. (disclaimer: I work for Intel) Bob
  15. BobK

    Need Transistor to relay help ASAP

    Uh, oh, that is not right. See my previous post. As CC posted would give you a 3V threshold. Edit: I see he fixed it, so never mind :) Bob
  16. BobK

    Need Transistor to relay help ASAP

    The 3 resistors in series would replace the top 10K resistor. The 1 resistor would replace the pot. This is called a voltage divider. If you have 4 units of resistance split 3:1, the 3 side will have 3/4 of the voltage across them, and the 1 will have 1/4. 1/4 of 24 = 6. It is that simple...
  17. BobK

    Solar charged LED light project

    That is a pretty bright LED. If you run it off a 6V battery, you will only need a resistor to complete the circuit. The forward voltage of the LED is 3.8V so the resistor needs to drop (6-3.8) = 2.2V at 30ma, which, translates to V = I R 2.2 = 0.03 R 73 = R. An 82 Ohm resistor...
  18. BobK

    Will this simple circuit work?

    Nope, the inverter's input is always at ground, so it's output is always high, so the transistor is always on, effectively bypassing the capacitor and turning the LED on permanently. Look up astable multivibrator for a circuit that will work. It can be made either from transistors or from...
  19. BobK

    Near signal detector

    Yeah, my wife wants me to build one of those for our cat. Bob
  20. BobK

    Near signal detector

    OMG. I was going to suggest velcro, but thought it was a bit tacky! Though I must admit, I wondered how he was going to attach it. Bob
Top