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  1. KrisBlueNZ

    interfacing gsm to avr atmega 8

    Hi Varuni and welcome to Electronics Point. Look in the data sheet for the GSM modem to see whether you need to send an initialisation string (so it can detect the baud rate you are using) and whether you need to wait in between sending command strings to it. Also, upload a schematic of the...
  2. KrisBlueNZ

    What purpose does this capacitor/resistor serve?

    The snubber is there to reduce arcing between the contacts in the relay on the hot tub control board when they open. Arcing causes early contact failure and can cause the contacts to weld themselves together. You have changed the load that those contacts drive. Previously they directly switched...
  3. KrisBlueNZ

    transistor and speaker help

    That schematic is very hard to read. Can you make it smaller, so the components are larger relative to the whole schematic, and repost it? There are no components to set the DC bias on the output transistor. You need a voltage divider using two resistors, like the one that sets the DC bias...
  4. KrisBlueNZ

    monolithic kernel

    Great! Those descriptions are pretty brief. I'm sure you found more detailed information about them as well. So you have answered your question.
  5. KrisBlueNZ

    How to find what value capacitor do you need?

    No, that's not right. It doesn't even make sense. The formula is: dV / dT = I / C where dV = change in voltage across the capacitor over a period of time (in volts) dT = the period of time (in seconds) I = current into, or out of, the capacitor (in amps) C = the capacitance (in farads). This...
  6. KrisBlueNZ

    monolithic kernel

    Have you looked up those words and phrases on dictionary.com and Wikipedia?
  7. KrisBlueNZ

    Resistance range to output voltage range

    I don't know about the CP94, CP95 and CP96 boards. If they're cheap, you could buy one and try it. Without seeing a schematic diagram for them, I couldn't say whether they'll work or not. I could draw up a circuit with about 10~15 components, including an 8-pin IC: ... which you can build up...
  8. KrisBlueNZ

    Circuit Help !!!

    No, I don't think so. This thread is going nowhere. I'm going to ask Steve to review it and offer some advice.
  9. KrisBlueNZ

    What's a good voltage tolerance for a small power supply?

    Very unlikely to be a problem. Most general logic ICs that are designed to run at 3.0V or 3.3V are specified with a maximum supply voltage of 3.6V. If the circuit has any large or expensive ICs, check the manufacturer's data sheet. If the circuit is designed to run from batteries, don't worry...
  10. KrisBlueNZ

    What's a good voltage tolerance for a small power supply?

    There's no definitive answer to your question. In practice any circuit that's designed to work from 9.0V will have no problem with 9.36V; a brand new recently manufactured 9V PP3-type battery could easily measure 9.36V with no load on it. But a 0.28V error at 3V is 9.3% which seems a bit high to...
  11. KrisBlueNZ

    Terminal voltage sensing

    That only applies when the battery is being charged from a fixed or limited voltage. This method is often used with lead-acid batteries and is called constant-current/constant-voltage charging. The charger limits both its output voltage and its output current. During the main charge time, the...
  12. KrisBlueNZ

    Resistance range to output voltage range

    Fuel gauges aren't linear anyway in my experience. They report less than half full when the tank is half full, to encourage you to fill up without waiting too long. And the sender isn't likely to be perfectly linear because of its design and manufacturing errors. Some non-linearity is sure to be...
  13. KrisBlueNZ

    Maintaining DC voltage across capacitor with signal?

    That's the story of our lives here on Electronics Point :-(
  14. KrisBlueNZ

    Choosing an Opto Isolator

    OK, so you're going to use the SSR that I recommended. That WILL need a resistor in series with the LED. I calculated the values for two different control voltages back in post #6. It doesn't matter which side of the LED the resistor is on, and it doesn't matter whether you connect the...
  15. KrisBlueNZ

    Terminal voltage sensing

    Terminal voltage isn't a reliable way to detect overcharge. At least, not for NiMH. There's a prescribed charging sequence that includes end-of-charge detection, which is best done by watching for a temperature rise. Anything that cuts charge based on terminal voltage is going to be rough at best.
  16. KrisBlueNZ

    Old folks in-home personal alarm system?

    Yes, that sounds more likely to me. My mother is not that bad, but she sometimes gets confused and we wanted to make sure it was very simple to activate. I think there is a good market niche for a product with those features. A compact cellphone with a few big colour-coded buttons that call...
  17. KrisBlueNZ

    Electronic power and resistance rules

    I always refer to the circuit common rail as the 0V ("zero volt") rail, not "earth" or "ground"; these have other specific meanings. Using the name "0V rail" also makes it clear that it is the default reference rail for voltages measured "at" specific points in a circuit (rather than between two...
  18. KrisBlueNZ

    Maintaining DC voltage across capacitor with signal?

    No, not across the capacitor. And if you connect the bottom end of the capacitor to 0V, then there won't be much, if any, 25 kHZ signal on the antenna. As you said, the diode allows the capacitor to charge up to 2000V DC. The voltage at the secondary of the transformer (the anode of the diode)...
  19. KrisBlueNZ

    Old folks in-home personal alarm system?

    Yeah, but in a fair and open market, what power would those medical emergency monitoring companies have over a technology products company like Uniden? As far as I can see, the only reason why Uniden would decline to compete with those companies would be if those companies paid them more than...
  20. KrisBlueNZ

    USB Chargers

    http://www.usb.org/developers/devclass_docs/batt_charging_1_1.zip http://www.usb.org/developers/devclass_docs/BCv1.2_070312.zip
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