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  1. BobK

    I want to connect a water sensor to a speaker

    Of course, you could make an alarm system work without transmitting through the water. You would transmit at a regular interval when not under water, and the receiver would detect no transmission for too long a time and sound the alarm. Bob
  2. BobK

    I want to connect a water sensor to a speaker

    In a way that you did not intend, this is correct. The only signals that get through water are very low frequency. Which means you need a huge antenna. https://en.wikipedia.org/wiki/Communication_with_submarines Bob
  3. BobK

    I want to connect a water sensor to a speaker

    If you are expecting it to transmit from under water, think again. Bob
  4. BobK

    "Variable Voltage Battery charger"

    You are scaring me. You don't have near enough knowledge nor experience to do thus project. You don't start with high power applications that are notoriously dangerous as a first project. Bob
  5. BobK

    Li-ion charging question.

    Salvage is fun, but not really worth the effort except for a few specialty parts. What you really need is a large collection of resistors ans capacitors, which can be bought very cheaply on EBay. These will give you several of each common value for less than a penny a piece. Bob
  6. BobK

    Li-ion charging question.

    They are not fully charged if they reached 4.2V and there is still 200ma of current. The charging should be terminated when it reaches 3% of C. I.E. if the battery is rated at 1000mAH it is fully charged with the voltage is 4.2V and the current is 30mA...
  7. BobK

    Looking for a specific transformer

    Potentiometers are not power devices, they are not used to reduce voltage at anything beyond milliamps. At typical potentiometer is rated at 1/2 W. You still haven't stated what battery chemistry you are trying to charge, so I will assume lead acid. These require roughly 14V for charging...
  8. BobK

    Headset music volume dimmer

    Look into anallog switches. Bob
  9. BobK

    Looking for a specific transformer

    Can you show us how you came up with 30VAC? Bob
  10. BobK

    Building an electronic foil surf board

    I would think the same way any boat does. By using an appropriate sized prop at an appropriate speed. Bob
  11. BobK

    Looking for a specific transformer

    If you really want a transformer that is 15V 30A, your best bet is to salvage a Microwave Oven transformer, remove the high voltage winding, and wind a new 15V winding. You can find lots of instructions for doing this on the net. Bob
  12. BobK

    Cool the temperature inside a perspex box

    A fan does not cool something unless the something is above the air temperature. Is there something in the box that is producing heat? If not, why would it be above room temperature? Bob
  13. BobK

    LED getting hot

    Some reading on determining whether electronics are hot: https://www.electronicspoint.com/resources/digital-temperature-measurements.40/ 110F would be 2 or maybe 3. Bob
  14. BobK

    LED getting hot

    That is not what I would call hot. Bob
  15. BobK

    Powering my project with a 7.4v battery

    Can you post a link to the device? 17.5W seems like a lot of power for a hand-held device, except maybe for a hand warmer. Bob
  16. BobK

    tip120 driving led array

    Then yes, they have to be UV. I use a standard under-kitchen-counter fluorescent lamp myself. The exposure is long (12 min) but It works fine. Bob
  17. BobK

    Beginner needing help - Simple timer circuit for reversing 5v motor

    I don't know what "running at that same time" could mean, other than they are synchronized. The question is, is there a time relationship between when the different motors run and reverse? Bob
  18. BobK

    tip120 driving led array

    Exposure of what? Bob
  19. BobK

    Serial communication problem.

    I was always amazed at how they came up with 25 wires to do 2 way 1-bit communication. RS-232 has to be the the best example ever of what is euphemistically called "design by committee." Bob
  20. BobK

    Now I am going to ask 3 stupid questions, please dont laugh. but i cant seem to find info

    No I did not say that. You did not state the the Variac was rated at 10A, you stated that the variac was pushing [sic] 10A at 120V: I said that the 12V secondary would have to be 100A if the 120V primary was pulling (not pushing) 10A. I guess I took your question too literally. I thought it...
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