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  1. BobK

    flip relay with 2n3904

    The datasheet says it is a 12V coil, so use 12V since it is available. Bob
  2. BobK

    Small RGB led cycle

    An analog circuit would be very complex, probably containing at least 10 chips. A microcontroller can be had for < $1, but you need to have a programmer, and be able to program it. A microntroller board (Arduino) will cost you about $20, and you still have to program it. There is no...
  3. BobK

    obtaining intermediate frequency with 74HC74

    You are not going the see the freqency difference as output when using random inputs, since random inputs do not have a frequency. Try putting a frequency of 3 times the clock frequency in as the input, and see what you get at the output. Bob
  4. BobK

    Small RGB led cycle

    Look up PICAXE. These are microcontrollers that have a bootloader so they can be programmed via a serial port on a PC (or USB to serial converter) and they are programmed in a BASIC like language. Good for beginners. The cost a little more, but do not require a programmer, which will set...
  5. BobK

    Driving an SSR input from a DC power adapter

    You could switch the 24V output instead of the in input. Bob
  6. BobK

    Question about voltage input before speaker

    Are you sure it is not 15 mV? Operating off a 9V battery it is not impossible to get a 15V reading, but I would question that the alarm clock is generating this higher voltage. I would also use the resistance setting to see if the black wire is really connected to the negative terminal of...
  7. BobK

    Question about voltage input before speaker

    Hi and welcome to the forum. I think you are probably measuring this wrong, so a couple of questions. Are you using AC volts or DC volts on your multimeter? You say you are measuring from the speaker to ground. The speaker should have 2 wires. Is the other wire connected to ground...
  8. BobK

    obtaining intermediate frequency with 74HC74

    Draw out a timing diagram with the CLK and D signals and then compute the Q output from that. That might give you a sense of what is going on. Bob
  9. BobK

    a reversing polarity?

    Then the input is floating when the switch is not in the activated position. Floating inputs are not good, you have to provide a valid logic level on an input at all times. In this case, you would use a pulldown resistor from each input to ground. Bob
  10. BobK

    Hot transistors in circuit

    Have you checked with a lawyer about the legality of this? I suspect that it would be considered a booby trap, which might be illegal. Bob
  11. BobK

    Open collector buffers VS Non Open Collector Buffers

    No, you would not be right. Modern logic families use open collector or open drain outputs as well, for the reasons that Kris stated. Bob
  12. BobK

    a reversing polarity?

    Thanks for the response, but you did not answer my question which is intended to lead to the way to correct the problem. When then switch is to the right, what is connected to the GAL input that is connected to the left side of the switch? Bob
  13. BobK

    a reversing polarity?

    So, when the switch is not connecting the supply voltage to the GAL input, what is driving that input? Bob
  14. BobK

    Home automation using DTMF technology

    If you know that, then what is your question? A relay switching AC can be driven by a small DC voltage compatible with a microcontroller. It will likely need a transistor switch to handle the current in the coil. Bob
  15. BobK

    Help making small squeaker

    Thanks Kris. And yes, a full H-bridge would give you 4X the power if needed. Bob
  16. BobK

    Home automation using DTMF technology

    A relay can switch and AC device. Just make sure the contacts are rated for the voltage and current needed. Bob
  17. BobK

    a reversing polarity?

    Show us a schematic. I am going to guess that it is floating when in one position. Bob
  18. BobK

    Help making small squeaker

    Just out of curiosity, this is not an offer, how much would you be willing to pay for something custom built to do what you want? Bob
  19. BobK

    Help making small squeaker

    Another alternative would be to use a small microcontroller. It could wait a random amount of time, then output a squeek as a varying PWM signal to a push-pull output stage, which is nothing more than 2 transistors, making a class D amplifier. You would use a wave table to approximate a mouse...
  20. BobK

    Coffee Temperature Detector

    An instrumentation amplifier is overkill for a thermistor. They are used wihen signal levels are much lower, as in a thermocouple. All you really need is a comparator. The set point is taken from a potentiometer connected to one of the comparator inputs. The other input is connected to the...
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