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  1. audioguru2

    Timer. What's wrong

    The diode across the relay coil (in parallel) is still backwards.
  2. audioguru2

    Automotive 12V to +-20V converter

    Hi Jonathan, Class-H is a mode of amplifier design that switches between supply voltages on demand. I wonder if the switching is audible at very low frequencies, where a zero-crossing has already past and a very loud phrase of bass music occurs. An ordinary amplifier would produce max power for...
  3. audioguru2

    Automotive 12V to +-20V converter

    Hi Jonathan, I am sorry that I misunderstood your load impedance. So your voltage converter project can power a 30W per channel into 4 ohms stereo amplifier. I wish it was designed for much more voltage and power output so it could drive National's Overture amplifiers to the much higher power...
  4. audioguru2

    Speed-limit Alert PLEASE HELP!!!!!!!!

    Hi Pieter, Sure, the pickup coil picks-up magnetic pulses from your monitor too. Thanks for letting us know. ;D
  5. audioguru2

    Keys finder circuit

    I think a 2-wire transducer will make a more sensitive microphone since its useful area is larger and in a single lighter-weight piece. ;D
  6. audioguru2

    Automotive 12V to +-20V converter

    Nice new project: http://www.electronics-lab.com/projects/automotive/012/index.html Its author claims that an audio amp can supply 50W to an 8 ohm speaker with a +-20V supply. I disagree. A typical amp can supply only 17.6W to an 8 ohm load at clipping when powered by +-20V. About the same...
  7. audioguru2

    Keys finder circuit

    Hi Lapsap, Just get a two-wire piezo transducer designed for external drive. Our project uses it as a microphone then drives it to make a sound. A CD4069 inverter is just a pair of push-pull complimentary Mos transistors. It makes a pretty good linear amplifier with the negative feedback...
  8. audioguru2

    Speed-limit Alert PLEASE HELP!!!!!!!!

    Thanks, Alun. ;D I think the 4 cascaded inverters will have a reasonable bandwidth and have much more gain over the bandwidth than an opamp. It is good to have the very low output impedance of the opamp as the 1st stage, to avoid positive feedback from the paralleled inverters output to the...
  9. audioguru2

    Speed-limit Alert PLEASE HELP!!!!!!!!

    Hi Pieter, 1) You selected a tachometer circuit that gives a linear voltage output with the speed of the engine. It would need an external comparator and driver to switch on an alert device. The circuit I posted uses the comparator part of the IC to "switch" on its driver transistor when the...
  10. audioguru2

    MULTIMETER

    Hi Prateek, Every DMM is different but mine beeps when I ativate it and only when the resistance is 40 ohms or less. It is not supposed to indicate low resistor values, but it does. It is supposed to be to indicate a short, or that wiring has continuity. ;D
  11. audioguru2

    why output voltage of LM317 decreases?

    Hi Alun, The LM317 and 78xx regulators reduce their output current with more than 15V across them to protect their pass transistor and keep it in its Safe Operating Area. Die temp affects only the typical 2.4A max current with less than 15V across it a little, down to about 1.6A typical.
  12. audioguru2

    Speed-limit Alert PLEASE HELP!!!!!!!!

    Hi Pieter, To get more range, decrease the value of R1 and R2. Also, use an inductor with more turns and a lower resistance. Increase the value of R9 so that D1's current doesn't load-down the output of IC2. I would try shorting C2 and removing R8. Then using a pot for R4 and R5 you could adjust...
  13. audioguru2

    why output voltage of LM317 decreases?

    My LM317 project has a minimum voltage of 1.2V like it is supposed to, and powers things with a very well-regulated voltage just fine without the voltage dropping. I usually use a 12VDC/1A wall-wart transformer to power it then its max regulated voltage is 9V. Without a load I set its output...
  14. audioguru2

    Deja-vu, reply to replies

    Hi Steven, Google is watching and recording our postings. ;D
  15. audioguru2

    Speed-limit Alert PLEASE HELP!!!!!!!!

    Hi Pieter, The instructions for the project say it can pick-up spark pulses from the engine when it is mounted in the cabin on the dashboard. I am amazed it is so sensitive. You probably must try it at many locations to find one that has the best pick-up from so far away. ;D
  16. audioguru2

    Keys finder circuit

    Hi Lapsap, Welcome to our forum. ;D You don't want a buzzer, the circuit needs a piezo transducer. D4, C7 and D3 are supposed to stop the transducer from whistling after about 1 second. The circuit is supposed to trigger from nearly any fairly high-pitch sound, not just a "fizzle", whatever that...
  17. audioguru2

    Timer. What's wrong

    Hi Karl, With the diode across the relay coil backwards, the 555 IC will burn again. I mentioned other necessary changes in your other topic. ;D
  18. audioguru2

    Speed-limit Alert PLEASE HELP!!!!!!!!

    Hi Pieter, Pins 4 and 7 on a TL061 opamp are its power supply pins. They aren't shown on some app examples because it is assumed that they are connected. I don't see how your circuit can work without pin 4 grounded. Most vehicle applications for a speed-limit alert use a shielded cable through...
  19. audioguru2

    2N3055 Vce(sat)

    Hi Alun, I disagree. Power BJTs are much slower than power Mosfets. The car DC-DC power supply posted with many 2N3055 transistors operates at an audio frequency and requires a huge transformer and big filter caps. The transformer's vibrating core laminations probably make an audible whine. Here...
  20. audioguru2

    Speed-limit Alert PLEASE HELP!!!!!!!!

    Hi Pieter, Many people have made a modern simple Speed Alert project with an LM2907 or LM2917 frequency to voltage converter IC. It is available in 8-pins and 14 pins packages, the 14-pins one has a built-in zener diode voltage regulator, their outputs can source or sink up to 100mA and one is...
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