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  1. audioguru2

    Audioguru FM Tx

    Hi Walid, I said "330nF passes frequencies down to about 40Hz". I did not say that 40Hz is the -3dB cutoff frequency. I design many audio circuits to have a flat frequency response down to a certain frequency, then the cutoff frequency must be much lower, 1/3 to 1/10th of the frequency where...
  2. audioguru2

    Problem in an H-Bridge

    When Mosfets are used for pulse-width-modulation to control a motor's speed, they switch on and off at up to 100kHz. Then a gate current of up to 1A from a Mosfet driver IC is used to charge and discharge the high gate capacitance quickly. The Mosfets are heating for the time that they are not...
  3. audioguru2

    Electronic Stethoscope 2

    Yes and it doesn't need good regulation if its filter capacitors are big enough for low ropple. Omly a 2-pole switch is needed to turn off both batteries. Post a new pcb and I will check it for you.
  4. audioguru2

    C1383 Datasheet

    It is probably a Japanese 2SC1383. You can download it and any other datasheet at www.datasheetarchive.com .
  5. audioguru2

    Electronic Eavesdropping Devices Detector

    I don't think the project works: 1) The value of R18 is too low. 2) C2 should be a 1k resistor. 3) The AM detector circuit and DC amplifier's input look weird. 4) The DC amplifier amplifies its own DC input offset voltage which messes-up the voltage-controlled audio oscillator. If it works...
  6. audioguru2

    Problem in an H-Bridge

    Yes, but 2V is approximate because each transistor has a slightly different base-emitter voltage. It is the base current that turns on a transistor. Learn about Mosfets; 1) The Gate Threshold Voltage (2.0V to 4.0) barely turns on (250uA) an IRF530 Mosfet. 2) Static Drain-to-Source On-Resistance...
  7. audioguru2

    Help!! My Door Alarm is not work?

    I think your circuit doesn't work because the pcb is too big. Radio circuits like this one must be small so that stray capacitance and stray inductance are small. My soldering iron is temperature controlled so that it never gets too hot. A solder joint takes only 2 seconds so the parts don't...
  8. audioguru2

    "Tenna" PS-20 power supply

    We have a 13.8V/10A power supply using the LM723 driving three 2N3055 transistors in our Projects section. http://www.electronics-lab.com/projects/power/027/index.html Your circuit might be the same but using more transistors.
  9. audioguru2

    Problem in an H-Bridge

    The base-emitter voltage is about 2V (2.5V max for some darlingtons) to turn on. The TIP127 has an emitter voltage of 24V so its base voltage is about 22V when it is turned on, and its base voltage is 24V when it is tured off. Saturation is when the transistor is turned on. The saturation...
  10. audioguru2

    need help with circuit resistance test question

    I think your answer is correct. Did you see that the resistors are in parallel?
  11. audioguru2

    Capacitors

    Hi Steve, I make many projects that are powered by a 9V battery and they have capacitors. I can't find 18V electrolytics so I use 16V and 25V ones. My metalized poly capacitors are 63V and 100V and my ceramic disc capacitors are 50V and 100V. A resistor must dissipate power as heat. A 1/4W...
  12. audioguru2

    Ft of the transistor

    Kevin, The RF amplifier in many transmitters operate in class-C for good efficiency. A transistor operating in class-C is completely turned off for some of the time of each cycle, but the high Q of the tuned circuit attenuates the harmonics so the output is a pretty good sine-wave.
  13. audioguru2

    7 Segment LED Counter, Need help!

    Hi Mukhalled, I haven't used old TTL ICs for many years and I have lost my TTL Cookbook. The datasheet for the 74LS90 shows an inverting clock input pin 14, so I think it counts each time it goes low.
  14. audioguru2

    Ft of the transistor

    Kevin, In a PLL, the carrier can be a sine-wave which is made by the VCO. The VCO's frequency is modulated. We are talking about the LC parallel tuned circuit reducing the harmonics of the 100MHz carrier frequency, not the modulating audio frequencies. The RF harmonics are AM but we are...
  15. audioguru2

    WHY A TRANSISTOR

    If you connect a transistor properly, bias it properly and add enough negative feedback to reduce its distortion then a transistor makes a good amplifier. Many transistors make an excellent amplifier like the ones in ICs. If you don't like transistors then you can make an amplifier with Mosfets.
  16. audioguru2

    need help with circuit resistance test question

    Hi Atticus, Welcome to our forum. ;D I hope your teacher won't get mad at you for not doing your own homework. I think the resistors are connected like this:
  17. audioguru2

    12DC / 220V AC INVERTER DIAGRAM

    Hi Aca, Welcome to our forum. ;D We have two simple old square-wave inverter projects in our Projects section: 1) The Inverter: http://www.electronics-lab.com/projects/power/018/index.html Its capacitors are backwards and explode, its transistors don't have enough base current and the...
  18. audioguru2

    white led

    Yes, all the LED current flows through the 3.9 ohm resistor. When the current causes a voltage of 1.25V then the LM317 reduces the current by reducing the voltage to the load.
  19. audioguru2

    Help!! My Door Alarm is not work?

    The parts are too far apart on your pcb. The wires to the switches and trimpot are much too long. The distance between parts and the long wires adds too much stray capacitance and inductance which is not wanted in this radio circuit. Check the datasheets for the transistors to see if you...
  20. audioguru2

    Problem in an H-Bridge

    You are wrong. 5V is the max allowed emitter-base voltage. The emitter-base voltage is a reverse voltage when the transistor is turned off. A base-emitter voltage is a forward voltage when the transistor is turned on. The Base-Emitter ON voltage is called Vbe(on) in the datasheet. It is listed...
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