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  1. Audioguru

    Contact Cleaner?

    Old or cheap electronic switches use silver-plated contacts that are good for high power electricity (the spark burns off the black tarnish) but poor for low levels. Modern high quality audio products use switches with gold-plated contacts (they do not cost more than silver-plated contacts)...
  2. Audioguru

    Ni-MH charging circuit query

    My new Duracell charger charges AAA Ni-MH cells fast at 775mA or slow at 388mA. It is a "smart" charger and it detects fully charged then shuts off the charging and lights a green LED.
  3. Audioguru

    Headphones from amp speaker wires

    The true power output of an amplifier is continuously with low distortion (no clipping) from 20Hz to 20kHz. Many amplifier ads exaggerate the number by saying the power at one frequency with 10% of awful clipping distortion for only a moment before the power supply voltage sags. Then they quote...
  4. Audioguru

    Headphones from amp speaker wires

    The resistor calculation depends on the impedance and sensitivity of each earphone that you did not say. Does the amplifier produce 45 Whats per channel or 45 real Watts?
  5. Audioguru

    Supply for 30 LEDs

    Current in a resistance causes heat. In my country we take sliced bread and put it in an electric toaster. The resistance wire in the toaster glows red hot. The resistance wire in an old fashioned incandescent light bulb glows white hot.
  6. Audioguru

    Supply for 30 LEDs

    I have a cheap Chinese flashlight with 24 white 5mm LEDs all connected directly in parallel. The flashlight is powered by three AAA alkaline cells in series (4.5V when new). The well-matched LEDs are in series with a 1.2 ohm 1W resistor. When I turn it on the battery (not new) measures 3.78V and...
  7. Audioguru

    Ni-MH charging circuit query

    Recently I bought eight Duracell AA Ni-MH batteries at a Black Friday sale and they came with a "free" charger. The charger box says "Fastest Smart Charger" and it charges very fast. It says on it that its fast charge current is 1550mA (2 cells) and its slow current is 775mA (4 cells). Its...
  8. Audioguru

    Shorting a Battery

    The Li-Po batteries for my radio controlled model airplanes are 325mAh and they can provide a continuous output current of 70C which is 22.75A or momentary peak currents of 45.5A! Each cell averages 3.7V during a discharge with a maximum charge of 4.2V just like a Li-Ion cell. Some of my older...
  9. Audioguru

    Transistor amplifier gain

    If you built it on a solderless breadboard then the messy wires all over the place are picking up all kinds of interference that are overloading the transistor and causing it to rectify them. On my FM transmitter I needed to add a little 100pF filter capacitor between base and emitter of the mic...
  10. Audioguru

    Ni-MH charging circuit query

    The charging voltage of a Ni-MH cell depends on the amount of charging current. Here is from the datasheet if an Energizer Ni-MH cell:
  11. Audioguru

    ECG front end simulation

    The circuit you posted will produce a lot of 50Hz mains hum and other interference on its output because the patient and the wires to the probes pick it up. Most ECG circuits cancel the interference by feeding out-of-phase AC and DC interference signals to the patient's right leg.
  12. Audioguru

    Transistor amplifier gain

    The gain is calculated with the collector resistor (parallel with the load resistance) divided by the emitter resistor in series with the internal emitter resistance (0.026/emitter current). If the gain is AC then the calculation uses the unbypassed emitter resistor value. Then a higher emitter...
  13. Audioguru

    Need help to make telephone tap coupler schematic

    A capacitor passes AC and blocks DC. It has microfarads, not ohms. Why do you ask about a capacitor? An audio transformer is needed to isolate the telephone system from picking up loud mains hum from the ground on your pc.
  14. Audioguru

    Need help to make telephone tap coupler schematic

    A 600 ohm transformer would connect to a 600 ohm telephone line but we do not know the impedance of the handset on your telephone so a higher impedance transformer can pick the signal from the earpiece and attenuate it. You must add a cancelling circuit to the telephone to cancel the sidetone...
  15. Audioguru

    Transistor amplifier gain

    A transistor can have the emitter resistor bypassed with a capacitor or it can have no emitter resistor. But the gain is not infinite because the transistor has an internal emitter resistance that is "the emitter current number in mA divided by 26". This resistance is in series with an actual...
  16. Audioguru

    Low cost electric skateboard?

    I said that a motor that is too small will be overloaded and will soon burn out unless you add a gearbox or chain and sprockets. Then the motor will run fast near its maximum allowed power but the skateboard will run slowly. Speed control is usually done with PWM (look it up).
  17. Audioguru

    Shorting a Battery

    You need to calculate (or experiment) about how much power you need to create the heating. The results will be the voltage and resistance needed. Then guess for the longest duration you need. Then find a battery that can produce the current for the duration.
  18. Audioguru

    Need help to make telephone tap coupler schematic

    To feed the earpiece signal of a telephone to the mic input on a pc then you must isolate them with a transformer and attenuate the signal down to mic level that is much lower than earpiece level. The mic input on a pc is designed to power an electret mic so a coupling capacitor must be used to...
  19. Audioguru

    Linking circuits help

    Correction. My circuit does not have pin 2 of the 555 connected to the LDR. The 100k resistor causes pin 2 to be high when there is no trigger signal. My 470nF capacitor provides a "proper timeout" (see my post about it yesterday). 1μF will be fine. Simply have pin 4 (the reset pin) connected...
  20. Audioguru

    LED SMD 10watt and Solar battery

    When the solar panel is loaded so that its voltage drops to 12V then its power to the LED is only 12V x 560mA= 6.72W, not 10W. The LED will not have full brightness. If you add a battery then you must adds a battery charger circuit and a circuit to detect low light levels, then switch between...
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