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  1. audioguru2

    Need help with a TV transmitter project.

    Hi Dazza, For my FM transmitter I made simple coils and used 5pF to 35pF variable trimcaps to tune from about 85Mhz to about 110MHz. You might need more range which is had with a variable inductor like in the project. My trimcaps are very difficult to tune to any certain frequency but an...
  2. audioguru2

    Re: Alun's low voltage alarm

    Hi Alun, For an opamp that switches fairly slowly, the capacitor probably won't make much difference. But for a high-speed comparator that oscillates when linear, you need it to switch with the positive feedback as quickly as it can. You can still filter noise from the supply by making the upper...
  3. audioguru2

    PWM

    Kevin, You can't use two unrelated signals for pulse-width-modulation. They must have exactly the same frequency and you create the modulation by producing a phase difference between them. Ordinary lamp dimmers and DC motor speed controllers work like that. In your circuit the AND gate is...
  4. audioguru2

    Re: Alun's low voltage alarm

    Hi Alun, Picture the non-inverting input at the moment of switching, with and without a capacitor there. Without the capacitor it has positive feedback for a quick "snap" action.
  5. audioguru2

    OPTOCOUPLERS

    Hi Prateek, I don't know about your country, but in Canada the ground wire is used for safety so that metal-cased appliances have their case grounded. If a live wire inside touched the case then the breaker would blow. The appliance is powered from the live and neutral wires. The neutral wire is...
  6. audioguru2

    Re: Alun's low voltage alarm

    Hi Alun, Your schematic is much better now, except: I think that the cap at the divider spoils the hysteresis. :o
  7. audioguru2

    AM to FM convertor

    Hi Andbor, It's great that you got it to work. I was glad to help you and thanks for proving my theory. Sure it will pickup a TV station and if you're near an airport it will pickup the planes and air traffic control.  ;D
  8. audioguru2

    Re: Alun's low voltage alarm

    Hi Alun, Yes, I completely forgot about the project's missing very important supply bypass cap.  ;D
  9. audioguru2

    OPTOCOUPLERS

    Hi Prateek, If the motor is powered from live and neutral but has leakage current to its ground, then a voltage would develop across its ground wire due to the current in its resistance. Therefore the ground at the motor isn't at the same voltage as the ground at the computer.
  10. audioguru2

    Puggy Potty Alarm

    Hey Alun, Silly, silly you again and again!  :o  Your 'fixed" opamp still won't work with its non-inverting input grounded. It needs to be at mid-supply.  ;D With its gain of -1, your opamp would mirror the non-rail-to-rail output of the 555. It needs some gain to go full swing.  ;D Nobody...
  11. audioguru2

    Need help with a TV transmitter project.

    Hi Dazza, 2.2pF is just about nothing. I use about 22pF in my FM transmitter that operates at the same frequency. Here you have C7 and C9 in series across the coil making 11pF which is about right. 1.1pF across the coil would be half of nothing?  ;D
  12. audioguru2

    I need a push button switch

    Hi Steven, Do you kill the engine and coast downhill to save gas?  ;D I had a car with a governor that would kill the engine when the car went faster than its tire rating. I didn't know about it until I was passing, and a big truck was coming at me! I don't know if my new car has a governor...
  13. audioguru2

    Re: Alun's low voltage alarm

    Hi Alun, I'm glad that you are learning here. 8) You have a very good idea to use the sagging supply voltage to help with hysteresis. I keep thinking of circuits with a regulated supply, but this application uses a non-regulated battery supply. Circuits with slowly changing inputs that use...
  14. audioguru2

    telephone forwarding

    Hi Doctor FC, Call forwarding is a standard feature on large private office telephone systems. If your office's phone is part of the hospital's new phone system then this feature just has to be programmed.
  15. audioguru2

    Puggy Potty Alarm

    Hi CRE, Did your startled little pug stop mid-stream? You can use any opamp for Alun's corrected circuit, but you didn't notice the elbow that I gave him. His latest circuit must be trying to save battery power because the opamp isn't the required inverter so the piezo gets nothing!
  16. audioguru2

    Puggy Potty Alarm

    Hi Alun, He, he. Silly you again! ;D ;D ;D  ;D ;D ;D  ;D ;D ;D  ;D ;D ;D  ;D ;D ;D  ;D ;D ;D  ;D ;D ;D  :o :o :o  :o :o :o  :o :o :o  :o :o :o  :o :o :o  :o :o :o  :o :o :o  ;D ;D ;D  ;D ;D ;D  ;D ;D ;D  ;D ;D ;D  ;D ;D ;D  ;D ;D ;D  ;D ;D ;D  :o :o :o  :o :o :o  :o :o :o  :o :o :o  :o :o :o ...
  17. audioguru2

    newbie pot usage

    To control the brightness of a 120V lamp, a pot feeds a small current to a lamp dimmer circuit with a triac in it. The triac switches full power to the lamp at 120 times per second using pulse-width-modulation. For full brightness, the triac switches the lamp on most of the time for its pulses...
  18. audioguru2

    Power Supply problem

    Kevin, If the regulator was hooked up wrong then how did the circuit provide 10A at its designed 5V?
  19. audioguru2

    Puggy Potty Alarm

    Hi Alun, I don't think that your opamp will work, it will latch-up.
  20. audioguru2

    OPTOCOUPLERS

    Kevin, An optocoupler passes audio and even DC pretty well. You can use a 2nd optocoupler for negative feedback to reduce distortion. They are not used only for digital. Maybe you don't understand how they work. If you wonder about pulse-width-modulation you can look at it in our Articles...
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