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    jasper's Avatar
    jasper Posts: 1, Reputation: 1
    New Member

    Jan 26, 2003, 01:27 PM
    Hi there,
    I have a question regarding particular components required for an electronics project I am working on. I hope that anyone with a reasonable knowledge of transistors gets a chance to answer this question as it would be a great help to me.
    I have a small electronic radio powered with a 9v pp3 battery. I cut the wires to the speaker (which is an 8 ohm variety) and am left with two wires which suppied the signal to the small speaker.
    What I am seeking to do is use this signal to power a mains light bulb so that it will flicker according to the signal the radio is receiving. I am not too picky about a hi-fi relationship, as long as the light flickering approximately mirrors the signal that is enough for me. The lightbulb is 40 Watts and I am in the UK using mains (240v) electricity.
    My question is: what are my options? I am especially interested if there is a particular high-powered transistor on the market which could work directly to amplify the small radio signal in series with the lightbulb. I do not know how to read transistor data sheets meaningfully. I have worked with mains before but lack the particular electronics knowledge to know how to do this. I do not for instance know if any diodes are required to convert the AC across the Collector/Emitter or into the Base from the speaker wires. Any 'workings' would be helpful for me to know about in case I change my requirements.

    Any help gratefully received!
    Locii's Avatar
    Locii Posts: 10, Reputation: 1
    New Member

    Jan 27, 2003, 08:41 PM
    I've never tried what you want to do, but I strongly suspect that to get any regular bulb to flicker in accordance with your radio signal is going to be virtually impossible; such bulbs just can't react quickly enough. I neon bulb might do the trick.

    Without seeing at least a partial schematic of what you're working with makes it hard to suggest more.
    speedball1's Avatar
    speedball1 Posts: 29,301, Reputation: 1939
    Eternal Plumber

    Feb 7, 2003, 10:18 AM
    If I were going to do what you wish I would forget the lightbulb. Purchase a used oscillosope and tie that to the terminals of the audio output transformer. Then you can "see" the audio signal. I did that for a science fair one year and it was a big hit. Good luck, Tom
    CommDweeb's Avatar
    CommDweeb Posts: 11, Reputation: 2
    New Member

    Feb 21, 2003, 12:50 AM
    Need more info.

    The first is trying to get an understanding of your level of expectation. The eye is lucky to see a fluctuation that lasts under 1/30th of a second. This would imply that you could not reliably see any fluctuations at a frequency of over 30Hz. This is well below the threshold of hearing, so it would not be possible to re-create the sound from the fluctuations in light level. You could at best hope to produce some kind of a brightness to volume relationship. The louder the sound, the brighter the light. Is this what is hoped for? If not, would this be acceptable?

    The next question, is how much signal is there to work with. It takes .3 to .6 volts to forward bias a PN junction. V^2 / R = P so V = sqrt(P*R) This means that if you have a 1 watt transistor radio, it will deliver 2.8 Volts. This is more than enough to reliably turn on a Transistor, SCR, or Triac.

    I would recommend using a Triac or SCR rather than a Transistor. This will preclude the necessity of rectifying the power to DC. Triacs and SCR's are turned on by a gate current, and turned off when the current through them drops to zero. If you feed them 50hz AC, they can only stay on for 1/100th of a second ( 1/2 of a cycle) and then they turn off. In your application, this would mean that they would be turned on for 1/100th of a second at a time when the sound level exceeds 43 miliwatts. Triacs SCR's are either on or off, so the light bulb will dissapate all of the power and you won't have to hassle with heat sinks. Get an SCR or Triac that is rated at 350V or more to leave yourself a margin of error.

    Be careful, get help! 240VAC can be very dangerous.

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