Aleks Posted June 15 Posted June 15 Hi everyone, I am making a controller using the following circuit: 1) powering 3pcs 9 volt batteries in series, which gives us 27 volts and a current of about 0.625 amps. 2) on / off rotary key 3) I am using buttons with circular backlighting. The backlight has separate from buttons outputs + and - outputs. Backlights work in the range of 9 to 30 volts, so it is not necessary to lower voltage for it to work in this circuit. I want the button backlight to work only when the circuit is closed, i.e. as a closed circuit tester. The safe current of my igniters is up to 0.15 ampere, so I need to reduce the current in the test line with the backlight and a resistor and make it less than 0.15 ampere. Can you please tell me how to calculate the correct resistor resistance for this circuit? I can't figure it out.
Arthur Posted June 15 Posted June 15 Assuming that the fireworks are to be set off at night, in the dark, a LED current of 2 milliamps should be bright enough, that is achieved by a resistor of 15,000 ohms and a tiny power rating. See whether you think 2ma is bright enough, I'd want to just see the lit LEDs not be blinded by a full brightness LED, You may think differently. Your job your decision. DO some tests before committing to a public show. 1
Aleks Posted June 15 Author Posted June 15 21 minutes ago, Arthur said: Assuming that the fireworks are to be set off at night, in the dark, a LED current of 2 milliamps should be bright enough, that is achieved by a resistor of 15,000 ohms and a tiny power rating. See whether you think 2ma is bright enough, I'd want to just see the lit LEDs not be blinded by a full brightness LED, You may think differently. Your job your decision. DO some tests before committing to a public show. Arthur, thanks! This remote is supposed to be not only for fireworks, but also for fountains and daytime pyrotechnics, so I'm thinking of making it bright enough that you can see them a bit in daylight too. Can you help me please, what wattage do you think the resistors should be?
Arthur Posted June 16 Posted June 16 If you reduce the resistors to 7500 ohms then the brightness will double. Try with 15000 ohms, if it's too dark add a parallel 15000 ohm resistor, which would double the current and hence brightness. The limits are getting too close to the all fire current, exceeding the 10 or so milliamps for the LED, AND reducing the lifetime of the batteries. Remember that small batteries don't last long with loads left on, and big batteries weigh a lot and use up space. Power dissipation in the resistor should be minimal ( 2ma 30v =60milliwatts ) fit whatever resistor rating you can come by. Don't go surface mount unless you are totally familiar with their use, and their circuit board design. DO tests first to be certain that this will all work at show time.
Aleks Posted June 16 Author Posted June 16 11 hours ago, Arthur said: If you reduce the resistors to 7500 ohms then the brightness will double. Try with 15000 ohms, if it's too dark add a parallel 15000 ohm resistor, which would double the current and hence brightness. The limits are getting too close to the all fire current, exceeding the 10 or so milliamps for the LED, AND reducing the lifetime of the batteries. Remember that small batteries don't last long with loads left on, and big batteries weigh a lot and use up space. Power dissipation in the resistor should be minimal ( 2ma 30v =60milliwatts ) fit whatever resistor rating you can come by. Don't go surface mount unless you are totally familiar with their use, and their circuit board design. DO tests first to be certain that this will all work at show time. Thank you!
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