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Resistance for a lightbulb circuit

  1. Nov 29, 2011 #1
    So I've made a circuit that lights up a light bulb whenever it's dark. I did this using a photoresistor and it is working. However, I have a relay switch with a 9V battery and a 7.2V lightbulb. When it was supposed to turn on, it lasted a couple of minutes and the battery is now dead. Should I place a resistance in that circuit so it lasts at least an hour? If so, what should I add?

    Thank you
  2. jcsd
  3. Nov 29, 2011 #2
    do you have any more info on the lightbulb? you are putting a little too much current through it, but it sounds like you wont get much improvement in performance as 9V dont have a lot of capacity. The top end ones have ~570mAh of capacity for the disposable kind. This means that if you take 570mA for an hour the battery will be flat. Seeing as you want it to last an hour, this is a nice easy calculation and you need your light to draw 570mA.
    Maybe you could try several 9V batteries in parallel, this will up your running time but it sounds like you are going to need a lot of them to run for any significant amount of time....

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