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"Casey Wilson" wrote in message ... "S Narayan" wrote in message ... Got the Radio Shack count up/down timer which can be set easily. It is big though (2" or so on a side). I connected an external AAA battery and put in a red LED across the speaker. I can now see when the timer expires with the flashing LED since I doubt I could hear the speaker go off. Can you provide a schematic? Please. I don't have the timer with me now, but IIRC, I experimented by putting several types of LEDs across the speaker inside, and it worked. In fact, the speaker works while the LED flashes. I didn't care if the speaker died, so I used the largest LED I had. I believe, there is some some kind of a charge-pump type circuit which steps up the voltage to tens of volts to drive the piezo type speaker. The somewhat high voltage won't hurt the LED, since the speaker driver cannot provide much current. The whole timer used to run off a 1.5V button cell. The standby current drain is around 5uA and putting an external AAA battery will keep it going as long as the battery manages to stay alive which may be a several years. The self discharge may be higher than what the timer needs! |
#13
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"S Narayan" wrote in message ... I don't have the timer with me now, but IIRC, I experimented by putting several types of LEDs across the speaker inside, and it worked. In fact, the speaker works while the LED flashes. I didn't care if the speaker died, so I used the largest LED I had. I believe, there is some some kind of a charge-pump type circuit which steps up the voltage to tens of volts to drive the piezo type speaker. The somewhat high voltage won't hurt the LED, since the speaker driver cannot provide much current. The whole timer used to run off a 1.5V button cell. The standby current drain is around 5uA and putting an external AAA battery will keep it going as long as the battery manages to stay alive which may be a several years. The self discharge may be higher than what the timer needs! Okay, now I've got it. I was confused over what you did with the battery. Now I see all you did was supplement the main unit battery -- it had nothing directly to do with the LED which simply parallels the pizeo transducer. Thanks. |
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