Hey. I am sure this is simple, but I dont know the proper electronics jargon to search for an answer. Basically, I want the completion of one circuit to activate a second circuit but not allow feedback to the first. I have an alarm clock running on a AAA battery, and when it sends power to the buzzer (detached) I want it to activate a second circuit running off a 9V battery. The problem is, I dont want the 9V circuit to get back to the alarm clock, otherwise it would overload it (Please correct me if I am wrong). Is there any way to do this without causing major problems? Thanks.
Sure -- it's doable. But a simple transistor alone won't cut it, I'm afraid.
Let's start our with your big question -- you're concerned that the 9V will get in to the alarm clock circuit and let the smpke out. That's a concern, but you can take care of it.
To begin with, your alarm is probably a piezo beeper which is driven by applying the voltage to one side of the beeper (1.5V), and 0V to the other side. It then switches the voltage back and forth at the sound frequency (probably a few thousand times a second).
This output is floating (separate battery). So from the outside, it looks like a 3V peak-to-peak square wave. You can feed that to a voltage doubler with schottky diodes, and that should provide a high enough voltage (about 4.5V or so) to drive a logic level MOSFET, which can turn on your circuit. Take a look at this (view in fixed font or copy&paste to Notepad):
Tack solder two wires to the beeper element in the alarm clock. Build the circuit with 0.1uF caps and 1N5819 schottky diodes. Then snip the negative (black) wire going from your 9V circuit to the battery, and attach the MOSFET in series as shown.
This will reliably turn on when the alarm is on. It may get a little iffy at very low AAA battery voltage, though. The MOSFET selected is beefy enough to drive any load a 9V transistor battery can supply.
If this is too complex, or if you need a quick solution and aren't able to come up with the parts on short notice, you can use an external 12V wall wart, use standard 1N4001-type diodes, and use a darlington transistor to drive a relay. The relay contacts can then be used to turn on and off your 9V circuit.
Please post again if this isn't clear, or you need more advice.
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