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How can I hook up a transistor to the speaker wires on a alarm clock to power a relay? Answered




Best Answer 8 years ago

You need to describe better what you want to accomplish.  I suspect you want to turn on a LIGHT in your room from your alarm clock so you can wake up in the morning..... RIGHT?  I did that to my alarm clock but i didn't connect to the speaker wires.  I connected a small 5 volt relay to the inside of the alarm clock to the same 2 wires that power the FM radio section.  When the alarm clock turns on the FM radio, it activates the 5volt coil on the RELAY.  Then I connected a 110 volt outlet onto my radio so i could plug in a desklamp.  Now when the alarm clock comes on... so does the LIGHT. 

ANOTHER way to do that would be to just buy a programmable timer that plugs into the outlet..  Then set it to go off a few minutes before your radio in the morning. 

That's what i'd suggest, too.

I should add to that; crack open the alarm clock and see what's running the show.  Chances are, there will be a single large IC that does most of the work, from running the display to keeping time to accepting input from the buttons.  See if you can find a datasheet for it.  That will help you figure out which pin to tap for your control signal.

The alarm signal is AC (at audio frequencies), and the signal desired for driving the relay coil is somewhat steady DC, thus some sort of rectification and low pass filtering is required, essentially some arrangement of diodes, capacitors, and resistors.

I like this doubler circuit, part of the circuit in the attached picture, because it should have zero response to a pure DC signal (because it has a capacitor in series with it).  Let me know if this diagram is readable.  One thing that often helps is to copy the picture's URL, and then replace the string "MEDIUM" with "LARGE". Then that URL points to the "large sized" picture.

BTW, one of the assumptions in this circuit diagram is that alarm clock is one of those powered by a single AA (1.5 V) battery.  If it is a different style alarm clock, like maybe the plugged-into-the-wall kind, then other options are available.  For example, for a plug-in alarm clock it may be possible to find power do drive the relay within the existing power supply for the clock, without need for extra batteries.

Another BTW:  I haven't actually built/tested the attached circuit.


It should work. I'd balk at calling the "voltage doubler" that, to me its a diode pump, but that's nitpicking. Some messing around will be required to find suitable values, and that relay needs a definite power diode across it, with the cathode to +ve supply.