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Hacking An Alarm Clock Answered

My sister (yes, the one for whom I made earrings) wants me to modify an alarm clock to run a small electronic device when it goes off. Preliminary research has given me two methods:
1) A digital clock with a 555 timer circuit
In this setup, a 555 timer circuit is used to translate the signal to an alarm speaker/piezo disk into a relay. When the clock goes off, the relay clicks, and any electrical device that is run through the relay has power.

2) An analog clock with a transistor amplifier
In this version, the analog clock seems to just have a straight DC output to the alarm. The transistor is used to amplify the DC to drive the motor of a fire bell.

Which of these would be easier to implement? The device I am supposed to use runs off of 3VDC, so I was just going to run the power line through the relay or through the transformer.

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randofo (author)2008-04-25

I did something like this once. I used an AQV210EH low-voltage relay (which from my understanding is just an opto-isolator). You need to be careful with these since they are static sensitive and crossing the pins with your fingers could short the chip. If the doesn't work quite right, try adding a low-voltage regulator coming from the alarm. This will create an even voltage from the alarm signal which will more consistently trigger the relay. Also, go to youtube and type in "watergun alarm clock"

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CameronSS (author)randofo2008-05-28

I just found a PC817 on an old fax machine board...would that work?

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randofo (author)CameronSS2008-05-29

There's a chance... try it with a AA battery and see if you could turn on and off an LED (with a different power source).

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CameronSS (author)randofo2008-04-25

I'm pretty sure I understand how it works, but where would I find one of these relays? Does Ripoff Shack carry something similar? Or do I have to special-order one?

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randofo (author)CameronSS2008-04-26

Digikey!

However, as tech-king pointed out... it probably can easily be solved with a switching transistor which you should be able to find at Radioshack.

Or other things to try If you want to go the Radioshack route:

You can probably get a variable voltage regulator at radioshack (1.5 - 18V) and hook that up the alarm at a reasonable setting for an LED. Then have the regulator power on and LED when the alarm goes.

You then need to have the LED pointed at a photocell hooked up to a 741 chip. See this for a schematic:

https://www.instructables.com/id/Night-Owl-Alarm/

And then have the 741 hit a 5V relay...

Or perhaps you could hook the 741 directly to the alarm. I've never tried this. You should try that since it will most likely work. Try setting up the alarm to trigger the 741. That should work. You still might need to put a voltage regulator in-between or something of the sort to increase and stabalize the voltage from the alarm output. I'm not sure what you can get at Radioshack to do that...

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tech-king (author)randofo2008-04-26

i checked out the firebell one; its needlessly complex. its uses a pnp attached to a battery to negatively bias itself and go on with the alarm, and also switches on the alarm, which needs a separate power supply. needlessly complicated and archaic. here is a simpler circuit, thats powered off a 9 volt battery.

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tech-king (author)randofo2008-04-28

i wrote firebell. although spell check does want me to put fireball. or do you mean it should launch fireballs in time to the alarm?

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randofo (author)tech-king2008-04-29

Maybe it was wishful thinking

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tech-king (author)tech-king2008-04-26
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CameronSS (author)tech-king2008-04-27

I'm not too good with transistors yet-could a 2N3904 and a 2N3906 be substituted for the 2N2222 and 2N2907, respectively?

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tech-king (author)CameronSS2008-04-27

sure. they are all general purpose switching transistors.

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guyfrom7up (author)2008-04-25

I'd do the second option. I think the lowest voltage relay from the shack is 12volts. Correct me if I'm wrong, but (I'm bad at transistors) can transistors not amplify voltage, they can only amplify current? Or am I wrong?

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tech-king (author)guyfrom7up2008-04-26

how can you be bad with transistors? they are the most important part. and it depends what transistor you use (bipolar, jfet, mosfet, ujt). transistors can amplify current or reduce voltage, but not amplify voltage. see my other post.

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guyfrom7up (author)tech-king2008-04-26

it's just that I don't get good practice with them and I just always have troubles in circuits where the main problems is i fried the transistor or I hooked it up wrong, or I chose wrong resistors, etc.

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tech-king (author)guyfrom7up2008-04-26

how can you choose the wrong resistor? although i agree that you should calculate the exact value, you can get away with a 1k resistor to the base.

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guyfrom7up (author)tech-king2008-04-26

well, it's just kinda like the one component where the operation of it is simple and definite, for example: capacitors hold a charge using 2 close plates that arn't connected a resistor resists current and there's ohms law a relay is a switch a transistor is kinda like a switch, but it has 3 terminals, etc. etc. Your right, they arn't that hard to use, just hard to imagine if you know what I mean.

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tech-king (author)guyfrom7up2008-04-26

actually. i have more trouble imagining the capacitors charging then the transistors switching and amplifying

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Ugh, I don't even bother with capacitors. (and somehow I get away with that) P.S.- How do you change text size on a forum, or do strikethroughs.?

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equals on either side (no spaces between text and equals)

big

not sure about strike through

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combining some symbols gives you things too:
for instance this text has 2 tldas followed by that up arrow thing for Superscript, and then reverse the order on the end

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The up arrow thing (whatever it's called) does superscript. To do subscript, use two commas. Strikethrough is two tildes (~)

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superscript & subscript & strike . Hmm, that's interesting

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Fo,r, some reason when I do shift 6 nothing comes up...

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Looks like to me it worked just fine.

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the little hat thing
i push shift and 6.
not sure for you

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tech-king (author)2008-04-26

why are your options so complicated? you could easily: 1) if you wanted a device to go one only while the alarm is (probably pulse though) add a switching transistor 2) if you want it to go on with the alarm but stay on, add an scr latching circuit 3) if you want it to go on for a short period of time when the alarm goes off, use a 555 in monostable.

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Goodhart (author)2008-04-25

I am wondering, if you need a simple DC current to run the "device" from the alarm (#1) if a simple bridge to smooth out the waveform would work ? If you need to step down the voltage, a regulator should work (the output to an alarm buzzer is probably not 115 vAC house current anyways).

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