Required:
Battery operated smoke detector
soldering equipment and skill, or somebody to solder for you
push button
wire (speaker wire?)
EAR PLUGS
Be aware that you might encounter a smoke detector where it is just too hard to get at the electrical contact points, or with too weird a setup. So it would be ideal to start off with one that you already have so as to avoid the possibility of trying to return a smoke detector that doesn't really have anything wrong with it.
Use ear plugs! Find the "TEST" button on the detector, and try it out. It'll give out an obnoxiously loud and annoying sound, bordering on painful, and very bad for your ears. That's what the test button is FOR - to confirm that it is still capable of being obnoxious and annoying.
Remove the battery, and open the case of the detector. Some have a sealed chamber - DON'T open that. You don't even want to know what is in there. With the case of the detector open, find the test button. There will either be two wires running from the test button to something else, or the test button will be mounted directly to a circuit board. If so, find the "solder pads" where it connects.
If the test button has anything other than two wires or two solder pads connecting it (such as one with FOUR wires or pads), then start over with a different brand of detector.
Solder one end of two long wires to those connectors of the test button. On battery operated detectors, there will normally not be any significant current, so you should be able to use speaker wire or "zip cord" (lamp cord).
At the other end of the wires, attach a pushbutton. A doorbell button should work OK. Your pushbutton is now "wired in parallel" with the test button. Therefore, whenever you press your pushbutton, it will be the same as it you had pressed the test button.
If you don't know how to solder, then find somebody who does, to connect the ends of the wires inside the detector. If you do use somebody else, then it is likely that they will understand what is needed if you use the sentence, "I want a remote switch wired in parallel with the existing test button."
An even better solution would be a wireless radio remote control. There exist some with a keyfob switch for use with automobile alarm systems. But the other end of it requires a source of power (12v typically) to operate, and you need one with a relay in order to get what is called "dry contact switch closure". I've done that with some special BSR X-10 modules, but that gets into some more money.
BTW, in training with it, some dogs will learn to watch your hand to SEE when you press the button, so a small pocketable button is ideal.
(c) 1998 Grumpy Ol' Fred