How to turn Smoke alarm into Ultrasonic Dog Repeller

I assume you mean an Ultrasonic Dog Repellent?

10-25kHz isn't really ultrasonic--most adult humans hear up to 20 KHz, and most kids well into their teens hear up to 25 kHz ( and many of them beyond).

I seriously doubt that that an old smoke alarm sounder can be modulated to sound in the ultrasonic range. In the USA, the high-pitched piezo sounders (as opposed to the older raucous buzzers) sound a lot like a Mallory SC628 Sonalert piezo sounder that operates at 2.9 kHz. Given that (as I understand it), that frequency is governed by a crystal integral to the sounder, I doubt it can be boosted up into the U/S range.

EDIT: Sorry, I didn't see your corrective post before I posted.
And an ultrasonic dog repellent is likely not to be a dog silencer--I suspect that dogs that have to endure it (i.e., can't escape it) might be noisy in their protests.

 

I don't know........our cat took one look and disappeared for 2 days the last time my burglar alarm went off(same sound) ...last I saw him he was about 300m away taking a look back to see what the hell was that//

Guess I should add in fairness, he was in a deep sleep in the middle of the day at the time

Then again, dogs seem to have attitude and you're likely that the response will be he don't give a fug.

I have seen where old smokies are hacked ( fairly simple) for use as water level alarms.

 

A piezo transducer is made to produce audible beeps, not ultrasonics.