This Instructable was inspired by an article from one of my first issues of MAKE. It can be applied to just about any noisy toy, although the details are specific to this one.
We have an infant mobile (Tiny Love's "Symphony-in-Motion" with remote) which plays annoying-after-the-Nth-repetition electronic versions of classical music fragments, at either high or low volume. Since our daughter really enjoys watching the mobile, the obvious solution to our annoyance was to install a mute switch.
Step 1: Parts and Tools
You'll need one "electronic" component, and a few tools:
A subminiature SPST three-position switch. My lab has a drawer full of C&K 7203 switches, which is actually SPDT. For a slimmer SPST version, order the C&K 7103 (pictured) for a few bucks from Mouser, Allied, DigiKey, Jameco, or your favorite distributor. This switch has three positions -- the middle one is open-circuit, just what we want for blessed silence.
Triangular screw bits. Our mobile's case has some, really weird "security" screws with triangular recessions in the caps. I ordered a set of four bits from McMaster-Carr Industrial Supply (items 5941A11 through 5941A14).
A soldering iron, solder and flux. You will need to know how to solder already.
Step 2: Open the Case
If you can't open it, you don't own it.
Disconnect the mobile's armature from the control case, and unscrew the box from the side of the crib. Remove the batteries. Undo the four triangular screws connecting the two halves of the case.
Inside you'll see two wires connecting from the control board on the front half to the battery compartment on the back. Be careful when you open the halves, and especially where you set down the back half. If you stress those wires and break one of the solder joints, you'll have to fix it yourself.
The "volume switch" (a simple two-position SPST switch with three contacts) is also mounted on the rear half of the case, with three wires (red, white, and black) leading from it to the circuit board mounted in the front half. Again, be careful not to stress these wires or you'll have a much larger project on your hands.
Step 3: Remove the Volume Switch
Carefully remove the little switch from its recess on the rear case, and take out its molded plastic cover as well.
There are three leads connected to the switch: the red lead is for "high volume", the white lead is common, and the black lead is for "low volume". Keep track of these wires so you can connect them to the new switch (Step 4).
With your soldering iron, release all three leads from the switch terminals, and put the switch and its molded plastic cover aside. If you have a spare-parts bin, that's a good place for them. If not, consider this a place to start :-)
Step 4: Install the New Switch
With the plastic cover gone, the new switch should fit through the open hole with no trouble. The plastic tab that was holding the old switch in place may be in the way. If so, you can cut it off with a sturdy knife, grind it down with a Dremel, or even just break it of in stages with a pair of nippers.
Push the switch through the hole from the inside, and secure it with the large keyed washer and nuts included. I used both nuts on the outside to make sure the switch doesn't come loose over time.
Solder the white to the center contact. Solder the red (high volume) lead to the right-hand "contact. This corresponds to flipping the switch to the left, matching the icon printed on the case. Solder the black (low volume) lead to the left-hand contact, corresponding to flipping the switch to the right.
Yes, it is a bit confusing. The fourth picture below should make it clear what's going on.
Step 5: Verify Functionality
Before you screw up (the case), make sure you haven't screwed up!
Put the batteries back into the compartment of the back of the mobile, and turn it on from the front panel. With the switch in the "high volume" position, the music should start up pretty loud. Flipping the switch over to the "low volume" position should quiet it down some, but it's still there. Finally, putting the switch in its center, vertical position should turn off the sound completely.
If you don't have sound when you turn on the mobile, start troubleshooting:
-- Are the batteries installed correctly?
-- Is the green ON light illuminated on the front panel? If the red "remote" light is on, push the switch again.
-- Check the exposed contacts and solder joints. Use a multimeter to confirm continuity along the leads from the speaker through the switch. and check for continuity or open-circuit between the contacts when the switch is in each position.
Step 6: Close It Up
Once you've confirmed that your modification works as desired, you're finished. Put the two halves of the case back together, and re-mount the mobile wherever you had it. Now you can turn off the music whenever you want, preserving both your sanity and some small fraction of battery life.