For today's How-To, you'll need:
- USB port (Mouser Electronics 538-67329-0031)
- LM7805 power regulator (Radioshack or Mouser)
- Heat shrink tubing (or electrical tape)
- Soldering iron
- Hot glue gun
To add our USB power port, we'll steal some DC voltage from the clock. We are going to run power through a LM 7805 voltage regulator to take it down to 5 volts. Adding a 220uF capacitor between the output and the ground can help smooth out the voltage if that's a concern. We referenced the USB pinout page at pinouts.ru
and the data sheet for the LM7805
Unsurprisingly, all the terminals are on the bottom of the PC board. The buttons on the side pop out, and a final screw holds down the board. The radio tuning hardware was pretty easy to align on ours, but be wary of more delicate designs.
With the board out, we cleared the area of conductors and plugged in the clock. If you're worried about the small exposed terminals carrying AC voltage, place some electrical tape over them before starting. We located 9 volts DC on the board at TP10 and TP11 (test pads). These were our favorite locations because they have nice big solder pads.
Soldering leads to the pads is pretty easy. Just heat up the wire and the pad and apply a bit of fresh solder.
Once the power leads are soldered on, re-install the PC board and run the wires through a good location. We brought them out next to the LED display.
Before finishing up, we did a quick test of the power setup. Flawless victory.
Deciding where to place the USB port depends on what you plan to do with it. Mounting it on the top is the most convenient location. There's plenty of room below for the port, and it'll give our USB light the most reach.
Dremel tools are great, but sometimes the quick, dirty (and stinky) approach is the easiest. We melted the hole with our soldering iron. Watch out for the fumes, but it gets the job done.
Test fit the port once the hole is cut. When you're happy with it, you can clean up the melty edges with some diagonal cutters or a utility knife.
Set the top of the clock down on a working surface (that can get hot glue on it). When the port is aligned, squirt in some hot glue to lock it down. You want to build up the glue for strength so take your time and let the glue cool down before adding more.
Once the socket is secure, solder on some leads to pins 1 and 4 once the socket is secure . Then follow up with some more hot glue. We waited until after the leads were soldered to add glue to the terminal side of the connector. The clear glue is still cooling off.
Match up the ground leads from the clock and the usb port. Slide some heat shrink tubing over them and
solder them to the center connector of the 7805. You might want to double check the polarity of the wires with your meter before soldering. Since we used two red wires, we marked ours with a black sharpie.
Wire up the other two connectors the same way. Positive power from the clock goes to the input pin and output connects to pin 1 of the USB port. If you're thinking of using the port to charge a low power device, you should consider adding a 220uF capacitor between the output and ground pins of the regulator.
If the 7805 was going to carry a lot current, we'd add a heat sink to the tab. In this case it'll be a light load, so we screwed it down with the PC board mounting screw. The tab is also a ground, so be careful that it won't short on anything. We had plenty of clearance. If you're worried, a bit of tape of heat shrink tubing over the tab is cheap insurance.
The hardest part of the project was putting the case back together. The speaker is pressure fit against the top with a spring, the switches all need to be properly aligned, and you have to watch out for your added wiring.
Success! The small switch built into the light turns it on and off. As a bonus, the light can run off the backup 9 volt battery in the clock. During a power outage, we'd at least have a little light to find the flashlight with. It's great for low power devices like the LED light. If you're thinking of putting a USB port in your clock to charge up your portable brain, we'd suggest installing a small power adapter or regulating and tapping reduced AC voltage from the transformer inside the clock to avoid drawing too much power off the circuitry in the clock. Enjoy!