Saturday, June 03, 2006

Skype Handset from Nokia HSU-3 Privacy Handset

If you prefer to talk to traditional style handset rather than hearing the other person inside your head using stereo headsets or using the fiddly earbud, here is one solution.
I bought few boxes of Nokia HSU-3 privacy handsets for a song. These are originally suitable for Nokia CARK-126 carkit but I thought they could be used for other applications.
I reverse-engineered the circuit inside the handset and draw a diagram. Then I came up with an idea to use them as handsets for cellphones and Skype.
This blog is about how to modify Nokia HSU-3 to Skype use. I have made quite few of these and they work well. The sound quality is better than some cheap headsets and the handset is light and comfortable to hold longer periods. Also having a volume control on the handset is a good idea. When not in use the handset is held on it's craddle and is always handy when you get a call.

What you need:
New or used Nokia HSU-3 handset
2 X 3.5mm stereo plug
50mm heatshrink tube
red insulation tape
Soldering iron and solder
side cutters

How to:
1. Cut off 6 pin RJ phone plug

2. Cut 100 mm length of the end off the cable. This becomes second branch of the Y-joint for the 3.5 mm plugs. Remove the wires and white string from inside making a hollow tube.

3. Cut a small nick 100mm from the cut end of the cable being careful not to cut the wires inside. Pull out black and white wire using a tiny screwdriver or small nail. These 2 wires are microphone cables.

4. Take the 100 mm hollow tube you made before and cut it 10 mm shorter to make it 90 mm long . Now thread black and white cable inside the leaving the wires exposed 10 mm on the end.

5. Expose the other strand by cutting the rubber shield back by 10mm. Now you have Red, Brown, Blue and Green wires. Cut back the unused Red and Brown wire leaving Blue and Green wire as speaker wires.
6. Cut 50 mm length of suitable size heatshrink tube, inner diameter about 8mm, big enough to fit those 2 cables through and small enough to shrink tidy over the cables. Pull this over the Y-Joint and do the shrinking last when everything else is done.

7. Place the plastic shields from 3.5mm stereo plugs on the both cables. This is a bit hard to do if you have already soldered the plugs on :)

8. Carefully expose the bare wires about 4 mm ready for soldering. Soldering through a plastic is a bit messy ;) Be careful with the side cutters not to cut the wires. Some people use their nails to expose the wires.

9. Solder the plugs. White goes on the center pin ( tip ) and Black goes to the ground terminal on the microphone. On the speaker plug solder Green to the ground pin and Blue goes to the tip. (center pin)

10. Add the strain relief by squeezing the metal wings on the end of the ground terminal over the cable.

11. Check the connections, you could use multimeter measuring on the tip and the ground terminal of the 3.5mm stereo plugs.
Speaker pins should read 146 Ohms and
microphone should read about 690 Ohms. If you don't get any reading check your work again.
Add Red insulating tape on the microphone cable.
12. Use lighter to shrink the heatshrink over the middle of the Y-joint making sure that you get tidy and secure joint so cables are not getting loose.
Test with Skype and enjoy chatting with friends for free.