By Jason Reilly.....

 

Handheld radios are a wonderful idea. They allow you to stay in touch where ever you go. They allow you to do whatever you want to do, without being restricted to within hearing range of your base or mobile set. 

 

There are, however, some activities where using a handheld on it's own can be a little cumbersome, tedious or even dangerous, when used in the conventional manner. The push-bike couriers seen in the CBD of large cities are a good example of where using a handheld can be dangerous. Could you imagine what might happen if, while pedaling at top speed through heavy traffic, one of these chaps tried to make a call by fumbling around, pulling a two-way radio off the belt and holding it to speak into?

 

 

In these instances, a speaker-microphone comes to the rescue. These handy devices allow your radio to remain stuck on your belt, harness or pack without having to remove it to hear or make a call. Looking similar to a conventional microphone, they also have a mini speaker built into them, so you can hear your radios received audio as well as speaking into it. The idea is that you can then have your handheld radio tucked safely out of the way, and have the lightweight and small speaker-microphone clipped somewhere convenient so that you can hear it and grab it without too much trouble. Sounds like a good idea, huh? You can buy these speaker-microphones from many outlets, but they really are a cinch to make, as I'm about to show.

 

 

Some time ago, a friend gave me a very, very, ill Philips FM620 UHF CB radio for parts. Parts indeed! I think the only useable parts were the microphone, the speaker and front panel. Looking at the Philips microphone got me thinking that it would be a nice size and shape for a speaker microphone. After taking the microphone apart and removing the dynamic microphone insert, I looked at trying to stuff a condenser microphone and one of those very miniature 27mm speakers from Dick Smith (cat. no: C2208) inside. The mini-speaker fits nicely into the recess made for the original dynamic microphone insert, and a condenser mic (Dick Smith cat: C1160) would just fit under where the PTT bar moves inside the case when depressed. That really is the hardest bit out of the way: confirming that all the bits required will fit.

 

 

The next step is to unsolder any wiring at the microphone end. The top mounted buttons will not be used, and we can cut off the DIN connector at the other end, too. Next, take your mini-speaker and use it to draw a circle in pencil on the front grille of where it will eventually fit. The front grille has a few holes drilled into it to allow sound to pass through to the old dynamic mic insert, but we want to drill many more holes to allow sound out from our mini-speaker. By drawing a pencil outline on the front grille of just how big the speaker is, we can drill the appropriate holes. I used a 1/16" drill to do this. I also made some holes in the appropriate area for the microphone. Clean up any burrs and remove any swarf.

 

 

The next bit is tricky - remove the PTT switch by undoing the small screw that holds it in place and then remove the PTT lever. Leaving the flying leads attached to your condenser microphone, smear a little contact glue around 1/3 of the circumference of it's case and using a pair of fine needle-nose pliers, locate the microphone with the glue contacting the old dynamic mic recess housing and the PTT lever stopper. Allow a few minutes for the glue to take hold. Reassemble the PTT lever and switch.

 

 

The next thing to do, is to select two wires from the microphone cable that will be used for the speaker. I used the red and black wires. I specifically did not use the white wire or the braid, since the white wire is shielded by the braid, and I wanted to use these for the microphone leads. I soldered the red and black wires directly to the mini-speaker terminals. Be as quick as you can soldering to this delicate speaker - any excess heat could damage it. Drop the speaker with it's newly attached wires into the old microphone recess.

 

 

Next, place a little bit of spaghetti insulation over the microphone's active lead. The active lead should be red, but if the microphone insert's leads are both the same colour, the lead that is not connected to the case of the insert is the active lead. Solder the active lead to the white microphone cable wire, and cover the join with the bit of spaghetti insulation. Next, take the earth lead of the microphone insert, and wire it to the centre tab of the PTT switch. Then, the braid of the microphone cable is soldered onto the outer tab of the PTT switch, so that the you have wired the switch to 'make' when the PTT is pushed.

 

 

Next, find something to hold the speaker in place so that it won't rattle around in it's recess. I used some double-sided tape and a plastic spacer to do this. I didn't use foam padding to fill the recess in and hold the microphone, although that could be done, it may reduce the sound output of the speaker a little.

 

 

Once all is neat and tidy, close up the microphone case. Give the microphone a good shake to see if anything is loose. If all is OK in this department, the last step is to put on the appropriate connectors at the other end of the microphone cable. A mono 3.5mm audio plug is used for the speaker output, and a mono 2.5mm audio plug is used for the microphone input. Strip the outer insulation of the mic cable back about 50mm. Strip and pre-tin about 3mm of the white wire. Don't forget to place the 2.5mm connector body over the white and braid wires. Place a short length of spaghetti insulation over the yellow wire, and solder the yellow wire to the centre tag of the 2.5mm plug. Move the insulation down the wire so it butts up against your solder join, but not over it. Solder the braid to the outer tag of the connector, and let it cool down thoroughly. Then using the crimp fins, fold these over to secure the wires you have just soldered. The spaghetti insulation is there to prevent the fins biting into the thin insulation of the white wire, and prevents a short across the connector. Be quick when soldering these connections. As I found out, too much heat or too much time taken to solder them will melt some of the plastic insulation within the plug and cause a short circuit.

 

 

Next, do a similar operation for the red and black wires for the speaker using the 3.5mm connector. I cut and stripped the wires a little shorter, since I want the speaker connector to take any direct strain when the microphone cable is pulled on. Again, use the insulation to protect the active lead when folding over those crimp fins, and don't forget to place the connector body onto the cable first.

 

 

By a wonderful coincidence, the hexagon shape of the connector bodies purchased from Dick Smith are just about perfect to glue together, side by side, and the spacing between the two connectors ends up being almost right for the standard speaker-microphone setup. Some double sided tape, about 1mm thick, will hold the two connectors together at exactly the right spacing. After doing this, I also opted to place some heatshrink over the two connectors to give some added strength in holding them together. I left some excess heatshrink extending up over the top of the connector bodies, so that I could fill in the space with some epoxy to really hold everything in place.

 

 

When the epoxy is setting, you can wipe any excess away for a neat finished result. Once the epoxy has set, trim any excess heatshrink, and smooth the surface of the epoxy with a fine grade sandpaper. If desired, the heatshrink can be removed and the hardened epoxy painted. If you think that the epoxy is a bit of overkill, you can rely on the heatshrink to do the job. To make a reasonably neat job of this, you can use some glue to seal up the heatshrink around where the mic cable exits.

 

 

So there you have it, a cheap, do-it-yourself speaker-microphone!