Project of the Month
A home-made desk microphone.
We will dismember one of those cheap indoor antenna's that you get from just about anywhere, fit a microphone and small microphone amplifier circuit, and make a desk mic out of it. These antennae, shown below, don't work very well as an antenna anyway, and this is a far better use for them. And the finished mic doesn't look half bad, either. The antenna shown below I got from 'The Warehouse' for $ 7. The amplifier circuit is a one-transistor amplifier running off two 'AA' batteries, with a quiescent current draw of 440 uA (micro-amps), so the batteries should last about 5 years with everyday use.
You will need:
A 'dish' type indoor TV antenna.
An electret microphone insert.
Some shielded microphone cable.
Two BC337 transistors.
A 50K log pot.
A pushbutton switch.
A 2 x 'AA' battery holder and 2 'AA' batteries.
Sundry resistors, capacitors, veroboard, wire, etc.
Step 1
Strip the antenna down.
Remove the sticker from the front of the feedhorn. Undo the screw inside the feedhorn. Remove the feedhorn and remove the dish. Cut the wires to the dish and throw the dish in the bin. Remove the dish mount. Remove the knob from the rotary switch. Remove the base. Remove the rotary switch and all the associated wiring, and throw it all in the bin. Remove the rabbit-ear-antenna mounts and throw them in the bin. Peel the label off the front and throw it in the bin.
Step 2
Assemble the mic head.
Feed the mic cable through the dish mount. Drill a hole just big enough for the mic cable, through the back of the feedhorn, beside the mounting screw hole. Leave enough room for the screw head. Feed the mic cable through the drill hole in the feedhorn. Solder the electret mic insert onto the end of the mic cable. Screw the feedhorn back onto the dish mount. Pull the cable back through the dish mount until the electret mic insert sits inside the feedhorn. You may have to wrap the insert in electrical tape to make a snug fit in the feedhorn so that it doesn't rattle. Push the mic cable down into the cable-run of the dish mount. Fit the dish mount back onto the base unit, feeding the mic cable down the neck.
Step 3
Fit the controls.
Fit the pot where the rotary switch used to be. Use a logarithmic pot. These are usually marked with an 'A', so your pot should be marked '50KA'. Linear pots are marked with a 'B'. If you intend to use the old knob, you will have to use a pot that has a shaft that matches the knob. The original knob may be a tight fit in it's recess, so you may have to fiddle a bit to get the pot in the centre. You may have to cut the shaft of the pot shorter, as I had to with this one. Drill a hole to suit your push button switch in the centre of the label recess.
Step 4
Build the amplifier circuit.
This circuit is not a RF circuit, so you don't need to keep leads short or build the circuit compact. Build it as sloppy as you like, as long as the connections are all good. Below is the final version I built on a small piece of veroboard, and the first test circuit built in mid-air. The veroboard is screwed to one of the mounting points in the base.
Transistors Q1 and Q2 can be any general-purpose small-signal transistor such as BC337, BC547, BC548, BC549, etc. Q1 and it's associated resistors form an amplifier with a gain of about 10. The mic insert is powered from the 3 volts by a 2K2 (two-point-two kilo-ohms) resistor. The audio from the mic goes into the pot, and then to the amplifier. Capacitors couple the audio into, and out of, the amplifier. When the push-button switch is pressed, 3 volts from the batteries is applied to the circuit to power-up the amplifier. 3 volts is also applied to the base of Q2 through a 10K resistor. Q2 turns on and pulls the 'TX' line to ground, sending the radio into transmit. If your radio works differently, you may have to use a double-pole switch for the push-button, and switch the 'TX' line as required. The LED is really optional, as it will drain the batteries quicker, but it will provide a TX indication and a battery status. LED's require 2 volts to operate, so if the LED does not light then it's time to change the batteries.
Step 5
Wire it all up.
Connect the positive of the batteries through the push-button switch, and to the amplifier. Connect the ground (battery negative) of the amplifier to the shield of the mic insert cable. Connect the pot as per the schematic diagram.
You will need a mic cord from an old mic, or some multi-core cable and a mic plug. The exact wiring of the mic plug for your radio I cannot give you, you will have to figure that part out for yourself. If you are using multi-core cable, make sure you use shielded cable, or RF interference may get into your mic and cause problems. Use an old mic cord if you can. Quite often in mic cords, only one wire is shielded. This wire is used for the mic audio, and don't forget to connect the shield to ground.
That's it. Fit two 'AA' batteries, screw the base back on, and plug it into your radio. Turn the mic gain up, lean back in your chair and key the mic with your toes.
Final Notes
This circuit should have plenty of gain for you to sit back and talk on the radio. With this mic connected to my Icom IC-400, and the mic gain turned up to maximum, listeners on the channel could hear what the people in my kitchen were saying. However if your radio needs more drive, then build the two-stage amplifier circuit HERE.
For those who don't want to over-modulate, build the add-on clipping indicator circuit HERE. (coming soon)
Or even better, the modulation meter circuit HERE. (coming soon)
Radios like the Icom IC-400 provide power at the mic socket, so you don't need batteries.
Get the full circuit of this mic for the Icom iC-400 HERE.
If you substitute the push-button switch for a toggle switch, you can lock the transmit on, for those long overs, or music-playing...
To give your desk mic the 'store-bought' look, get one those foam wind socks and fit it over the feedhorn. Make a label on your computer to fit into the old label recess, print it out, cover it with clear 'contact', and stick it down with double-sided tape.
