An inexpensive toy called the "Spy Ear" may be easily modified to make a super-sensitive microphone for an ordinary PC microphone input. The Spy Ear shown below is from an eBay lot and cost about a quarter after shipping and they are commonly seen on racks of party favors. The circuit consists of a nice electret microphone connected to a three-transistor amplifier featuring high gain, low noise transistors. For this modification, the output transistor that drives the ear buds is removed (the transistor nearest the audio jack).
There are at least two versions of the board with variations in how the earphones are connected:
There are at least two versions of the board with variations in how the earphones are connected:
The version on the right connects the two ear buds in series and two jumper wires are needed. First, remove the transistor near the audio connector and jumper the collector pad to the battery positive. I used a short blue insulated jumper for clarity but a short bare piece of wire is fine since the traces are next to each other. Next connect a jumper (longer blue wire) from the center pin of the audio jack to the ground (switch terminal). Note that on this version, no trace goes to the center pin of the audio connector.
The PC will supply power to the circuit and the switch and volume control will still work. For even more gain, select "microphone boost" in the soundcard menu. Expect feedback if the mic. is in the same room as the computer!
The schematic below shows the goal of the modification. The tip and ring of the connector are connected together and to the point where the positive battery terminal connected. The shield connects where the negative battery terminal connected. It might be a good idea to remove at least one of the battery terminals so that a battery isn't inadvertently installed in the future. More gain may be had by increasing the 680 ohm resistor to 2.2 k.
You will need a long cable and a monaural cable is fine as long as you use a stereo plug for the computer end with the tip and ring connected together. Otherwise, the circuit will not get power.
The PC will supply power to the circuit and the switch and volume control will still work. For even more gain, select "microphone boost" in the soundcard menu. Expect feedback if the mic. is in the same room as the computer!
The schematic below shows the goal of the modification. The tip and ring of the connector are connected together and to the point where the positive battery terminal connected. The shield connects where the negative battery terminal connected. It might be a good idea to remove at least one of the battery terminals so that a battery isn't inadvertently installed in the future. More gain may be had by increasing the 680 ohm resistor to 2.2 k.
You will need a long cable and a monaural cable is fine as long as you use a stereo plug for the computer end with the tip and ring connected together. Otherwise, the circuit will not get power.
