Cheap Telephone Microphones

[phurm]

I have salvaged several microphone elements from cheap electronic telephones.<p>I need information as to impedence, are they electret, etc. I would like to be able to use one of these mics with a tape recorder and possibly later with a digital recorder of some kind.<p>Is some circuitry needed to interface or just plug it in?<p>TIA.<p>Paul

[russlk]

Who knows? You will have to experiment. Try attaching to an amplifier, if it works, great. If not, then measure the resistance at the microphone terminals. If it reads open circuit, you will need to supply some voltage. Try 1 meg to 12 volts and see if that works.

[rshayes]

The shape and size will tell you something. Electret microphones are the most likely. These are usually in a small cylindrical aluminum case around 3/8 inch in diameter and about that long. The terminals are usually either leads or solder pads on one end. There may be either 2 or 3 leads.<p>Carbon mikes are a possibility. These were usually about 1-1/2 inch in diameter. Connections were usually made by pressure to a round contact and ring on the back. These are more likely to show up on non-electronic phones.<p>Crystal and ceramic mike s were made in a wide assortment of packages, but probably were not used much in electronic telephones. They require a high impedance load for best results.<p>Dynamic microphones have some type of magnetic structure. This makes them fairly heavy, and the case is usually steel, since it is often part of the magnetic path. Some dynamic microphones look like small loudspeakers.<p>The illustrations in the Mouser or DigiKey catalogs will give you some idea of the sizes and shapes of the various types.<p>The electret microphones have a permanently polarized dielectric element between two capacitor plates, one of which is moved by the incident sound waves. The charge generated by the diaphragm motion changes the voltage on the capacitor, and this is buffered by a simple amplifier, usually consisting of one MOSFET. This is all located in the cartridge. The 3 lead cartridges usually bring the source and drain of the MOSFET out as well as ground. This allows the MOSFET to be used as a source follower or common source amplifier. The 2 lead versions have the MOSFET wired as a common source amplifier with the ground and drailn brought out. This is probably the most common.<p>The drain lead needs to be connected through a load resistor to a positive power supply. The audio signal appears across the load resistor. Typical values for the load resistor appear to be in the 2K range. The supply voltage is in the 3 to 5 volt range. Some units appear to have maximum ratings of 10 volts, but 5 volts is probably safe. Operating current is in the 1/2 milliamp range, so the drop across the load resistor will be about 1 volt.<p>I would expect the ground lead to also connect to the case. This can be determined by visual observation or an ohmmeter. If you use an ohmmeter, use a digital meter. The analog meters can deliver excessive currents on the low ohms ranges and excessive voltage on the higher ranges.<p>Ceramic and crystal mikes will appear as very high resistances shunted by capacitance. Carbon microphones will be 100's of ohms, while dynamic elements will be 10's of ohms. The electret mikes will probably show different resistance measurements if the leads are reversed, although I haven't checked this. The other types should measure the same when the ohmmeter leads are reversed.