Commercial units often use a rotating magnet and either e reed switch or a hall cell. Units are available which screw onto where the cable usually goes and the cable screw into the back of them. A magnet and a coil could be used allowing an opamp to register speed down to some lower speed when the signal becomes too small to be reliable. Differential input and appropriate magnet and coil would allow this to be very low. If you have access to the inside of the speedometer you could possibly sense the existing magnet. Depending on speed of response (and the purpose you could sense the odometer (distance meter) movement in some manner. RM -----Original Message----- From: Riaz Ahamed To: PICLIST@MITVMA.MIT.EDU Date: Tuesday, July 13, 1999 5:23 PM Subject: Re: [OT] Speed Sensor > You can use a reed relay for this pupose. It's cheap! > >thanks >Riaz > >Des Bromilow wrote: > >> Is it worth building soemthing based on exisitng speedo ideas? >> >> a rotating magnet pulls the needle against a spring.. >> >> Why not use the rotating magnet to generate pulses, or to generate an average DC voltage (which could be fed into a PLL VCO) >> >> Just batting ideas around. >> >> Des >> >> >>> Wagner Lipnharski 7/13/99 12:32:07 am >>> >> Does anyone knows some supplier for "speed transducer" to be installed >> at the car speedometer cable? >> My circuit needs to receive a frequency proportional to the car speed, >> and I need to find out ... well... cheap sensors... :) >> >> Thanks >> Wagner Lipnharski >> http://www.ustr.net >