Count wheel rotation sensor

[Quintin]

I am currently working on a project that uses a magnetic pickup sensor and a magnet glued to a wheel of a RC car.  The magnetic pickup sensor returns a voltage signal that is later converted to RPMs.  The voltage is above 3.5 V when the magnetic pickup "sees" the magnet and below 1.5 when it doesn't.  I would like to replace this magnetic pickup with some type of sensor that could just send a high voltage (analog or digital) when it "sees" the spokes of the wheel.  Does anyone out there know of the sensor I need?

thanks in advance,

Quintin

[Otis]

HI Quintin,

It sounds like the system you have now will work fairly well, but you are wanting to increase the resolution of your measurement.

The first place I would look is swing by a hobby store and see if they have any sort of device that could show you the 'speed' of the RC car.  It would probably be easier to use a solution like this and then use the sensors in it, than to make your own circuit.

However, if you are wanting a high amount of accuracy and weight is not a huge concern, then you may want to consider such device as a Quadrature Encoder or a Hall Effect Sensor (These work really well).  Keep in mind that these solutions will most likely be larger (once you add in all the extra signal conditioning) and definitely more expensive.

If anybody else has any ideas feel free to post them so that Quintin gets a more well-rounded perspective on what's available.

Regards,

[AnalogKid2DigitalMan]

Opto-interrrupters:

Devices have a light source, typically a visible or infrared light emitting diode (LED) and a phototransistor which recieves the light from the LED. Typically when the light is fully blocked (from a spoke for example), the phototransistor is OFF and when not blocked output is ON. ON/OFF states can be inverted depending on how it is wired up and some can even provide a continous analog voltage output. readily available from any electronics house (Allied, Newark, Digi-Key, Mouser, etc)

Can be sensitive to ambient light in some cases and light source must be chosen so that spokes completely block the light. One side of the device will have to reside outside of the wheel- don't know if this could be an issue or not.

There are also capacitive and inductive proximity sensors/switches, but without specifics of the car they may not be a good choice.

As Otis suggested, an encoder may be the way to get rolling