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Speed Sensor Interface For a Microcontroller

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What would be a good way to interface a frequency signal from a transducer (speed sensor whose output is a varying frequency signal) to a microcontroller? The peak to peak voltage of the signal varies from a few volts to 120V but probably very low current. Is an instrumentation amp the best choice here?
 

dr pepper

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You could divide the volatge down to a manageable range like 10v with a resisitor divider, hen use a comparator, or window comparator to square up the signal.
Pretty much whats inisde the chip semaphore mentions.
 

Semaphöre

New Member
I'm not sure what's in it but if that's how a VR sensor behaves I guess it is, and it does have 2 wires. Is that the same as a hall effect sensor?

Thanks for the help!

No problem!

A VR sensor isn't quite the same thing as a Hall effect sensor: VR sensors are passive devices that generate a current proportional to the speed of the metal encoder tooth whizzing by them (due to the lower reluctance of metal). Hall effect sensors are active devices (they need external power) that detect metal in front of them using the Hall effect and an internal biasing magnet. The big difference is that (automotive) Hall effect sensors don't need the metal to be moving and produce a microcontroller-friendly square wave.

I'm not really a physics expert, so maybe someone will chime in with a better explanation :)
 

Mosaic

Well-Known Member
The OP needs to define 'few' peak to peak volts. If this is in excess of 6Vp-p then a schottky rectifier, a 10K resistor & 5.1V zener clamp effectively makes the signal 5V logic ready. Input this into the T0CK counter pin of a PIC and run a fixed time loop or interrupt to determine the pulse rate. Job done.
 

MikeMl

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What would be a good way to interface a frequency signal from a transducer (speed sensor whose output is a varying frequency signal) to a microcontroller? The peak to peak voltage of the signal varies from a few volts to 120V but probably very low current. Is an instrumentation amp the best choice here?
Try the circuit I posted here.

V1 is your sensor (needs to have d.c. continuity). Circuit works from mV to hundreds of V
 
The OP needs to define 'few' peak to peak volts. If this is in excess of 6Vp-p then a schottky rectifier, a 10K resistor & 5.1V zener clamp effectively makes the signal 5V logic ready. Input this into the T0CK counter pin of a PIC and run a fixed time loop or interrupt to determine the pulse rate. Job done.
Well, the sensor in question is a Ford Vehicle Speed Sensor (VSS), the type that has a gear on it that is driven by the output shaft of the transmission. The signal starts at 0V then goes up to around 80V pp at 100 MPH.
 

atferrari

Well-Known Member
You could divide the volatge down to a manageable range like 10v with a resisitor divider, hen use a comparator, or window comparator to square up the signal.
Pretty much whats inisde the chip semaphore mentions.
Why not 5V instead? Wouldn't be more consistent with common input pins of a micro fed with +5V?
 

dr pepper

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Most Helpful Member
Would that be an mtx 75 transmission?
I dont know the actual circuit however its common for ecu's to clamp the voltage to a sensible level.
Kinda buffer the signal with a resistor, then clamp to gnd with a zener then go to a transistor level shifter or if you like an opto.
vrs type sensors are now being replaced with magneto rsesitive, the o/p's of which however are analogue.

Are you making a speed restricter disabling device?
 
Would that be an mtx 75 transmission?

No, it's a C-4.

I dont know the actual circuit however its common for ecu's to clamp the voltage to a sensible level.
Kinda buffer the signal with a resistor, then clamp to gnd with a zener then go to a transistor level shifter or if you like an opto.
vrs type sensors are now being replaced with magneto rsesitive, the o/p's of which however are analogue.

Are you making a speed restricter disabling device?

I'm making a digital speedometer. The vehicle in question does not currently have a VSS but I can get one from a later vehicle which uses it for cruise control.
 

dr pepper

Well-Known Member
Most Helpful Member
Sounds like your in the US, isnt the c4 a classic auto box, if they were made with a vss they must have a long heritage.
Some schems on the net just show a shunt resistor across the vss sensor, looks as though they just load down the sensor to a low voltage, maybe you could try something along these lines using a battery drill to turn the sensor.
My car vss goes direct to the speedo, maybe theres some in the US that do the same, the i/p circuit from one of these might be a good donor, looking up the datasheet for the chip would be usefull if you went this route.
 
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