How to detect (12v) pulse from two wire meant for speedometer/Taxi meter

[bikashh]

Dear friends,
I am trying to design a circuit,to detect (12v) pulse from wires meant for speedometer/Taxi meter interface.but don't know how to make a circuit.please help.

Thanks in advance

 

[randallfarmer]

Needsome more info. what are you wanting to interface to, theres alot of different ways to do it but it all depends on the outcome you are intending.

 

[bikashh]

these two wires are available in every vehicle which comes from the speed sensor connected in the vehicle.i want to count these pulses and later send to the server using vehicle tracking system device.


thanks a lot for the reply

 

[randallfarmer]

Ok, helpful but I assume you want to store this on said device and retreive the info later? Also, I am assuming you are trying to detect the sppen at which your taxies are going? Or does this pulse only happen at every x distance? Therefore you want the total distance traveled for his/her route.

 

[bikashh]

yes i just want to read the speed at a particular time and than send it to the server.also if possible please help me in converting pulse count to Km/Hr.

 

[randallfarmer]

I found this... **broken link removed**

Basically, you need to change your pulse to an analog signal, you can then send this to a uC and with the use of timers and some code convert this to your desired outcome. When converting your pulse pay attention to the output voltage that will go to your controller most controllers will only handle 5 VDC.

 

[keith1200rs]

That is not a good way to do it. Converting a digital signal to an analogue one to convert it back to a digital one!? Just level shift the signal and measure the pulse rate/count pulses with a microcontroller.

Keith

 

[swordfish01]

Hi,

Choose a micro controller that has a capture-compare unit, that will allow you to measure the time between single pulses. From the time between single pulses you can calculate the speed, from counting the pulses you can integrate the traveled distance. It might also be helpful to have the possibility of raising an interrupt when the signal changes.

As keith1200rs said, you'll have to level shift the signal before connecting it to a uC, that can be done via dedicated ICs (more expensive), or via some simple circuits (use g00gle or forum search to get an overview)

Regards,
Andreas

 

[bikashh]

i've found this site please have a look

Microcontroller-based Speedometer-Cum-Odometer | Engineering Projects

 

[swordfish01]

The schematic won't work in your case since it uses a reed-relais and a magnet mounted to the wheel and does not decode a pulse from the bike. Apart from the the circuit seems ok, and the logic after the optocoupler should be the same as in your case. I wouldn't use such an old controller if I don't have a good reason (existing programmers, etc.) for it though.

 

[luben111]

What is the problem simply to put one 10K resistor - the protective diodes on the input will limit the high voltage to Vcc and GND will be still GND. ANd then to read this data with interrupts or directly in the timer

 

[bikashh]

i think the pulse received will be disturbed.will need a shaper

 

[luben111]

A simple RC filtering can completely remove any kind of glitches. I don't see any reason why not to use resistor and RC filter. It's cheaper and more robust solution.

 

[bikashh]

can i use an optocoupler which will provide a better isolation also.

 

[keith1200rs]

Yes, an optocoupler would be a good way of providing isolation and doing the voltage level conversion.

Keith

 

[bikashh]

I am not sure about the voltage i'll get from the cable.
can anybody tell me the specification of the pulse that i'll get at the cable end. please help.
even if i detect the pulse how should i count it.should i check the pin all the time.or should i write an routine and execute it when i get an IRQ signal

Thanks in advance