Problem with high noise in a digital speedometer

[ansh1208]

Hello everybody....first of all i'll like to tell you I'm not very good in basic electronics. I'm a mechanical guy, but i love doing electronics. I'm trying to make a digital speedometer for my car. I tried this circuit (diagram attached) for feeding the pulse from sensor installled in vehicle to microcontroller. This is working absolutely fine on my Table, but when i try to install it on vehicle the noise is so high the 16x2 alpha-numeric LCD shows abnormal characters & hangs up. I tried putting different capacitors, inductors in sesnsor supply. I even tried placing a MOV but none is helping out. As the source of supply to the circuit & sensor is same I think there's no use placing a opto-coupler? How can i reject this vehicle noise.

Waiting eagerly for your reply & suggestions. Thanks

 

[Freddybaby]

digital speedometer circuit

Looks like you could use a little filtering on the hall effect output to the transistor. Also, whats the frequency response of the hall sensor ?

Added after 5 minutes:

How long are your wires going from the sensor to the uC ? An opto might be what you need, or possible run the return through a common mode choke, worked for nme with noise issues from long sensor cables.

 

[DrBob13]

digital speedometer for car

Connect the sensor to your circuit with a twisted pair of wires and ground your electronics on the clean side of the common mode choke so that the hall sensor and the electronics see a common clean ground point. As suggested in the previous post a small capacitor across the hall sensor output to clean ground may help, try 1nF to start with and increase it a bit if necessary, the data sheet of your hall-effect sensor may help you decide if this is a permitted option.

I hope this helps.
Bob.

 

[millwood]

schematics digital speedometer

it is a miracle that the design had worked: Q1 has no bias so the output should always be high.

you should figure out a way to bias Q1. The simplest is to put a resistor between Q1's collector and base, and then replace D3 with a resistor. the values will depend on the sensor used and your desired gain.