Turn Digital Signal into Solid 12v Signal

[Adam60]

If one wanted to take a digital signal or sine wave signal crom a vehicle spped sensor and turn it into 12v dc, how would you go about it. So I would need a 100% solid 12v signal. I am thinking it would have to be something controlling a relay or some type of switch that would latch when it sees a signal, and unlatch when it does not. Any ideas appreciated. Diagrams??..

[james_s]

What you want is effectively a low pass filter, there are lots of ways to do this. A very simple way would be to use a R-C filter with the output of that feeding one input of a comparator. The other input of the comparator could go to the wiper of a pot connected between 12V and ground, or you could figure out the threshold you want and just use a divider made of fixed resistors.

[Housedad]

I don't know for sure, but I have a question about doing that.  Would it not make a difference to the speed sensor circuit if the speed sensor detection is current based?  Is it current or voltage based?  They are hall effect sensors of some type on the tone rings.  If current based sensing, would it not screw it up to use that signal directly for anything else?

Someone around here would know the answer I hope.

[Adam60]

Vehicle speed could be Hall Effect or variable reluctance so I need a a way to change both if possible. I can't take away too much current from the circuit or I will run into more problems.

[ArthurDent]

Use an opamp configured as a buffer so you don't load the driving circuit. The opamp output can drive a diode with a cap to rectify the output to get a D.C. voltage to turn a transistor on/off to control a larger relay or drive a sensitive relay directly.

[Adam60]

Would you have a diagram of such a circuit? That sounds like something that I am looking for. I was thinking if I could run the 12v through a relay, I would just need a low amp circuit to energize the relay coil. It takes roughly 100mA to do that.