I have a little project I'm doing and I'm not sure if the way I'm doing it will cause problems or not so I wanted to ask someone with more experience than myself.


I have a reluctor wheel that gives a nice 12v square wave signal to a tachometer. The problem is I need a 5v signal. Can I use a simple voltage divider to do this safely without hurting the tachometers performance? If so how should I go about it as I do not know the load of the tach etc..



Sorry for such a stuiped question!

 

Would a simple transistor work ok? I'm concerned with 12v being on the base with only 5 v coming down from the collector.

 

Making a continuous 5V signal (if that's what you want) from a 12V square signal is not very simple as you will need some kind of rectifier and a voltage regulator...

 

thanks for the reply..

No, I need a 5v signal of the same type of the 12v. Basically I have another tach device whose input needs to be no more than 5v.

 

Clamp it with a 5V zener?

 

You could use a small FET, like a 2N7000. Set it up as a source follower, with the source to ground. Put the 12 volt signal to the gate through a 100 - 470 ohm resistor (whatever is handy), and use a 360 ohm resistor to pull the drain up to the 5 volts. Take the signal off the drain. It will be out of phase, but tachs don't care.

 

You could use a small FET, like a 2N7000. Set it up as a source follower, with the source to ground. Put the 12 volt signal to the gate through a 100 - 470 ohm resistor (whatever is handy), and use a 360 ohm resistor to pull the drain up to the 5 volts. Take the signal off the drain. It will be out of phase, but tachs don't care.

360 ohms might be a waste of current, depending on how much drive the tach needs, but it's an OK place to start. A simple attenuator might work, or, as mrmeval suggested, a series resistor and a shunt 4.7V zener. It's hard to design an interface between two pieces of hardware when you know nothing about them except voltage levels.
"Source follower" is misleading. A source follower takes the output from the source, and the voltage out is the same phase as the input, but level shifted.

 

Thank you guys for the replys.

Can you give me a little more info as to a series resistor and a shunt 4.7V zener? Can you share a schematic with me?


Would somethingl ike this work?

12V square wave unknown current---------------Tach 1
                                                     |
                                                     |
                                                     |
                                                     |
                                                     +--Resistor----Zener diode---Tach 2

 

Should also mention this is DC

 

This shows implementation of both suggestions, zener above and FET below:

 

This shows implementation of both suggestions, zener above and FET below:View attachment 1924

That is fantastic!!!! Thanks a ton!

 

Just want to let you guys know that we ran it tonight and it worked flawlessly! Thanks a ton.


Next question.. and I might have to open a new topic for this.. I've got a schematic that I would like to produce around 50-100.. Does anyone work in this field? I would like professional review of the schematic and schematic tracing etc.

 

A separate topic would be best.

 

hey,
i bet a simple resistors voltage divider would do just fine, alvin.
a zener will slightly change the duty cycle, right?
the resistor voltage divider will do just fine, cause your 5v tach device should've been eqquipped with high impedance input.

 

hey,
i bet a simple resistors voltage divider would do just fine, alvin.
a zener will slightly change the duty cycle, right?
the resistor voltage divider will do just fine, cause your 5v tach device should've been eqquipped with high impedance input.

But if the 12V is directly connected to the vehicle battery, it will fluctuate, causing the output of the voltage divider to fluctuate. A zener or transistor solves this potential problem.

 

you're right there ron . i didn't think about that.

 

you're right there ron . i didn't think about that.

I suggested it earlier (the "simple attenuator"), and I didn't think of the potential problem until later.