I am putting a late model engine into an older model car and need some help regarding the vehicle speed signal.
The car produces a 12v 50% PWM signal for the speedometer and transmission. The engine management system for the new engine requires the speed signal to be 5v 50% PWM.
Rather than fork out $200 for a brand name signal converter box, I would like to find a simple alternative I can put together myself. I believe using 2 resistors to make a voltage divider would work, but I'm worried that it may have a negative effect on the existing signal.
Can someone please help me out with an easy circuit that won't be taxing on the existing signal?
I'm a mechanic, so my knowledge of creating this type of stuff is minimal. Any help (preferably with a diagram) would be much appreciated.
Best Answer
Probably doable very simply.
Definitely doable with not TOO much effort.
2 resistors should work OK.
For 12v-5V with no loading by the output, the resistors would be in the ratio 7:5 (7V dropped across the top one, 5V across the bottom for 12V total).
7/5 = 1.4:1
So eg a 1500 Ohm and 1000 ohm (12 to 4V)
or 1500 + 1200 (12 to 5.333v).
Depending on how variable the 12V is and how critical the 5V is such a simple solution may work.
If the output is low resistance it may load Vout down. This can be compensated for by changing resistors. A zener diode "clamp" on the output keeps the Vout more constant and provides a degree of protection against over-voltage.
simulate this circuit – Schematic created using CircuitLab
If the 12V may be eg 11-14V and/or the %v must be 5 +/- 0.1V or ... then more precise solutions are not much more complex. If you have a better guide to voltage ranges and if you can provide vehicle and equipment model/brand details it may help.
Safer methods which better isolate the 12V from the 5v circuits can be designed if desired.