# Inverting a negative PWM signal to a Positive PWM

#### Bobcata300

Mar 25, 2020
5
HI GUYS
I’m a automotive with 40 years of experience, pretty good with diagnosing and repairing electrical circuits and components that are OE installed. Heres my issue. My personal project car, a 1969 Mustang Mach 1 has a OE Ford cruise control servo in it out of a 2000 Mustang. Its uses a positive PWM signal from the ECM to calculate the speed the car is traveling, heres the rub: I’ve installed a Holley Performance Terminator stand alone computer that sends out a negative PWM signal. I need to invert the signal. I’ve tried to figure it out on my own. I guess a NOT gate would do it. Is it something I can build easily? Or is someone willing to build me a module that can perform the task? Willing to pay. I belive its a 5 volt DC circuit

Oct 5, 2014
6,707

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,476
What do you man by "positive pwm signal" and "negative pwm signal"?
Is the polarity of the signal positive or negative, or do you refer to the active state of the signal (pos being high pulse and neg being low pulse)? See my sketch:

In case 1 you need an amplifier with a gain of -1.
In case 2 an inverter does the job. A CD4014 IC, for example, works from 5 V to 15 V operating voltage and does the job. Or build a simple inverter from a MOSFET. The latter is probably more robust and therefore suitable for application in your car. This is a circuit similar to the one @Bluejets linked, only using a different type of transistor. Both circuits will work equally well, I guess.

#### Bobcata300

Mar 25, 2020
5
Harold,
It would be the first scenario, The output signal is negative polarity, and I need a positive signal. What is an amplifier with a gain of -1?A part number, wiring diagram, etc.. would be helpful. Your first picture looks perfectly like what I want to do.

#### Bobcata300

Mar 25, 2020
5
I'm also interested in a signal generator, so I can test what I make before installing in car. Would like something handheld that can output 5 and 8 volt signals, frequency signals and a/c wave forms.

#### WHONOES

May 20, 2017
1,217
Your description is a little confusing. Modern cars have a negative earth. This means that the chassis is connected to the negative terminal of your battery.
In technical parlance, the negative terminal would be called 0V. Circuit voltages would then be referred to 0V be they positive With Respect (WR) to 0V or negative WR to 0V.
Your description seems to infer that the signal is below 0V. Is that the case. If so there is a simple solution to your problem. The other scenario is that the signal is positive WR 0V but that the mark and space needs to be changed (inverted). If that is so then the solution again is simple. We just need to know which it is.

#### Bobcata300

Mar 25, 2020
5
The computer sends out a ground signal, you electronic guys call it 0v I guess. In automotive language is just ground, so I think it looks like a square wave form, except its pulled down. I might need to ck with the Holley engineers as to the exact signal. I just know what the manual says. It should look like Harolds first diagram.

#### WHONOES

May 20, 2017
1,217
So you are saying that the signal goes below 0V.

#### Bobcata300

Mar 25, 2020
5
Now that you say it like that, probably not. I believe its justs a pulsed open to ground signal. be kinda hard to go below 0v I believe. let me call the tech line tomorrow and get back to you guys. Thank for taking time to help me.

#### WHONOES

May 20, 2017
1,217
It could go below 0V but it is unlikely.

#### Kiwi

Jan 28, 2013
462
I think what he means is that the Holley output is a sink type. It pulls voltage down to negative ground.
The cruise control requires a source type input. It pulls the voltage high: 5v in this case.

What I have done before is to connect the output and input to a 10kΩ resistor. The other end of the resistor is connected to a 5v supply, eg 7805.
When the Holley output is low(on), the cruise control input will be low.
When the Holley output is high(off), the cruise control will be 5v.
Might seem a bit crude to the purists, but has worked for me when I have had inputs and outputs that are not totally compatible.

#### Bluejets

Oct 5, 2014
6,707
What I have done before is to connect the output and input to a 10kΩ resistor. The other end of the resistor is connected to a 5v supply, eg 7805.

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,476
According to this manual (and I assume these units are all similar), the pwm outputs are configurable:

In any case there are no negative voltages involved, only ground (0 V) and +12 V.

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