Turn on at Negative Trigger

B

bdiddie

New Member
Use signal wire to determine output

Here is my problem; I have a PWM signal that controls a negative trigger solid state relay to control voltage to a light source.

When the PWM signal is On-Low, it shows 0.0V and triggers the relay. At Off-High it shows 12.8V

I need an ic that will monitor the PWM signal / voltage. When input is 0.0V then output 0.0V to complete circuit. If input voltage is anything else; i.e. 1V - 11V then there will be no output. In essence I want to stop varible voltage from passing to the relay, turning it into a standard on at 0V off at 12.8V signal.

Would something like this work? NC7SZ384 - 1-Bit Low Power Bus Switch

Thanks
 
Last edited:
As I said on the other forum.

Please post a schematic.

Is this what you want?
>0 output high
= 0 output low

How near zero does the voltage need to be?

There has to be a switching threshold, it could be 0V but in order for the output to be off, the input would need to be below 0V i.e. a negative voltage. So if you want 0V I suggest settling for just a bit above 0V i.e 0.1V.
 
Why use PWM? Sounds more like On Off keying. Agree with Hero, post a diagram.
 
This is actually the circuit for an automobile's daytime running lights. The wire I have circled in red is what I am trying to monitor for negative ground.

When the head lights are off the wire shows 12.8V.
DRL mode is at 70%Duty cycle so - approx 9V
When the head lights are full power wire shows approx 0.0V


So I can get by with

>0.5 output high
= 0.5 output low

**broken link removed**
 
Last edited:
This circuit detects voltages above 0.6V.

It has some hysteresis for noise immunity.
 

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After reading my original post I guess I did clearly state what kinda circuit I need. Although my signal wire I will be monitoring is PWM, I think that is a non issue. Sorry for the inconvenience, hope I can still get a solution.

Here is what the circuit needs to consist of.

1) Signal wire
2) Battery(+) Voltage wire; (as this is automobile voltage will be between 12.5V - 14.8V)
3) Ground (I would assume!)

Here is what I need to accomplish:
Monitor "signal wire" ~~~> Voltage on this wire will be between 0.035V to 14.8(or = B+)

If signal voltage is > ≈ .75-1.0V Then
output = B+ Voltage

Else output = signal voltage

Does this make sense?
 
Last edited:
Tried a comparator yet?
 

Sorry I don't know what you mean?

What's the B+ voltage?

It's PWM, the voltage will either be nearly 0V or nearly +V. If you want to change that to different voltages, use a level shifting similar to what I posted previously.

If you want to convert the PWM to a DC voltage, use a low pass filter.

If you want to trigger an output when the PWM is above or below a certain duty cycle, add a comparator to the output of the low pass filter.
 
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