Pressure-activated circuit

[anasasisxenophontis]

I'm working on a circuit for an automotive application. It will use the voltage output from a manifold absolute pressure sensor to activate a relay and turn on a solenoid. I'm rather new to circuits, so I'm unsure of what diode and resistor to use in the circuit.
I want the V-out to run through a zener diode so that the relay will only activate when the MAP sensor passes enough voltage through the resistor to ovecome the zener voltage of the diode. The input of the MAP sensor/relay side of the circuit is 5v @ 850mA.

**broken link removed**

 

[Reloadron]

Typical MAP sensor output I think is in the .5 to 4.5 range give or take between idle and wide open throttle. You would need one small and sensitive relay for your circuit to work. The breakdown voltage of the zener, the value of R and the resistance of the coil would all figure into it.

For what you want to do I would consider taking the MAP sensor output and running it to a comparator circuit using maybe an LM339. You could then easily set up a reference voltage on the comparator to change states, drive a small transistor to close a relay for a pretty precise manifold vacuum. You could even calibrate it.

Ron

 

[anasasisxenophontis]

Thats awesome, this actually simplifies my circuit by using a little more complicated part. So what would the pins for this IC be? Correct me or add to this if I'm wrong:

2 pins for resistor reference voltage
2 pins for MAP sensor input voltage
2 pins to energize the transistor

Anything else? It's been several years since I've done any work with circuits so I've forgotten most of it, but the help is really appreciated!

 

[tcmtech]

A lot of this will depend on what type of output your MAP sensor uses. Many use various frequency or PWM type outputs or even full digital logic and not a proportional voltage output.

What kind of vehicle is it?

 

[anasasisxenophontis]

The MAP sensor is from a late 90s Toyota Camry, I don't really know what year. I just found it in a junkyard and ripped it off. It produces a DC signal though, proportional to how much pressure I apply to it. It runs on a 5v DC input and puts out about 3.33v on the return line at standard atmospheric pressure.

 

[Boncuk]

I suggest to go with Ron's suggestion.

Am I right assuming that intake manifold pressure is negative (vacuum) pressure?

Boncuk

 

[anasasisxenophontis]

I think I will. It does sound like a better idea. Intake manifold pressure is typically negative, but this sensor will not be in the manifold.
So, if I use this IC chip, what else do I need for the circuit? Just the IC, transistor and relay and that's it?

 

[Reloadron]

The MAP sensor is from a late 90s Toyota Camry, I don't really know what year. I just found it in a junkyard and ripped it off. It produces a DC signal though, proportional to how much pressure I apply to it. It runs on a 5v DC input and puts out about 3.33v on the return line at standard atmospheric pressure.

This link will give an overview of several sensors including the sensor you have I believe. Then **broken link removed** has a good overview of comparator circuits using the LM339 I mentioned earlier. Sometime today I will try and make a basic schematic. Depends on after work time. The links should give you an idea of what I was getting at.

Also:

Typical Readings:
Sensor output voltage range is 0.5 to 4.5 volts.

Output voltages between 0.5 and 1.5 volts indicate a high vacuum (low pressure) situation, such as idle or deceleration.

Output voltages between 1.5 and 3.0 volts indicate a medium level of vacuum (pressure) such as a cruise or slight acceleration condition.

Output voltages between 3.0 and 4.5 volts indicate a low vacuum (high pressure) situation such as hard acceleration or a mechanical failure.

Any reading of 0 volts or over 4.5 volts indicates a problem.

Something else I have seen done is using a LED bar graph driver chip with manifold pressure sensors.

Ron

 

[Reloadron]

Attached is a drawing using two of the four comparators found in a LM339 chip. It should help you get the idea. One circuit goes active when the signal is above refrence and the other goes active when the signal is below refrence. Likely a better choice would be to use a LM393 as it is a dual comparator rather than the LM339 quad chip. Since I am only using two of four comparators on the LM339.

Ron

 

[Boncuk]

I think I will. It does sound like a better idea. Intake manifold pressure is typically negative, but this sensor will not be in the manifold.
So, if I use this IC chip, what else do I need for the circuit? Just the IC, transistor and relay and that's it?

You'll need someone who sucks on the sensor pressure port.