All the examples Ive found for 555's show how to source current (turn on an LED, etc). How can I get it to sink current? Ideally, my circuit is going to sink an unknown amount of mA (shouldnt be very high, but 555's are cheap anyways), from 5v (signal is pulled to 5v inside PCM) down to GND @ 57Hz, running off 12v. If someone would toss this together real quick, that would be nice But just telling me how to sink current would work just as well.

EDIT and ha ha to everyone who says "this sounds like homework". Im using it to emulate a Flex-Fuel sensor on an '00 Ford Ranger:

 

It will source current if you connect the load to the low rail and it will sink current if you connect the load to the high rail (then the load is ON when the output is low).

 

Basically, make the flasher ground through the 555 timer (lookie at the picie)

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MechTronics

World of Warcraft is a disease

 

So really theres nothing different I need to do? Just hook it up to the sensor wire and it will sink current?

 

Yes, as long as it is grounding through the 555 timer, it will work just fine. I generally design circuits around Source, but to make simple flashers just have 1 LED sink and source, making it alternate as the square wave rises (5V) and grounds (GND).

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MechTronics

World of Warcraft is a disease

 

Note: 555 timers can sink and source 200mA.

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MechTronics

World of Warcraft is a disease

 

Since the sensor wire is pulled to +5v by the PCM, I should be using an LM317T to drop the supply voltage of 12v to 6.7v, since Output = VCC - 1.7v, right?

 

Actually now that I think about it, would it be better for me to just supply the IC off 12v and use a transistor connected to the output of the 555 to pull the signal wire to ground?

 

The output of a 555 does not go to ground. With a load of 200mA its output low voltage could be as high as 2.5V to 3V. with a low sinking current then the output voltage goes close to ground. See the graph on its datasheet.

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Uncle $crooge

 

I wasnt running 200ma off it, only about 20.

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MechTronics

World of Warcraft is a disease

 

So would it be better to be using a transistor then? It has to be swinging from 5v to gnd, not 5v to 2.5v.

 

With a 10mA load, a 555 can sink to typically 0.1V or 0.25V max.
with a 50mA load, it can sink to typically 0.4V or 0.75V max.
With a 100mA load, it can sink to typically 2.0V or 2.5V max.
With a 200mA load, it can sink typically to 2.5V but there is no max.

It can source current up to 200mA but the voltage drops are a little higher than when it sinks current to a load.

A transistor with a base current that is 1/10th the collector current has a much lower saturation voltage loss than a 555.

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Uncle $crooge

 

For some reason its hard for me to visualize this; Ive got the 555 circuit figured out (R1 = 4.7k, R2 = 10k, C1 = 1.0uF, f = ~58Hz), but I dont know how to hook up the transistor (NPN/PNP, base resistor value, that kind of stuff) to the sensor wire (which is pulled to 5v inside the PCM) so that it pulls the sensor wire to ground.

 

The base resistor for a switching transistor is calculated by using 1/10th the collector current in the bese resistor. Then the value of the base resistor is calculated by Ohm's Law (the voltage across it divided by the current in it).

Of course you should look on the datasheet for the 555 and for the transistor to determine the voltages.

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Uncle $crooge

 

How do I find out the current? I dont know the value of the resistor pulling the signal line to 5v.