Help Me Understand This LM311 Circuit

[bdiddie]

I would just like to know if this will work?!? It is supposed to act like a on-off switch for a relay in a car. I need pin 3 to always hover around .5v -.7v. The non-inverting input(pin2) actually has three different possible values coming from the automobile: 12.8v , 2.5v ,or 0.020v. Are the resistor values sufficient? I'm an electronics beginner, trying to wrap my head around this circuit. I have read up on the LM311 and understand the operation of the IC but I'm not sure what the all the capacitors are for and also if the resistor values are going to give me the correct values for my input signals.

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[Brevor]

The resistor values you show should give you about .56 Volts at pin 3, that would be the approx. switching threshold. You list 3 different voltages that may be applied to the input but you didnt say at what voltage you want the circuit to change states.
The voltage in a car varies depending on if the engine is running or not so your .56 volt reference will change some also. A way to avoid this would be to replace the 220 Ohm resistor with a diode like a 1N4148 with the cathode connected to the ground. This will always have approx. .7 Volt drop across it and will not change when the supply voltage varies.

Also offhand I don't know if a LM311 will directly drive a relay, and you will also need a snubber diode across the relay coil either way.

 

[ericgibbs]

I would just like to know if this will work?!? It is supposed to act like a on-off switch for a relay in a car. I need pin 3 to always hover around .5v -.7v. The non-inverting input(pin2) actually has three different possible values coming from the automobile: 12.8v , 2.5v ,or 0.020v. Are the resistor values sufficient? I'm an electronics beginner, trying to wrap my head around this circuit. I have read up on the LM311 and understand the operation of the IC but I'm not sure what the all the capacitors are for and also if the resistor values are going to give me the correct values for my input signals.

hi,
At which voltage of the 12.8v , 2.5V , 20mV to want the relay to operate at.??
On the circuit diagram you show 12.8V or 20mV, a simple transistor would switch the relay using these as base drive voltages [via a suitable base resistor]

 

[bdiddie]

The strange thing about this relay is that its "on" at near ground(0.020v) and 12.8v will trigger it "off". This is why I was using a reference voltage(pin3) of approx .56v as my switching threshold; so to answer everyone's question I need the relay to operate (turn on), when the voltage is below the reference voltage.

So below approx .56v my output(pin7) would equal approx the voltage at pin3 and the relay would kick "on"?

 

[bdiddie]

I finally gathered the parts to make this circuit. Only problem is - it doesn't work!!!
I took Brevor's advice and substituted a 1N4148 diode for the 220 Ohm resistor on Pin3; this gives me a stable reference voltage of 0.64V which is great!

I'm having an issue where my signal going in to Pin2 is always 12.8V+. I guess I should have mentioned that this signal wire is actually a pulse width modulated ground signal. I have also read up on the "relay" and its some sort of solid state electronic relay module.

Pull down to 0V turns On, High 12-14V is Off. And low speed setting is 15% duty-cycle so, 1.8 to 2.1V.

I'm trying to completely bypass the low speed, and have a basic on and off system. So if the Input coming into Pin2 pulls down to 0V then BAM! it should switch.

Is there something wrong with the portion of my circuit going in to Pin2? It just seems like its always getting 12.8V and not utilizing the PWM signal coming in to the circuit.
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Thanks.