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Discussion Starter #1 (Edited)
Now that I've built (or rather am still in the process of building) my warning lights into my Tach, I'd like to have my own custom temp light on/off sensing circuit.

I found a few possible examples but not entirely sure of the details.

Here's a simple Schmitt trigger circuit -



And here's a temp switching circuit -



In that second circuit it's not fully clear to me which IC they're using, anyone know the full name?

Obviously I need to keep it as small and simple as possible, and in both those circuits the relay would simply be replaced with a low current red LED.

Thanks :).
 

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Ic 4001 uses 4 nor Gates... Ic 4011 uses 4 nand gates...Logic is shown on the bottom of your circuit diagram... The diagram shown is using IC 4001 (4 nor Gates) ...This information is given by virtue of looking up the data. I have no idea how you will use this and wish you good luck in your experimentation.
 

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The second circuit uses a quad two input NAND gate. There are dozens of specific numbers depending on what qualities (speed, low power, etc) you need. The family is called 7400 series. 4011 may also be a family number. I'm not sure what, if any, the difference is. Logic circuits, especially CMOS logic circuits, are sensitive to voltage spikes, so don't mix it with your 200 volt tach signal! There are Mil Spec versions available that are more rugged.

Pins 7 and 14 are ground and +5 volts respectively.
 

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Upon further reflection I see that the inputs on all the gates are tied together. This makes them into inverters (if the input is high the output is low and vice-verse). Either a NAND or a NOR (7402) gate will do this (you can also get a straight inverter chip, 7404, but it has 6 inverters and the pin out would be different).
 

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Who's Ron?
Ron is the one that posted the diagram. There's a "Contact Ron" link on the page for questions.

My son like's Mike's Diagram because it actually has the model # on it.
 

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The first post mentioned a 'warning light' as opposed to a fan; if just switching a light, you can drive an LED directly in place of the Relay/Diode

I would personally go with the comparator circuit if it was me - in fact you can drive the LED directly without even requiring the transistor (up to 50mA)
(if not using transistor swap the + & - so the output is low vs high for the light 'on' condition)

That gets you down to the 8-pin DIP, resistor & potentiometer on a mini-board (add a resistor on the output if using a bare LED in the instrument, not if a 12V LED)

Edit - from this thread I see you commoned the LED cathodes - would be simpler if you connected the anode to +12 and sink the current through the common-collector LM311 output i.e. turns on LED when output transistor is on
 
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