Switch & Fan voltage detect project

Are you sure that the speed control is a variable dc voltage and not pulse width modulation?

Maybe

Do what I did, design and build your own, which has its own LEDS built in...

Yes get real. Ardui or similar is cheap and flexiable and when you don't like the way it works you can change it.

Dave

flexible at the same price.

Possibly. I was actually going to suggest a PIC chip as these are also cheap...

Dave

The controller is already in place so just want to 'supplement; with LED

don't want to use Ardui

1) Sense current flow in wire, make a status that way. Ground referenced sense resistor, ground referenced capable opamp. 2) Extra contact pair on switch (which you have dismissed as a solution). 3) Optoisolator, which works as long as the load will share 25mA with you. 4) Tachometer output on fan. Use a series resistor and a zener, to condition the tacho output for input to your controller. You can use a monostable and some logic gates, to measure the period between fan pulses (instead of measuring the period in software with your controller), and that is how you "guarantee a minimum fan speed is met".

With a 4538, I can build just about any timing function you can think of. I used to use those for bench automation, so I could go home while a lab circuit did repetitive procedures.

The original ones run over a wide supply range (5V to 15V?)

Fans come in the following configurations:

2-wire Basic fan, runs 5V to 12V or runs 7V to 12V. You should not feed them 3V, as they may draw some amount of current (stall) and not spin if you don't meet the fan_min input. As you lower the voltage, the current flow should increase a bit. Test for this! Your driver must be able to supply stall current, if you insist on driving at 3V. 3-wire Two pulse per revolution tachometer output. Based on output from brushless DC coil drive. 4-wire Fan now receives a constant 12V power. The PWM signal, uses pulse width to indicate what the desired fan speed is. At "0 PWM", an Intel Spec fan continues to spin at fan_min speed. At "0 PWM", a non-Intel fan would spin at zero speed. The PWM signal operates at a high enough frequency, so that there is no acoustic effect coming from the fan hub. (PWM fans sometimes "sound different" than 2-wire or 3-wire fans.)

Paul

It will be more work to modify it than to start from scratch.

My controller went live two hours ago, so far so good.

Well try praying to god then

Both require knowledge & skills I don't have, and expensive to turn ion or off 4 LEDs My thoughts are use an LM319 voltage comparator to get me a 12V signal anytime there is any Fan +ve Remove switches from controller and use them to power DPST reed relays .. outputs complete fan circuit and drive LED

Cost is under a Tenner

Looking like:

Your schematics don't display. A LED just needs a resistor from switched 12v to indicate. A comparator is another way, where needed. A transistor can be used as the comparator, a resistor divider on the input setting the threshold. For mains voltages a neon is easier. There are plenty of free parts in e-waste.

I did and he said don't use a sledgehammer to crack a nut ....

After suggestions here and elsewhere I can achieve what is needed with 2 x DPST reed relays and a pre-built voltage comparator module or even this simple voltage detector

I don't have experience to use an open source electronic prototyping platform such as Arduino, I'm not knocking it but don't have time to learn to use it. Plus I can achieve what I want for under a tenner.

If I was building my own system controller from scratch that would be a different proposition, but the existing one is under warranty for a year, so will keep with it for now.

Thanks its simple 2 wire 12V DC