Electronics 555 and LEDs

I have built cpemma's Diode Bus, but I can't be bothered to make the LEDs dim appropriately, so I figured I'd make something different. As you may have inferred from the title, that different idea is blinking LEDs. What I want to do is have the LEDs light on an off at a varied amount of time depending on voltage being fed to fan . If you know anything about the 555, you know that I need to come up with some way to either adjust the capacitance or one of the resistances electrically to vary the rate at which the LED blinks (I know that there's frequency and duty cycle to deal with, but that's a minor detail right now). Question is, how would I go about doing this? The only thing remotely close is to use varactor diodes in reverse polarity, but I'll be damned if I know if that'll work in practice, let alone how to shop for one for what I need.

Then again, some of you might have a simpler idea that I haven't even thought of. I'm all ears for those suggestions as well.

 

you could use an array of zener diodes with identical resistor before and after each one and connected in parallel so as the voltage increased, more zeners would switch on and so the resistance would drop therefore increasing flash rate and leave the second resistor and capacitor constant... maybe that would work??

guzzler

btw, i can make a schematic if you like

 

Nah, I don't need a picture. That sounds like an idea, especially since it *should* keep things fairly linear. Also, I can get Zener diodes in a fairly wide range of voltage ratings as well.

 

Rather than a 555, a 566 might be useful.

It's a voltage-controlled oscillator ic, possibly hard to find as I think NS have discontinued it. LM331N (voltage to frequency converter) is another option.

These are designed to run at fairly high frequencies, so you may also need a divide-by-lots ic (eg 4020, 4060, etc) to get a led flasher.

There again, if you've made a diobus with a 2-pole rotary switch, you could use the other pole to switch resistors in a simple 555 circuit.

 

Afraid I can't do that last one, as I went cheap and bought 1 poles (I suck at planning things out ). If they were two pole, I probably wouldn't be asking.

I looked at the 331's specs, thankfully Jameco carries it (but not Mouser, must be a galactic joke or something), and, *shocker*, I think I understand how it works! Now I just need to finalize some numbers and get my list ready.

edit: Question, does the value of Cl matter at all or do I just need to worry about making sure it's voltage rating is sufficient. Also, what type(s) of capacitor should I use?

 

If it has a sensor on the Fan a counter would be simpler to build, and give a more acurate rating of the speed.

 

I can't do that with my current setup. Right now, I have their connectors are RCA jacks. Plus, one of my fans is a 4-pin molex.

Plus I thought the fan's RPM sensor didn't work when it's below its usual voltage, or is that the motherboard's sensor for the fan that I'm thinking about?

 

Two or three possible problems, first is PWM control which interrupts the supply so the signal is patchy. Then if the fan controller is between fan negative and ground, the pulse is the wrong voltage level for the mobo sensor. The third possibility is related to slow fan speeds, more common when you've swapped in a 80mm for a 60mm, and depends on your hardware monitor chip.

On many mobos the fanspeed is calculated like this :

Speed = 1350000 / (FanByte * FanDiv)

FanDiv is the number of pulses the fan sends per rev (usually 2).

FanByte is related to the time from one pulse to the next.

The highest value that the memory space allowed for FanByte can contain is the value 255 (FF). A value of 255 indicates that there's no fan connected, so the motherboard says there's none available.

So if your fan runs at a very low RPM let's say 2500, the FanByte should contain 1350000 / 2500 / 2 = 270. But the max value a byte can contain is 255, so the chip turns your 270 reading into 255 and translates it as a "no fan available". The solution to this problem is to set the divider to a higher level.

On a clear day, I can understand this stuff.

 

Question before I finalize the parts: I want it so that at full power (12v), the LED blinks at such a high rate that it looks solid, but I don't know as to whether or not the frequency should be 30hz, 60hz, or some other number I'm not aware of. Sure, I could test this with a pot, but it'd be helpful if someone else already knew this.