Freq to voltage converter for use with computer HDD LED

[asellith]

I want to convert the signal from a motherboard HDD LED signal into an LED array. The array will have several stages to inidcate an raise in the frequancy of the HDD LED signal. IT's for a custom case mod I want to do. I was thinking a unity follower to prevent damage to the motherboard. then I was going to try a freq to voltage converter to get a voltage then run into several comparators. This will then run my LED array. The only problem I am having is I have no clue what kind of current and voltage is comming from the HDD LED signal. I am guess because it is designed to run a red LED it might be some where around 5 volts or possible 3 because that would be the voltage at the board. If anyone has an information on this signal or a better idea to accomplish my goal it would be greatly appreciated.<p>Thanks

[rshayes]

Most of the cases that I have seen had LEDs with current limiting resistors on the front panel. I suspect that the positive lead of the LED drive is the positive 5 volt supply. The neagtive lead would be shorted to ground by either an NPN transistor or an N-channel MOSFET.<p>The alternative is connecting the negative lead to ground and switching the positive lead, but this is unlikely, since it would require either PNP transistors or P-channel MOSFETS.<p>The current level for an indicator LED would be in the 5 to 15 milliamp range. If your circuit uses less than a few hundred microamps, then a buffer should not be necessary.<p>A 2 or 3K pull up resistor might be necessary to be sure that the high signal level is actually +5 volts. The low signal should be within a few hundred millivolts of ground.<p>The LED would be on when the signal is at the low level. This signal can be filtered with a time constant of a second or so. This can be obtained with a 10K resistor and a 100 uF capacitor. The signal will be inverted, 0 percent will be around 5 volts with 100 percent being ground. A rail-to-rail op amp could be used to invert and scale this signal to the levels needed by a bar graph driver integrated circuit.

[Robert Reed]

Jon
I am assuming you want to bar graph frequency.Once you come out of the frequency/voltage converter, you can drive an LM3914 with it.This is a 10 level bar graph driver chip. You can google this up for specs and see if it is what you need. National also makes a nice freq./ volt. chip. The number slips my mind at the moment, but if this is the way you want to go, I could look it up. I hope this helps you.<p>[ June 10, 2005: Message edited by: ROBERT REED ]</p>

[Gorgon]

Hi Jon,
If you use an optocoupler as the interface by connecting the optocoupler LED in the HDD LED circuit, you can use the output transistor either in a source or sink configuration. I'm not sure what you want to display, is it the number of accesses or the time the HDD is accessed.
If it is the time, you could just charge a capacitor through a high value resistor. You could then display the voltage rising over the capacitor in your bargraph. You need to discharge the capacitor through another resistor to make the bargraph display turn off when the disk is not accessed.<p>If you use the optocoupler, your circuit will be galvanicly isolated from the electronics of the HDD. This is a nice thing if something goes wrong in you circuit, and can save you a harddisk. 4N35 is an example of an optocoupler.<p>Good luck!<p>TOK