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LEDs to control fan off/low/high.

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mpearce

New Member
Hi,
I have a LED array connected to a serial port of a pc, I can control which LEDs are on by sending a value to the serial port. I would like to remove the LEDs and replace them with a transistor, which will be connected to a PC fan. Ideally, I would like to be able to use two of the LEDs to give the fan a high or low option. So basically, if both LEDs are off, the fan is off, LED1 on, the fan would be low, LED2 on, the fan would be high.

A pc has +5, +12 and Ground; I was reading an article that said I could us the +5 as the ground for the +12 and end up with +7. Is this true? I was thinking I could have the first LED control on or off, and have the second one control the speed by switching the ground from +5 to actual ground.

Also, I'm not sure what type of transistors to use in this situation.

Any help would be appreciated.

Thanks,
Mark P.
 

kinjalgp

Active Member
How have you connected LED's to serial port? Are you using any serial to parallel converter or USART IC?
 

mpearce

New Member
The board I have is a pre built LCD with a set of 8 LEDs across the top. I have written a VB program that can write text out to the LCD and send patterns to the LEDs. So, I'm not really sure how the board was designed, but it's a simple matter of sending a code and a number between 0 and 255 to control the LED pattern.

Something that might help with determining what type of transistor I need is that they are small surface mount LEDs.

Thanks,
Mark P.
 

bogdanfirst

New Member
i think that you are using the paralel or printer port ...the one with 25 pins.
you can use a transistor for the first led, to connect the fan to 12V and not necessary with an optocoupler(though it will be safer)
and for the second output use 2 transistors and an inverter, one transistor and the output of the optocoupler(or from the computer) to the input of the gate and ther other transistor to the output of the gate
or you can use 2 resistors from the leds and a transistor to regulate the voltage across the fan and have the circuit with the fan in series with the transistor connected to 12V and GND. this way you can make it with 3 speeds
 

mpearce

New Member
Hi,
First I would like to thank everyone for their help..

bogdanfirst said:
i think that you are using the paralel or printer port ...the one with 25 pins.
Actually it is the serial port, I am a sys admin, so I know PCs.. It's the electronics I am kind of a newbie with. I took a couple of digital electronics classes while in college, but that's about it. Here is a quick scan I took of the board, showing the front, back and cable that connects it to the pc.

bogdanfirst said:
you can use 2 resistors from the leds and a transistor to regulate the voltage across the fan and have the circuit with the fan in series with the transistor connected to 12V and GND. this way you can make it with 3 speeds
This sound like it might be what I'm looking for, can you describe it a bit more for me and my limited electronics knowledge.. :)

kinjalgp said:
How about doing it with 16-step digital PWM?
I looked at the schematic for this and I think that is a bit complicated for what I want to do. I have to make at least four of these (8 LEDs, 2 per fan = 4), so cost may become a factor.

Thanks again,
Mark P.
 

Sebi

Active Member
In circuit work a processor, and communicate with PC via MAX202 serial converter.(very sharp picture) If the processor drive the LED-s directly (not with multiplex method), you can drive another circuits via transistors, but i recommend to use optocouplers.( connect the opto LED serial with display LED. If the display LED is blue, and supply is 5V, not working.)
 

Sebi

Active Member
I see Your problem, but can't solved with only one transistor. Need a relay instead of switch, or a switchable supply with transistors.
 

Sebi

Active Member
No failure in sematic. Have You checked the LED driver circuit? (multiplex or direct?)
 
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