Multiplexing circuit help

[brobl]

Hey all,

I'm trying to figure out a way to multiplex three circuits(12V, 0.5A MAX) using 2 LEDs from a DSP evaluation kit as the decoder bits. When the LEDs turn on, the voltage change is a little over 1V (2.5 - 3.6). I'm stumped as to what to do with these specifications. Any help would be appreciated. Thank you!

btw, These three circuits are used to control fan speeds on a standard 12V fan.

 

[duffy]

So you have 2 leds, and you want to use the binary states to switch on or off one of 3 circuits? A 74HC138 sounds like what you might want. It takes up to 3 binary inputs and decodes them to 1 of 8 outputs. (ground the unused input).

Is the supply 5V? Are you reading across the LED itself, or the LED and its resistor? If you trace the connections back on the LEDs, you should see each of them is connected to a resistor. You want to tap in to the other side of that.

 

[brobl]

thanks for the response, duffy!

The supply I need to power the fans is 12V. I would need to use the 12V as the supply to the decoder then, correct? Thanks for the hint on the LED resistors, that makes more sense.

EDIT: I can change the fan to be 6V if needed. I'm just concerned about current load. I don't want this thing to burn up. A 6V fan, at load, would probably draw about 250mA?

 

[ljcox]

You don't need to change the fan voltage. You can switch it with transistors that are driven by the decoder.

However, the 74HC series has a maximum supply voltage of 6 Volt.

So you will need a voltage regulator to derive 5 Volt from 12 Volt.

 

[crutschow]

You could use a CD4028 1 of 10 (BCD) decoder which works up to 15V.

 

[duffy]

Don't try and drive the fans directly. Connect the outputs of the decoder to the gate of an N-channel MOSFET, like an IRLD024. Connect the source of that MOSFET to ground. Connect the drain to the (-) side of the fan. Connect the (+) side of the fan to +12V.