I had designed a transistor logic based switch for a LED based project recently at my day job. I discovered that the design I had, N-channel mosfet triggered by a PNP transistor, worked, but it didn't bring the voltage down far enough to turn off the LEDs I was working with. The LEDs in question had a forward voltage of 2.1V, but did they turn off after the voltage dropped below that? They did not. In fact, they would even be dimly lit as low as 1V. In other words, I was dealing with a situation where the LED's properties didn't follow the datasheet. You will always have a little voltage & current flowing through a transistor logic circuit, but I needed it clamped way down in this case.
I was in a bind, the project was due, I did what we all have to from time to time, pulled an all night design session. I ended up simplifying the overall design, made the final product cheaper, and brought the temperature down as well. I switch to a P-channel mosfet switch, bringing the "off" state voltage down to ~41mV. I then drew it up in Multisim, and the simulation worked perfectly. I had the PCBs changed per my new design, and built 4 perfect prototypes.
Below I am showing a single case of the mosfet switch.
When "Line 1" is high/on, it will be cut off when "Line 2" is high. So that only LED2 will be on at that point.
R1 & R3 are resistors to limit the current for the LEDs.
The mosfet & diode are actual components I picked for the simulation. Final components I chose had similar properties.
If you have more LEDs or conditions, just replicate the setup for each.
I thought I would share this design for all those faced with a similar task.
Enjoy.
Cabe
