Society of Robots - Robot Forum

Electronics => Electronics => Topic started by: ken52787 on February 22, 2007, 04:29:50 PM

Title: Mosfet woes
Post by: ken52787 on February 22, 2007, 04:29:50 PM
I thought I had a grasp of these mosfets, but I can't get my circuit working.

What I have is two Sharp IR distance sensors mounted on opposite sides of my robot. I have both data outputs running into one input of an op-amp (running as a comparator) so I can tell when its a set distance away from the wall. I want the robot to go back and forth, so I have a D-latch set up to keep track of which direction it's going. To keep the sensors from conflicting, I figured I use the Q and Not Q outputs to power the sensors and prevent them from both spitting out data at the same time. That didn't work (chip can't supply enough current?), so I figured I'd use two mosfets.

So here is what I have. Two P-channel mosfets (IRF9530 is that matters). On the D-latch, I have Q going to one gate, and Not Q going to the other. I have both Drains connected to a regulated 5V source. The Sources are connected to their respective Power input on the distance sensors. Judging from this (, I thought I had it all setup. What I found when I ran the robot is that both sensors are getting power no problem. I took a multimeter to it, and I can't find where I went wrong. One gate has 5V, and both the Drain and Source have 5V, as I'd expect. I then check the other mosfet, and the gate is 0V (actually 0.2V), but the Drain and Source are both 5V, which I didn't expect.

I double-checked everything, and it's not a simple wiring mistake. The only way the sensors can get power are through those mosfets. Doesn't matter what the gate is (0 or 5V), the drain and source seem to be connected regardless.

Anyone have any ideas?
Title: Re: Mosfet woes
Post by: Admin on February 25, 2007, 04:51:00 PM
well, the pinout I had on my page was for a N-channel . . . I probably should have clarified that . . . sorry!!! (its now fixed)


someone correct me if im wrong, but i think you need to put ground on drain and power on source . . . im making this judgement purely by the direction of the fly-back diode . . .

me being someone who has never used a P-channel mosfet, and who gets headaches thinking about flipflops, not sure if i can help you here . . .