ZoneMikel's Blog

Problems with transistors

I have been working on this line following bot using only two transistors. It barley worked with 2n3906 and 2n3904’s driving the motors, which is to be expected since the are “small signal transistors” not for driving motors. On top of that the sensors would not even allow the pnp transitors to really reach ground so there was not enough current to really get the motors going. I had to remedy all that by using some npn transistors to allow the pnp’s to get a good ground. In case you are confused,

sensor -(output goes to base of)> npn ‘04 -(collector goes to base of)> pnp ‘06 -(emitter goes to one side of)> motor

so basically the sensor drives a npn tranistor that drives a pnp transistor that drives the motor. It didnt work well at all, I knew it would not but i was impatient waiting for other transistors to arrive. In the video it only shows the bjt transistors once, its the part where i kick it to keep it moving.

Eventually i got some mosfets, really little buggers you cant really see them in the video. Hooking up one didnt do the trick i had to solder 3 of them in parrallel to get enough umph to the motors. They work 10 times better than the bjt’s and simplify the circuit greatly. With the mosfets I just have the sensors driving the mosfets that drive the motors, so the entire circuit consist of 2 transistors, 2 sensors, 2 motors and some resistors. Really the transistors are 3 in parrallel but they act as one, they are so small they fit on the end of the 3 pin male headers in the video. I need to get some different fets that allow me to use just one, or we may go to a h-bridge ic. This was just for testing/prototyping.

Problems with following the line

As you can see it overshoots the line a lot. The fact that once one motor shuts off the otherone gets a extra boost of power makes it “jerky” but should actually help it. The right motor turns a little slower than the left motor, this causes it to go right when its not really following anything, it will not just go in a straight line. Anyway once it overshoots the line it has nothing to go off of so it just goes, the paper was to stop it from running under the sofa. A third sensor would come in handy to see if there is a line or not, but in this case it wouldnt really help because it would just stop.

line bot video on youtube

ph(z)
Func
If 3.5>=z and 0<=z Then
(.49998132583533+z*(.25704798516144+z*(-.013028431968512+z*.013909872543518)))/(1+z*(ª.28527797782298+z*(.21079685756164+z*(-.027639384901111+z*.0029194748417015))))»z
Return z
EndIf

If -3.5<z and 0>z Then
z*-1»z
(.49998132583533+z*(.25704798516144+z*(ª.013028431968512+z*.013909872543518)))/(1+z*(ª.28527797782298+z*(.21079685756164+z*(ª.027639384901111+z*.0029194748417015))))»z
Return 1-z
EndIf

EndFunc