Maker Pro
Maker Pro

Pull down resistor for ultrasound sensor



Jan 1, 1970

my robot from robowatch ( several interfaces,
where I can connect different sensors. I have connected an ultrasound
sensor. The robot's manual mentions a pull down resistor, which is
supposed to be connected to the ground. Why is this necessary? I find
this information in connection with many different sensors.

Many thanks for an explanation.


Some Guy

Jan 1, 1970
In most TTL logic, an open input acts like a high. Some sensors and logic
chips will effectively open their outputs when they are not actively making
them high, or when they are not "enabled". That is, the system may be
powering down the sensor whenever it is not actively using it. A powered
down sensor would appear open. This open state will be seen by the next
chip down the line as a logic high, when it is actually intended to be a
low. The pull down resistor forces the line between the open output and the
input to be held low. (If the input is unused, we just tie it directly to
ground) When the output goes active, it will no longer be open, but will
now have a positive voltage. The resistor now drops enough voltage
(effectively all of it) to keep the line high.

So the pull down resistor is there to keep errors from creeping into the
A pull up resistor does essentially the same thing, but holds the line high
unless it is being made actively low...