AR40B schematics

Thread Starter

vick5821

Joined Jan 27, 2012
54
Hey there,
recently I read up AR40B schematic diagram and I got some doubt.
My doubt is about the sensor part.Why is there a pull up resistor (4.7k) there?



Can I just eliminate it ? or do not have the 5V there?
 

Andreas

Joined Jan 26, 2009
82
Perhaps to keep the SEN data lines high if no sensor is fitted?
I agree with Paultreg and it prevents the sensor's output from floating with no connection especially. What sensors can it use, are they specific to the controller board or generic types?
Note how the two encoder outputs don't need pull-ups.
 

Thread Starter

vick5821

Joined Jan 27, 2012
54
I agree with Paultreg and it prevents the sensor's output from floating with no connection especially. What sensors can it use, are they specific to the controller board or generic types?
Note how the two encoder outputs don't need pull-ups.
If I didnt use the 4.7k resistor ? Ok ?
 

SgtWookie

Joined Jul 17, 2007
22,220
The sensors are most likely have open-collector or open-drain outputs. This means that they can sink current, but cannot source current.

They frequently use open-collector outputs so that the output voltage levels can be different from the supply voltage, like yours is.

You need the 4.7k resistors in order to have an output that is between 0v and 5v. If you leave the resistors off, the outputs of the sensors will always be at/near 0v, and the circuit will not work as it is supposed to.
 

Thread Starter

vick5821

Joined Jan 27, 2012
54
The sensors are most likely have open-collector or open-drain outputs. This means that they can sink current, but cannot source current.

They frequently use open-collector outputs so that the output voltage levels can be different from the supply voltage, like yours is.

You need the 4.7k resistors in order to have an output that is between 0v and 5v. If you leave the resistors off, the outputs of the sensors will always be at/near 0v, and the circuit will not work as it is supposed to.
why it will be 0/near 0 ?
 

Andreas

Joined Jan 26, 2009
82
why it will be 0/near 0 ?
Let's suppose the output from the sensor has an NPN Bipolar Junction Transistor as its final output drive stage. When the required base voltage is reached, current flows through the device (Collector / Emitter pathway) causing it to conduct and shunt the collectors voltage to ground. Thus the final output you will see is zero volts. When the transistor is not conducting due to no base voltage, the collector will be at the rails potential, in this case 5V. The resistor is there to limit the current through the transistor when it conducts.
Refer to the AAC VOL III - Semiconductor pages.
 
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