@Peter, programmer here with only experimential experience in electronics. Just read this thread and, it appears I cannot get my head round this concept, it would be world changing (from an interfacing pov).. see if i have this right. with this layout..
12v-------470k------Prop pin
|
150k
|
0v
...the prop would be ok?... Because the 470k makes sure the current flow is trivial, and the propeller's internal protection will deal with the voltage disparity?(not lookin for a guaruntee, if you would do it with your prop thats good for me
also
same at 5v with same circuit?
how high a dc voltage would You connect to it and feel confident of getting away with it?
If i've grokked this correctly, its incredible and thankyou very very much for opening my electronics eyes a little wider ;-)
Glad you got it and that is what it basically means, as long as you limit the current you can feed much higher voltages into the Prop safely. Obviously there is a practical limit but usually from a safety viewpoint although the circuit and those values will work equally well from 5V to 50V and higher. Theoretical limit for those values and assuming we do not want more than 500uA current is 238V but of course you start having safety and rating issues way before then.
The 150K acts both as a pulldown in the absence of a signal and also as an optional voltage divider which is only effective when the input voltage is low enough that the I/O pin would be below 3.6V. Once the input voltage is high enough the bottom resistor has little effect due to the clamping action of the I/O "diode". So the bottom resistor is useful for setting the minimum high threshold voltage say in the event that you don't want 5V tripping the circuit but you are happy if 10V does etc.. Otherwise you could make this bottom resistor 2M for example and the circuit will trip from 2V upwards.
So that makes MUCH more sense now! I never did notice the Note in the prop data that says input voltage may be higher if forward bias current is not exceeded. So the "voltage divider" resistor (the one pulled to ground), does not really divide the voltage in this case, it increases the current needed to pull the prop pin "HIGH". Since I want to draw as little current possible from the existing circuitry, I could use the 470K and 150K resistor values with what I am using on both the 5V and "12V" inputs.
@Peter... That's mind blowing, it has simplified completely many cases where i just want to sense a signal without messing with it much.....1000x Thanks ;-)
@eagletalontim: Glad you're as blown away (no pun wanted) by this revelation as me, learning is best done together ;-)
Nice Peter, I did play around on the falstad with this layout, and measure the volts.. but as I hit the 14v plus input, saw too much at the prop pin. However, didn't see the bit about current. Great result, and a nice resolve. Regards to my posting of various methods previously, ALL the methods have been used on production prop powered boards, one sold my Parallax themselves. Such a variation!:) Ill try the above KISS method in the car next few days
This is the same reason why you can use a simple resistor input in place of an RS232 receiver. Although RS232 could pump out +/-12V you will find that the voltage is more like +/-6V or so in practice due to actual operating conditions and loading etc. So the Prop input trips at practically the same thresholds as an RS232 receiver which only ever looks at the positive voltage. Knowing that the input only requires a positive low voltage you can do the same for transmit and simply feed a transmit signal from a Prop pin perhaps through a low value resistor for protection. Just invert the signal in software of course. Works every time, although maybe not if you are somehow interfacing to equipment >30 years old (ancient "real" RS232) but why?
I also use the current limit method for industrial automation where I am sensing 24V inputs.
Glad you got it and that is what it basically means, as long as you limit the current you can feed much higher voltages into the Prop safely. Obviously there is a practical limit but usually from a safety viewpoint although the circuit and those values will work equally well from 5V to 50V and higher. Theoretical limit for those values and assuming we do not want more than 500uA current is 238V but of course you start having safety and rating issues way before then.
The 150K acts both as a pulldown in the absence of a signal and also as an optional voltage divider which is only effective when the input voltage is low enough that the I/O pin would be below 3.6V. Once the input voltage is high enough the bottom resistor has little effect due to the clamping action of the I/O "diode". So the bottom resistor is useful for setting the minimum high threshold voltage say in the event that you don't want 5V tripping the circuit but you are happy if 10V does etc.. Otherwise you could make this bottom resistor 2M for example and the circuit will trip from 2V upwards.
Sorry for bringing this thread alive again, but I have one quick question. Now that I have my board all soldered up and testing each function, I noticed that my 5V output from my 555 timer RPM signal generator is not quite enough to trigger the RPM signal correctly. The "actual" voltage output is 4.76V. Would replacing the 150K with a 1M still keep the prop safe in the event the RPM signal ever sees a 16V pulse? Would this be calculated by Current = Voltage / Resistance? If I calculated this right, this would be right about 34uA to the prop pin if 16V pulse.
Sorry for bringing this thread alive again, but I have one quick question. Now that I have my board all soldered up and testing each function, I noticed that my 5V output from my 555 timer RPM signal generator is not quite enough to trigger the RPM signal correctly. The "actual" voltage output is 4.76V. Would replacing the 150K with a 1M still keep the prop safe in the event the RPM signal ever sees a 16V pulse? Would this be calculated by Current = Voltage / Resistance? If I calculated this right, this would be right about 34uA to the prop pin if 16V pulse.
As mentioned before that resistor is not absolutely necessary for "safety" but useful for both adjusting the threshold voltage and also to ensure the Prop pin doesn't float in the event of a disconnect. The 1M series resistor will ensure that practically any voltage you use as a signal cannot possibly damage or affect the Prop. So the pull-down or divider resistor can easily be 1M if you like. You will probably find that if you are trying to test the circuit with less than 5V that the divider resistor (the one to ground) needs to be around 270K or more.
Comments
Glad you got it and that is what it basically means, as long as you limit the current you can feed much higher voltages into the Prop safely. Obviously there is a practical limit but usually from a safety viewpoint although the circuit and those values will work equally well from 5V to 50V and higher. Theoretical limit for those values and assuming we do not want more than 500uA current is 238V but of course you start having safety and rating issues way before then.
The 150K acts both as a pulldown in the absence of a signal and also as an optional voltage divider which is only effective when the input voltage is low enough that the I/O pin would be below 3.6V. Once the input voltage is high enough the bottom resistor has little effect due to the clamping action of the I/O "diode". So the bottom resistor is useful for setting the minimum high threshold voltage say in the event that you don't want 5V tripping the circuit but you are happy if 10V does etc.. Otherwise you could make this bottom resistor 2M for example and the circuit will trip from 2V upwards.
@eagletalontim: Glad you're as blown away (no pun wanted) by this revelation as me, learning is best done together ;-)
I also use the current limit method for industrial automation where I am sensing 24V inputs.
Sorry for bringing this thread alive again, but I have one quick question. Now that I have my board all soldered up and testing each function, I noticed that my 5V output from my 555 timer RPM signal generator is not quite enough to trigger the RPM signal correctly. The "actual" voltage output is 4.76V. Would replacing the 150K with a 1M still keep the prop safe in the event the RPM signal ever sees a 16V pulse? Would this be calculated by Current = Voltage / Resistance? If I calculated this right, this would be right about 34uA to the prop pin if 16V pulse.
As mentioned before that resistor is not absolutely necessary for "safety" but useful for both adjusting the threshold voltage and also to ensure the Prop pin doesn't float in the event of a disconnect. The 1M series resistor will ensure that practically any voltage you use as a signal cannot possibly damage or affect the Prop. So the pull-down or divider resistor can easily be 1M if you like. You will probably find that if you are trying to test the circuit with less than 5V that the divider resistor (the one to ground) needs to be around 270K or more.