Can the Proellor handle Negative volts??

krazyideas

Hello

I am wanting to use the CTRA mode %11010 to measure a pulse from a light show that goes +5 volts -5 volts and I want to know if I can plug that stright into the propellor board or if I need to bring the volts down??· I know that anything above 1.6 volts it reads as high and anything below that it reads low, and that the propellor puts out·3.3 volts when a pin goes high.· But will it hurt it if it is fead +5 volts to read as high and -5 volts to read as low as the input for CounterA to read????

Also kinda a dumb question but what does the % infront of the mode you set for the logics do, like in %11010.

Anyway thanks so much

SRLM

You could use two I/O pins and two diodes to choke the voltage so that it doesn't drop below zero. If you do, then you'll have three states: really high, really low, and middle.

Mike Green

1) The "%" indicates that the number that follows is in binary.· Similarly, a "$" indicates that a hexadecimal number follows.

2) You can potentially damage the Propeller chip if you connect a negative voltage to an I/O pin without some kind of current limiting resistor.· Similarly, you can damage the chip if you connect a positive voltage greater than 3.3V without some kind of current limiting resistor.· For voltages like +5V or -5V, a 1K resistor is adequate, but something more like 10K is better.· Use Ohm's law to see how much current is involved.

FrameShift.

the % prefix indicates the number that follows is in binary format.

krazyideas

Thanks guys That helped alot

Philldapill

Just a side note, if you ever need to read the voltage when it is negative, you can use a summing amplifier(Opamp circuit) to bring that voltage range from +5 to -5, to +10 to 0, then use a voltage divider to scale it down. I had to do that for a project recently.

Beau Schwabe

krazyideas,

Here is a resistor divider circuit that will convert (5V...0V...-5V) to (3.35V...1.65V...-55mV)

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Leon

Here is a more general version of Beau's circuit for converting a bipolar signal to a unipolar one:


Leon


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Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle

Post Edited (Leon) : 11/5/2008 1:35:52 PM GMT

Beau Schwabe

Leon,

That works if the Bi-Polar input is 5V and Vcc is 3.3V, the Uni-Polar output is 3.32V.· However·if the Bi-Polar input is·negative 5V and Vcc is 3.3V then the Uni-Polar output will be·negative 1.68V


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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Leon

It's useful for input voltages between, say, -1.5V and +1.5V. The output will be +0.5V to 2.0V, with a 5V supply.

Leon

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Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle

Leon

It's useful for input voltages between, say, -1.5V and +1.5V. The output will be +0.5V to 2.0V, with a 5V supply. It's often used with ADCs.

Leon

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Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle

Tracy Allen

Another option is an opto-isolator. Especially recommended if there is any reason to keep the "light show" power supply ground isolated from the Propeller ground.

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Tracy Allen
www.emesystems.com

Ken Peterson

The data sheet calls out a maximum of 500 microamps current if you are going to drive an input beyond the rails. I have a project where I'm detecting an input of +12V, and I'm using a 40K ohm resistor in series just to be safe. The input impedance is very high, so you can get away with a fairly high series value. Only thing you need to worry about is noise. Keep your resistor close to the propeller, perhaps with a small capacitor on the propeller pin too.

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·"I have always wished that my computer would be as easy to use as my telephone.· My wish has come true.· I no longer know how to use my telephone."

- Bjarne Stroustrup