Reversing the polarity of output signals

BonanzaMan

I am newish to electronics and microcontrollers. I am trying to re-create the following two signals using a Propeller based circuit (see osc.jpg).

Note that these signals are shifted by 90 degrees, have a max voltage of -700 mV and min voltage of -2.01 V and have a frequency of 60 Hz. I have created a Propeller circuit that gives me the needed signals but with positive voltages peaking at 1.74 V (see parallax osc FREQ 242.jpg).

So, I need to reverse the polarity of the two output singnals.

Any ideas on how I can do this?

Many thanks.

Phil Pilgrim (PhiPi)

BonanzaMan,

Welcome to the forum!

What kind of current do your outputs need to be able to supply to their loads?

-Phil

BonanzaMan

Minimal, application is an external time base for a clock.

BonanzaMan

Thanks for the welcome Phil - good to be here. Earlier reply was a quick one as I was overdue in a meeting. Thanks for your consideration all.

Mike Green

The simplest way to do this is to use an optoisolator with the input running off the Propeller I/O pin. You'll need a current limiting resistor for the optoisolator's LED. The exact value depends on the optoisolator's specs, but 100 to 150 Ohms will give you an LED current around 10mA.

You'll need a power supply of approximately -3.0V with the + terminal connected to ground. The optoisolator's NPN transistor should have its emitter connected to -3.0V and the collector connected through a 220 Ohm resistor to ground. The collector should be connected through a diode to your clock circuit to yield a voltage that goes from -0.7V to -2.0V. Different diodes have slightly different voltage drops and you can check the diode datasheets or measure the actual voltage drop across the diode until you get one that does what you need.

Phil Pilgrim (PhiPi)

Here's another option:



Here's the output I got from it:



The yellow trace is the input from the Propeller; the cyan trace, the output.

I supplied the -5V to the LM358 from an ICL7660 voltage inverter, driven from the Propeller Demo Board's +5V supply.

-Phil

Ale

For circuits not involving microcontrollers, I find PSpice (and for example Linear's version LTSpice from www.linear.com) very helpful. You draw a schematic and you simulate it for a definite amount of time while you measure current and voltages at any point.
Phil's solution is not only easy but flexible, many op-amps can be used. At some point you will have to take into account the frequency because it will start to act as a low pass filter! (you can also simulate this behaviour). (Don't forget that nothing beats building the actual circuit and testing how it works!!).

BonanzaMan

Thank you very much Ale, Phil and Mike! Parts on order, I'll give both approaches shot and report back.

Kindest regards,
Greg

BonanzaMan

Phil - Finally getting back to this project. Ended up implementing the circuit as you suggested. Works GREAT. Thank you again so much. Sorry for the delay in getting back to you on this.

Greg