Voltage range change..

Hello all,

Just got the arduino and am starting to play with it. My question is an 'EE' one in general but should apply to anybody using this kit..

We have analog outputs which can be set between 0..5V. I actually need to change this range to accommodate for 2 different analog voltmeters with the following scales:

A. -3V to +3V; hence when the arduino pin output is 0V the voltmeter should measure -3V, when it's +5V the voltage should read +3V. B. 0V to +30V; when the arduino pin output is 0V the voltmeter should measure 0V, when it's +5V the voltmeter should read +30V.

The reason I mention specifically that an analog voltmeter panel is used is that (i think) the load is small due to the high impedance of the voltmeter.

I've looked around and it seems I might need an inverting OpAmp circuit, a non-inverting one, etc. Usually OpAmps take +/-15V for operating, which is another problem..

Any suggestions?

Thanks, Gilad.

To get an output voltage below 0 you'll need an op amp with dual power supplies like +/- 9 volts or so.

To get an output value of 30 volts you'll need a high voltage op amp similar to this one:

http://focus.ti.com/docs/prod/folders/print/opa452.html?lpos=See_Also_Container&lid=Alternative_Devices

Thanks, I looked at the datasheet and plan to get this OpAmp (OPA452).

Still, the question is circular.. My input to the whole project is +12VDC, how do I get +/-10V (to operate this opamp)? Clearly I can't use an opamp since it requires negative voltage also.. I looked at single supply circuits (for reference if anybody is interested the link is:http://www.eng.yale.edu/ee-labs/morse/compo/sloa058.pdf). But i'm not sure this will work since the total voltage range available from the wallwart is +12VDC, which is not enough to drive the opamp..

Any ideas?

hi

to create dual power supplies you'll need dual regulators- try a 78L10 for the positive rail and a 79L10 for the negative. Then you need a positive and negative raw supply rail- something like 12-13V minimum on each side. The standard way to do this, which is pretty old school, is to use a center-tapped transformer, with the CT going to ground. The other two wires go to a bridge rectiifer , with the +/- outputs of the rectifier creating your +/- rails.

Here's the basic diagram, but done with single rectifiers: http://www.elecfree.com/electronic/tag/7915

That's an "old-school" solution because it uses a kilo of stuff ( i.e. a heavy transformer) to produce the few milliamps needed by the op amps. These days you can just poke around for a switch mode/charge pump/ super duper chip that takes in five or ten volts, and with the help of few capacitors or an inductor or two, produces what you need.

The MAX680 does just that, with one ic and four capacitors: http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1530

D

To get +30 volts out of the op-amp you'll need to supply it with more than +/- 10 volts to run on. The spec sheet sez Vo is 2 to 4 volts less than V+/V- so you'll need at least +/- 32 to 34 volts to run it. Prolly 36 volts would be fine.