Hi,
I am working on a project that requires me to take a 0-5v DC signal from an encoder and convert it to a 4.1-6.3v DC signal that will be read by an RC transmitter. I have though about using an OP-AMP, but that will only multiply my signal it will not make it start at 4.1v. If you understand my situation can someone please give me an idea.
Any help would be greatly appreciated.
You actually need to do two things, divide the 0-5V down to 0-2.2V and then offset that by 4.1V. Easy enough to use a voltage divider, then use an op-amp to add the offset voltage (and you get a buffered output as a bonus).
What do you have available to power the op-amp?
Another possibility is you “may” be able to do this with just a voltage divider… let me crunch some numbers and I’ll get back to you around lunch.
It is difficult to understand what you are asking in your post.
Dou want a square waveform with the volage range of 4.1V low and 6.3V high?
If so will need to boost the voltage with a DC-DC converter…you can buy a nifty little DC-DC converter designed to increase the voltage to something arbitrary from Dimension engineering:
dimensionengineering.com/AnyVoltMicro.htm
but this only solves part of the problem…
Mark
Two things are confusing:
0-5vdc from an encoder, sounds more like a potentiometer.
4.1vdc to 6.3vdc to be read by an RC transmitter, never heard of an RC system that requires these values. If it’s an older unit with a vdc of around 10vdc this could be the values from a joystick pot, which has a reduced range due to mechanical limitations of the joystick assembly.
An op amp iirc require a double ended power supply, like +/- 9vdc, GND. Not sure if these is an op amp that can be used with, or was designed for, a single ended power supply +9vdc, GND. It’s simple enough to make an adjustable gain amp that reads in the “encoder” pot on one leg (input) and a trimmer pot on the other input (offset). Would have to break out my old op amp books to be more specific. But the idea is to adjust the gain and offset to get 4.1-6.3vdc out from 0-5vdc in.
That the idea?
It’s lunch time… actually it came and went. Where are those cruched numbers? I’m interested as to what the solution might be.
Robot Dude,
I am using US Digital shaft encoders usdigital.com/products/encoders/absolute/rotary/shaft/ma3/ in the analog setup. So it kinda is a Potentiometer, USD just calls it an encoder.
Yes the RC transmitter is quite old, it is the joystick pot that i am replacing and it does run on 10v. So I have to use a dc-dc converter to make a 5v power source for the MA3 and then boost the output from the MA3 to the range the existing POT is putting out.
EddieB,
I have never used an OP-AMP before and was hoping it could be powered by either the 5v from my DC-DC converter or right off my batteries in the TX (8 cell NI-MH).
Cheers,
Well until EddieB gets done eating… 
The best solution for you would be to use the raw battery voltage and use an inexpensive 3 pin regulator such as the LM 2937 ES -5.0 to get the 5vdc for the MA3. Cool part btw. I wasn’t thinking of that when I posted. Then another 3 pin regulator for 9vdc could be used to get the power for the op-amp. I do think there are single ended op-amps, but it been a long time… The op-amp circuitry is really simple to implement, and you will only need to adjust two trimmer pots gain and offset to set it up.
Unfortunately I don’t see being able to just scale things with simple resistors as particularly safe. You can get close but it would require a pull-up resistor to a voltage >> the 5V the MA3 is operating on which could be painful to the MA3 if something wasn’t connected correctly or came loose. Also sort of an unknown is the output impedance of the MA3 so it’s difficult to calculate the exact resistors needed since that would factor in.
An example would be to use a regulated 7.2V reference with a voltage divider set between the 7.2V and the MA3 output consisting of a 10K and 13K resistor.
7.2V — 10K — x — 13K — MA3 output, the ‘x’ junction between the two resistors being your output to the radio.
At 0V input the voltage divider sees 7.2V so (using 10K at the top and 13K at the bottom) the input to the radio sees 7.2V * 13K / (13K + 10K) = 4.07V.
At 5V input the voltage divider sees (7.2-5.0) = 2.2V so the radio sees the 5V + (2.2V * 13K / (13K + 10K)) = 6.23V.
It’s pretty close and if you used a small potentiometer at the ‘x’ you could probably tune it in.
You also need to make the 5V for the MA3 and the 7.2V reference voltage… depending on what you have for a battery… if you have a 9V battery you could probably use LM317 adjustable regualtors to make the voltages easily enough.
I guess a reasonable question is just how precise you want this to be. I worked out a circuit using op-amps and a single 9V battery supply but it’s not trivial to build if you’ve never wired up a breadboard/plugboard before. That circuit actually uses 5V as a reference so you kill two birds with one stone since that same 5V could power the MA3 easily enough.
With respect to ‘eating’… I’m at work guys. Some days are more involved than others. 
Just good natured fun Eddie. Can’t this be done with a single op amp? All my op-amp technical reference books are at home. 
If you didn’t care about 0 to 5V generating 6.3 to 4.1V out (rather than 4.1 to 6.3V) you can probably do it with a single op-amp on a single supply. Maintianing the requested 0-5V producing 4.1-6.3V relationship in a single op-amp would require a negative voltage of some sort, however a single quad op-amp package lets you flip the slope of the incomming signal which in turn lets you add the required offset. something like an lm324A would work nicely and is pretty forgiving to wiring/layout atrocities.
Most radios have the ability to reverse the throw for the joysticks, so it shouldn’t be a problem.
Guys,
It doesn’t matter if the output is 4.1-6.3 or 6.3-4.1, my transmitter can reverse the signal. If I could get away with using the Quad amp-op that would be great. If I understand this correctly you can adjust the factor and add an offset, so the unit would multiply my voltage by .44 and then add 4.1 volts to that.
I’m not to worried about the circuit being a little complex. I am planning on using stripboard to make up the circuit so it will be easy to work up, assuming I can lay it out nicely.
Cheers,
