# Designing a display for Watts, Volts and Ohms.

by Browndustin
Tags: designing, display, ohms, volts, watts
 P: 12 I am working on a small project and having those values shown in real time on the device will be helpful. I don't have much room as it is. I currently have one of those little 1s-6s volt meters used by RC hobbyist and use my multimeter to check resistance. My goal is to have something around that size, but show all three values. I have experience, but unfortunately it was mostly just having my boss hand me a schematic and a pile of parts. A couple of engineer friends helped me some with theory but not much practically. They are Nuclear engineers. One of them frequents this website regularly and suggested you guys/gals for this. I was hoping for an off the shelf part, but hours of searching came up with nothing. Some specifics; The power is coming from two IMR LI ON 3.7 volt high drain batteries in series. A buck converter with a 20 amp/ 120 watt limit. The output voltage range is 3.3ish up to 6 volts. The variable resistance will have a low end of .3 ohms and an upper limit of about 1 ohm. I know it's possible because I have seen it exactly in another device. I thought about cannibalizing it for the display, but it would cost about 100\$ for a small piece of circuit and a tiny oled screen. Furthermore that's not a real solution.
 P: 867 It isn't clear to me what, exactly, you are trying to do. In order to display watts, you need to measure two things (V, I or V,R, or I,R) and operate on them. In order to display resistance you need to connect an ohmmeter to each end of a part not in a circuit. Now, given a specific circuit, you can measure things about it in different ways if certain things are known or constant. Also, you want three displays? You want to move probes? You want it to cycle through the readings? You want to throw a switch? I'm sure it is all clear in your mind. Write it down for me.
 P: 12 I really appreciate you helping! I apologize for not being clear enough. I understand Ohm's law and know this isn't a difficult idea. What I want is one display showing all three values possibly all on screen at once or cycling through by up and down buttons. I want a circuit that is fixed in the device without having to move probes. The voltage will change from fresh batteries at 4.2/8.4 down to about 3.3/6.6 volts. The chip I'm using is a naos raptor by GE. THe 20 amp limit one. I will have to look at the data sheet to see if can pull a constant voltage down from one of the outputs. Right now I'm using a 200ohm potentiometer to have a voltage range of 3.3-6volts.
P: 12

## Designing a display for Watts, Volts and Ohms.

The resistance is a heating element that will change periodically.
 P: 12 The entire thing is controlled by an off/on Pushbutton switch. I would like it if the display could be operated independent of the switch.
 P: 867 I still don't know what nodes you want to measure. But it really doesn't matter I suppose. The problem is how to convert circuit values to a display via some sort of user interface algorithm. The most fun would be to build an arduino based (or other uC) display controller. The Arduino A/D converter could be switched to measure various things and display them as you desire. It's not a trivial project though. You can start here http://www.electroschematics.com/935...tal-voltmeter/ Or you could use something like the icl7107 (google for digital voltmeter IC - there are quite a few) and control it with an arduino or other uP. As you narrow the search feel free to ask more questions.
 P: 12 Exactly the kind of thing I'm looking for! Now to be able to measure the resistance of the heating element as well as power across it...
PF Gold
P: 2,608
I agree with meBigGuy as to where to start but just to give you an idea of what's out there, Texas Instruments has this .pdf application report that's cool.

A Digital Multimeter Using the ADD3501

 Texas Instruments ADD3501 is a monolithic CMOS IC designed for use as a 3 (/2-digit digital voltmeter).
Edit:
Now the chip is manufactured by National Semiconductor. Their data sheet can be found here: