J.R. FERGUSON'S DIGITAL VOLT METER

I began this project because the voltmeter gauge on my '79 Wing was grossly inaccurate when there was a load on it. It would register 10 volts when my mult-tester would show 12.2 volts. I began to wish that I could mount my multi-tester in the dash. Even took it apart to see what I could do. Decided that it wasn't feasible. Not long after that, I saw a voltmeter module in the Radio Shack catalog, but it was by order only, so I did. When it arrived, I read the instructions and realized that I had no idea what they were talking about. I solicited the help of friends on the "Wings on the Internet" Discussion list. Gerry Pophal provided me with the basic instructions on the wiring. Not wanting to "fry" this thing, I asked the list for more help and along came Dave McElderry (from Ohio, no less). He took a GREAT deal of time and "taught" me an awful lot about electronic basics and theory. He double checked my wiring as well as answered may questions about the operations of the module itself. Mind you, he did all this via e-mail. I've never spoken to nor met either Gerry or Dave, but I'll bet when I do, we'll feel like we've known each other for years. Many thanks to these "Cyber-Friends".

The size of this unit requires a cut-out of 1.31 X 2.68 inches.

I know that there may be some of you who wish they had one of these, but you have NO desire to build it. If you send me e-mail to that effect, I think that I could build one for you. We'll talk. Contact me at jrferg@airmail.net.

SUPPLIES:

(1) Digital Voltmeter/Ammeter Module -- Radio Shack Part # RSU 11461498 (order only from catalog).
(1) 7805 5 Volt Regulator -- This steps the 12 v from the battery to power the unit and backlight.
(1) Roll or tube of solder -- Best to use a very fine or thin solder for the linksand CN2 connections.
(1) Roll of wire -- a light gauge will do as the unit pulls a very small load. I used 3 colors.
(1) 1 Megohm Resistor -- Make sure it is 1% tolorance. This is Resistor R(a).
(1) 10 Kilohm Resistor -- Make sure it is 1% tolorance. This is Resistor R(b).
(1) 4 inches of ribbon cable with a 14 pin "head" connector (female).
(1) A "pencil" soldering iron -- should have a fine tip. My iron is 14 watts.
(1) Digital Multi-Tester -- This is for calibrating and for checking connections and continuity.
(1) Small "Project Board". This is a perforated board that lets you place and solder parts. Mine is 1 1/2 inch square.

NOTE: R/S normally doesn't carry the 1% resistors. You can order them, though, at the same time you order the unit. Your best bet is an electronics suppy store for these items (except the unit). Check the Yellow Pages.

Let's Build The Thing:

Step 1 --- Assemble ribbon cable to head connector. Insert into Connector CN1.

Step 2 --- You need to close Links 1,2,3,4, and 6. Do this by putting a dab of solder across each link.

Step 3 --- Connector CN1 pins 2 and 4 are connected to ground 12v (-). Also, connect the ground (or common) pin on the 5v regulator to this ground.

Step 4 -- Now, connect 12v (+) to the Vin (Voltage In) pin of the 5 volt regulator AND one end (doesn't matter which) of R(a). Remember R(a) is the 1 M Resistor.

Step 5 -- The other end of R(a) is connected to one end of R(b) (the 10K resistor) AND Connector CN1 pin 1 wire on ribbon cable.

Step 6 --- The other side of R (b) is connected to the ribbon cable wire for CN1 pin 2.

Step 7 --- Connect the Vout pin on the 5 v regulator to the wire from CN1 pin 3. Connect CN1 pin3 to CN1 pin 6".

Step 8 --- Finally, connect the wires fron CN1 pin 10 to the wires from pins 12 & 14. Connect CN1 pin 9 to pin 13.

That's it for the wiring. Now we have to do just a little more soldering.

Locate the annunciator pads on the left hand side. Refer to the instructions. This is what symbol will be shown on the display. We want the "V" for Volts.We want to put a dot of solder across the "V" pad and the "selected" pad. We then want to solder all the other annunciator pads to an adjacent "not selected" pad. Take a look at page three of the instructions to see the example of how they did the soldering for "C".

Congratulations...you've done it! All we have to do now is to calibrate the unit. Because of the resistors we used, the readings are not perfectly linear. We need to calibrate it for the range that we expect to use.

Decide where you want to hook into you power. I found that to get the best readings, you need to hook into the battery itself. Other places have a voltage drop of up to .5 volts. I installed a relay. They are easy to install...all you do there is follow the instructions with the relay. Nonetheless, you now need to hook the + wire from our module to the + terminal of the battery and the - to the - terminal. Also, hook up the muti-tester set to measure 20v. Locate "VR1" (refer to the instructions). This can be turned with a small screwdriver. Slowly adjust VR1 until you get the same reading on both voltmeters. You won't actually get the same reading, but you will get pretty close.

Now you can either make a cut-out in your dash or panel of your choice or do what I did. I used a "project" box that was about the right size and made a cut-out on the top with the screws. I made sure that it was waterproofed with clear caulk and mounted the "box" part under the pocket cover. I also drilled holes to allow air circulation and to prevent accumulation of condensation.