Radio Shack Meter Modifications
Tony Alfrey
tonyalfrey at earthlink dot net
Jan. 23, 2011

For many years, Radio Shack stores have sold electronics components for the hobbyist and experimenter.  Certainly this is now a small fraction of the stuff they sell, but they still offer at least some components.  At one time, they offered a full range of analog panel meters, but this list is now down to just one offering.



Radio Shack Part Number 22-410


As you can see, this meter appears to be a conventional analog 0-15 V DC meter.  But not quite.  It is actually a 0-1 milliamp meter with an additional series resistor of 15 kohm that is supplied in the package with the meter.  The user is instructed to install the resistor in series with the voltage to be measured.  With a simple change in the selection of the resistor, one may change the voltage range or, by placing a resistor in parallel with the meter connections, one may instead convert the meter to a DC ammeter.

Of course, then one has the problem of the meter scale being in disagreement with the actual meter function.  To solve this problem. I've created artwork for a whole range of alternate meter scales that may be used with this meter.

Why would one do this?  For example, one may purchase a variety of Chinese-manufactured analog panel meters from several on-line vendors for less than the $12.99 price of the Radio Shack meter.  But the Radio Shack meter, which, by the way, has remained unchanged in physical dimensions for over 30 years, has one very important feature which is apparent in this view from the back.



The enclosure for the meter movement itself measures only 1" in diameter by 3/4" deep.
Compare this with the enclosure of other typical analog meters at about the same price.



This means that, when you are drilling a hole into a panel on which to mount the meter, you can use more conventional tools, including the typical
Greenlee knock-out punches which are perfect for very clean holes in sheet metal:


1" Greenlee Knock-Out Punch #730BB-1

If your not familiar with these punches, the user first drills a smaller hole, in this case, about 3/8", the punch components are assembled through the hole and around both sides of the panel, the cup is tightened with a wrench, and a clean hole is "knocked out" from the panel.  The "knock-out" piece is shown to the left.  These punches are not cheap (about $45) but they are perfect for repeated use.  A conventional drill bit can also be used, but in thin metal panels, it is best to use either a punch or a hole saw.

Changing the Range and Function

It would be nicer if the meter movement offered by Radio Shack were a 0-100 microamp movement (the standard used for virtually all analog meter movements prior to the advent of the digital panel meter) but we work with what is available.  The resistance of the meter movement itself is advertised to be 85 ohms, and I measured 86.4 ohms in the lab.

To make voltmeters from the 1 milliamp meter, one must add a resistor in series with the meter movement:

Range Added Series Resistor 0-1 Volt 931 ohm/1% 0-10 Volt 10.0 kohm/1% 0-25 Volt 15.0 kohm/1% 0-50 Volt 49.9kohm/1% 0-100 Volt 100kohm/1%

The series resistor for the 0-1 Volt movement is reduced from 1000 ohms to acommodate the meter movement resistance.  Correcting for this error is unnecessary for the other ranges;  the error is insignificant.

To make ammeters, one must add shunt resistors in parallel that are closely matched to the meter resistance.
For example, to make a 0-10 milliamp meter, 9 milliamps must pass through the shunt, while 1 milliamp passes through the meter.  If 1 milliamp passes through the meter resistance of 86.4 ohms, this represents a voltage drop of .0864 volts.  Therefore, the shunt resistance must be .0864 volts / 9 milliamps = 9.60 ohms. This value can be obtained by putting standard 1% tolerance resistance values of 19.6 and 18.7 ohms in parallel to yield 9.57 ohms.

Range Shunt Resistor 0-10 mA 19.6 and 18.7 ohms in parallel 0-50 mA 1.8 and 91 ohms in parallel 0-100 mA 1.8, 1.8, 56 and 56 ohms in parallel

Shunt resistors for higher current values can become tricky to fabricate.  But if one can use standard values for the shunts (such as 1, 5 or 10 milliohms), then the problem becomes easier.  How could this be achieved?  The solution is to modify the internal resistance of the meter itself by adding an extra series resistance.  Then this assembly is connected to the shunt resistance.  Here is the circuit to use:




We start with the 86.4 ohm meter resistance and then add 13.7 ohm/1% standard resistance in series, to obtain a total meter resistance of 100 ohm (100.1 ohm).
Now, this meter will read full scale with an applied voltage of 0.1 Volts.  If a shunt resistance of .01 ohms is used, a current of 10 amps through the shunt will yield a voltage drop of 0.1 Volts across the shunt (the meter itself will draw negligible current).  So the table now becomes as follows:

Range		Add Series Resistance	Shunt
0-10 mA		0 ohm	19.6 and 18.7 ohms in parallel
0-50 mA		0 ohm	1.8 and 91 ohms in parallel
0-100 mA		0 ohm	1.8, 1.8, 56 and 56 ohms in parallel
0-500 mA		13.7 ohm/1%	0.5 ohm
0-1 A		13.7 ohm / 1%	.1 ohm
0-10 A		13.7 ohm / 1%	.01 ohm
0-50 A		13.7 ohm / 1%	.002 ohm

Of course, when making ammeter shunts, the shunt itself must tolerate the higher current you are measuring, especially since the resistance of the shunt can change with temperature.  Refer to a Digikey or Mouser catalog for suitable current shunts.

Display Change

Now that the meter function and range have been changed, you'll need a new scale.  The clear plastic meter face is easily popped off of the meter body with a small screwdriver.  Once this is achieved, you can either remove the old meter scale with a razor blade, or affix a new scale over the old one.  There is not much clearance between the meter needle itself and the face of the scale, so you may find that it is necessary to remove the old scale.  Of course, you must use extreme care in doing this so as not to damage the meter needle or movement.



The original meter scale is printed on a sheet of thin aluminum, and affixed to the meter body with double-sided tape.  So after the razor blade is thoroughly inserted under the scale, the scale is removed as a whole unit without much difficulty if you work slowly and methodically.  Most of the old adhesive can be removed from the meter body with a tissue and some denatured alcohol.  The new scale is attached with thin, double-sided tape, such as Scotch #665.

This is what a typical page of meter scales looks like. 




Download the Adobe Acrobat page of meter scales from here.
Simply print out the whole page of scales on a sheet of high-quality photography-grade paper and cut out the scale you want with scissors or an Xacto knife.  I use a circle template and Xacto knife to cut out the half-round portion of the scale surrounding the meter movement, and I use double-sided tape to affix the new scale to the meter body.  Each meter scale should be exactly 2.6 inches (66 millimeters) wide at the top (there is a slight taper).  You should make sure that your printer configuration is printing to the correct size before you waste expensive photography-grade paper.  Tweak the printing scale of your printer or print utility before committing to a final print.  Here is the typical end result:




Important!  When you snap the meter cover back onto the meter body, be very careful to realign the meter zero adjust (the thing that looks like a small slotted screw on the face) with the meter zeroing lever that is attached to the meter return spring.  The tiny black plastic pin protruding from the back of the zero adjust must align with the slotted lever affixed to the actual meter movement.

Here is the Adobe Illustrator file used to construct the various meters so that you can make your own scales or customize them with fancy colors or fonts.
The "VU" meter face in the collection of faces is a classic style used for audio recording and broadcasting and requires a special driver circuit that I'll provide soon.  Remind me if "soon" turns into "not-so-soon".