Milliammeters From Surplus VU Meters

I had a bunch of these beautiful VU meters lying around. About time I made something useful out of them. At the same time using digital multimeters for measuring current was a huge pain. Had to change the measuring lead to a different socket and then clipping the leads to crocodile clips, etc. So decided to make two standalone milliammeters to use in the lab, one reading 0-10 mA and the other 0-100 mA. Ideal range for working with LEDs.

I disassembled the VU meter, glued an aluminum foil tape on the blue plastic meter face and then put the meter together but without the cover for the meter face.

I connected the meter to the circuit shown. A 100 ohm 10-turn potentiometer was clipped in parallel to the meter terminals. A programmable current source that I had constructed was then connected in series with the VU meter and a quality digital meter set to read mA. The pot was set to maximum (100 ohms), current was set to 100 mA - the digital meter confirming that 100 mA was flowing through the circuit. The resistance of the pot was decreased till the meter needle swung to full scale.

The pot was disconnected from the meter and the rest of the circuit making sure that the resistance was not changed during all the disconnects. The resistance of the pot was then measured with a multimeter to be 2 ohms.

A 2 ohm resistance was soldered across the meter terminals, and the meter was connected in series with the current source and digital meter. Different amounts of current were passed through, from 1 to 100 mA, and the position of the needle at each current was marked on the aluminum foil tape meter face.

For the 0-10 mA meter, I decided to solder a 20 ohm resistor across the meter terminals assuming that the meters are identical. If 2 ohms gives 100 mA full scale deflection then 20 ohms should give 10 mA full scale deflection. I then put this new meter in the circuit as shown and calibrated the needle positions for different current levels.

This is an unorthodox way of figuring out scale readings; for a more formal and detailed approach I highly recommend two videos by w2aew. The fist video from w2aew shows how to characterize an analog meter movement. The second video shows how to calculate the shunt.

My calibration procedure showed that the meter response was nonlinear.

I imported photos of the hand labeled meter faces into a drawing program, then drew the scale on top of the squared photo and sized the scale to the correct dimensions. The scales were printed on an inkjet printer, cut out and carefully aligned onto the meter face (after removing the aluminum foil meter face). The scale was held in place with double sided tape. The printed meter face was covered with shipping tape. The feter was the fully assembled locking the new meter gace in place. Looks great, doesn't it?

Measured the dimensions of the meter and built a little wooden box to encase them. Based on the measurements I drew out the wood panels in a drawing software and cut them on a laser cutter out of 3mm thick plywood. The wooden pieces were attached to the meter by using double sided tape for temporary positioning and by wood glue to get a permanent bond.

After the wood glue had cured, I wrapped the four sides of the small box with shipping tape to add further strength to the whole unit.

Small holes were drilled into the top of the meter case. Female header sockets were placed into the holes with 2 sockets per meter-terminal in case I wanted two wires to connect to each terminal. I soldered the sockets to the terminals - which turned out to be quite difficult due to the tiny space. Might have been better to just attach the top wooden panel and then solder and then assemble the other panels. You can see the bit of plastic I had melted with my hamfistedness.

Soldering on the 2nd meter went better as I had figured out the contortions required to solder the terminals.

Did a final test on both meters. Worked great with an accuracy of about 5-10%.

Glad I did make these as am using them quite a bit for figuring out LEDs for a colorimeter I am working on.

NOTE: You can wire multiple shunts into a single meter and use a switch to switch in a specific shunt or use additional sockets that connect to a specific shunt. I needed two meters so decided not to use multiple shunts per meter. I may add a 0-1 A range on the 0-100 mA meter (which will need a 0.2 ohm resistor) and most likely will use an additional socket that connects to the 0.2 ohm resistor. A switch wont work as the switch resistance may be significant.