DIY: Digital Multimeter

Build a meter like this from a kit.

Building a multimeter from a kit is a rite-of-passage for most aspiring electronic technicians and dedicated hobbyists. The process teaches attention to detail, good soldering practices, and simple alignment procedure. In a few evenings, an accurate multimeter can be constructed from a kit.

Things You'll Need

• Soldering iron
• Solder
• Flux
• Flush cutter
• Tweezers
• Screwdriver
• Alcohol
• Cotton swabs
• Known good multimeter - for calibration
• Variable power supply - for calibration

Instructions

1. Check the parts list against the parts included in the kit. Be certain that all the parts are present and their values are correct. This is a good opportunity to learn resistor color codes and the nomenclature for other components. If anything is missing, contact the manufacturer or retailer for replacement parts. In a tech school environment, spare parts may be readily available from previous classes.

2. Read the instructions regarding lead-free soldering. If you have previous experience with lead-tin solder, be aware that the newer lead-free product produces solder joints that appear different. Compared to lead-tin joints, they are lumpy like a cold solder joint. Soldering irons have to operate at higher temperatures, necessitating regular tip tinning to prevent corrosion.

3. Follow the kit instructions. Work on just one component and solder joint at a time. Form the component leads and place the part into the printed circuit board (PCB). Cut off the excess component lead, but don't cut it flush with the board. Cut it about 0.030 inches - the width of a resistor lead - above the surface of the PCB. This makes it easier to de-solder if it must be removed later.

4. Bring the soldering iron in contact with the joint. Watch for a quick color change to bright silver, indicating the metal is hot enough to take solder. Bring the solder in contact with the pad, not the iron, and allow the molten solder to flow across the joint, filling the hole in the PCB. The process should take no more than 3 seconds. The solder should form a concave fillet between the pad and lead on both sides of the board.

5. Clean the zebra strips with alcohol and a cotton swab before installing them. Clean the contact points on the LCD and clean the corresponding contacts on the PCB. Zebra strips look like a piece of black rubber, but they are composed of multiple parallel conductors. After cleaning these parts, avoid touching them with your fingers. Position the zebra strips with tweezers. These strips are commonly used with LCD displays, so learning how to handle them is a good practice.

6. Calibrate the meter according to the manufacturer's instructions. Use a known good multimeter to measure the output of a variable power supply, then check the kit meter against it. The calibration is done by turning a small potentiometer until the kit meter and known good meter agree.

Tips & Warnings

• Work slowly and carefully so you get it right the first time. But don't be dismayed by mistakes. Troubleshooting electronic components involves finding defects and failures in order to repair them.

• Observe precautions for preventing damage from electrostatic discharge (ESD). It is responsible for failures and degraded operation.

References

• Elenco Electronics: M-1007K Multimeter Kit
• Elenco Electronics: M-1007K Assembly Manual