"That's nifty," said Mike as he picked up an object from the workbench and turned it over in his hand.
"That's my new digital multimeter from radioshack.com," I replied. "It's a Metex model ME-11. Radio Shack's part number is 910-4092.
"It certainly looks nice," he commented. "What did it set you back>"
"Would you believe twenty dollars?" I grinned.
"No, I wouldn't believe it" he replied sneering. "What did it really cost?" I pulled a paper from a pile on the computer desk and handed it to him. "Twenty bucks," he agreed staring at the receipt. "So how good is it?"
"I can tell you it is well made," I replied taking it from Mike and laying it face down on the bench. I picked up a phillips screwdriver, removed the battery compartment cover and extracted a small but thick plastic bag. "You don't often see a battery cover attached with a screw anymore. And I have never seen a plastic guard before to cover the battery and protect the meter in case the battery should leak."
Mike studied the plastic. "That is neat!" he agreed.
I picked up a test prod, unscrewed the handle and slid it back on the wire. "The test prods are soldered on instead of molded," I pointed out. "So they can be repaired when the wire breaks. And the wires always break eventually!" I added. "But the real reason I bought it is the computer interface." I pointed to a DB-9F on the top of the meter. "With this I can set up an experiment and walk away while the computer graphs the voltage for me for as long as I like."
I picked up a tiny rattail file and started filing a notch in the battery compartment cover.
"Hey! What are you doing?" exclaimed Mike aghast.
"I'm making a slot to run a small wire out of the battery compartment," I explained and slid a tiny piece of perfboard attached to a wall wart to the center of the workbench, "for this battery eliminator. The one disadvantage I have noticed about this meter is that it eats batteries. A dollar store nine volt battery didn't last twenty-four hours in it. I plan to run experiments that last for days and I need to keep the meter running and this seemed like the best way."
"That's slick," said Mike admiringly. "What's in it?"
"It's nothing but a 12 volt D.C. wall wart from my junk box, a simple voltage regulator made from a nine volt zener diode and a one hundred ohm resistor, a 100 MFD capacitor to filter out ripple and a .1 MFD cap to take out hash. It looks like this," I added drawing out the simple schematic. "I built the regulator on a piece of perfboard the size of a nine volt battery so it would fit inside the battery compartment. I used a nine volt battery connector on the output to plug into the battery connector on the meter. So the only modification I have to make to the meter is a tiny hole to run the wire out. And I have done that by filling a slot in the battery compartment cover so I don't have to damage the meter itself," I added triumphantly.
"It looks like you've thought of everything," observed Mike respectfully.
"There's one more thing," I said warming up to my lecture. "You'll notice that the leads on the battery connector appear to be connected backwards."
Mike examined it closely and cleared his throat. "Yes, I noticed that," he said. "I was about to ask you about it."
"That's because we are using it in reverse. The negative terminal of this connector mates with the positive terminal on the one in the meter so our black wire must go to the positive side of our battery eliminator."
"I knew that," said Mike somewhat confusedly.
I slid the perfboard into the protective plastic and snapped together the battery connectors. Then I stuffed it all into the battery compartment. "That's all there is to it," I said replacing the battery compartment cover. "Let's try it." I plugged the wall wart into a bench outlet and pressed the ON button on the meter. We were rewarded with high contrast numbers on the LCD display.
"So now you can use just like it had a battery in it?, queried Mike.
"Not quite," I cautioned. "You wouldn't want to put the common lead of the meter on more than a hundred volts or so with repect to ground. You have to keep in mind that any voltage on the meter common is also impressed across the insulation in the wall wart and I wouldn't trust it too far. In addition there will be some capacitive coupling to the power line which could induce hum into sensitve measurements. You need to always be aware of the limitations of your test equipment and I have just added a couple more to this meter. But it is worth it to not have to worry about a battery running down in the experiments I have in mind."
"So what are we waiting for?" asked Mike excitedly.
"No reason at all to wait," I said plunking a Hodowanec capacitor in front of Mike and sitting down to load the meter software into the computer.