Need easy instructions on how to use GRT-3500 Gardner Bender

I found one!

GARDNER BENDER Toggle Switch, 20A 125VAC, On/Off, ...

Home Depot https://www.homedepot.ca > ... > Light Switches

The Gardner Bender 20/10 Amp Toggle Switch features spade terminals and on-off single-pole, single-throw circuitry. This switch fits standard 1/2 in.

..

As with any single pole switch, there are two wires, or else two screw terminals. One is for the wire (or wires) that is always "hot", and the other is for the wire that goes out to the light fixture.

AA means double A , AAA means triple A, 9 volt has a double lead with two wires on it. which do you have?

I don't think you will find a manual for a Pentasonic. I was conned into one in Tenerife in March 2013 and there is no address of any sort on the packaging or instruction manual. I am sure that is against EU rules. They seem to be manufactured to mimic a Panasonic model. That's what I thought I had bought until I got it back to the hotel.

check online at GB's site, some manuals are available as a pdf

so do I
I will call them and post the answer

Two LR44 button batteries.

Google " gardner bender GVT-392 manual".... there is plenty of hits good luck

Set it to OHM or resistance and read the element

To measure the voltage at an electrical outlet requires the use of a multimeter. Always test your test equipment for proper operation before use. Set the multimeter to AC voltage. The markings may appear as VAC, AC V, or a V beneath a wavy line. Choose the AC voltage closest to the voltage you will measure. Standard current in home and businesses in the USA is in the range of 110 to 120 volts. Some circuits used for equipment such as dryers, air conditioners, electric stoves, ovens and other large equipment may use voltage in the range of 220 to 240 volts. These heavy-duty circuits can be identified by their non-standard outlet plugs. While grasping the insulated portion of the probes (never touch the metal conductor during testing) place one probe into each of the two terminals. The multimeter will display the voltage. Carefully remove the probes, being careful not to touch the metal part of the probes to anything or each other. The voltage should typically test in the range of 108 to 121 volts for most circuits. If voltage is higher or lower, professional electrical service may be required.