This PROPTACH does not have any contact to the aircraft – no hook-up is required. It is purely optical. It works on ambient light and displays directly any two or three blade prop RPM. It is hand-held point and read. Or simply laid on most any horizontal surface that will expose it to the prop. It is small hand size. The specifications state a one RPM resolution. The range is 250 to 4500 RPM. It is battery powered and will indicate a low battery. It has a quartz time base. The stated target (prop)acquisition range is up to ten feet. It uses a 9 VDC battery. It is manufactured by Cardinal Electronics Inc.