Automatic impedance matching conduit locator

In apparatus for coupling an electrical search signal to a concealed electrically conductive element for the purpose of assisting in locating the same

• a pair of output terminals operatively connectable to such an element,
• electrical signal-producing means including transformer means having primary and secondary sides, with said primary side being operatively connected to a signal generator and with said secondary side being operatively connected to said terminals for supplying a search signal thereto,
• current-monitoring means operatively connected to said primary side for monitoring the level of primary-side current, said current monitoring means including filter circuit means constructed to allow such monitoring only of currents having a frequency substantially the same as the fundamental frequency of said signal generator,
• changeable-condition impedance-adjusting means operatively connected to said secondary side, changeable to adjust, to different selected amounts within a preselected range, the output impedance of said secondary side as coupled to said terminals, said impedance-adjusting means including, for defining said range and said selected impedance amounts, a digital bidirectionally gatable up/down counter which is gated in response to selected changes in the level of primary-side current monitored by said current-monitoring means, with different counts stored in said counter effecting different output impedance amounts within said range, and
• means operatively interconnecting said current-monitoring means and said counter, whereby the level of monitored primary-side current is effective to change the count stored within said counter so as to produce an output impedance amount within said preselected range which accommodates maximum output-current flow from said secondary side through said terminals.