This circuit would be used to hold a BLOCK OCCUPIED condition as a train passes out of the block even if the normal detector is not sensing a train.
The Helper circuit uses phototransistor type light activated detectors to sense that a train is still in the block, even though the engine has left that block.
The only reason to use this circuit is if there are no conducting wheel sets at the tail end of the train that would keep the normal occupancy detector activated until the train has completely exited from the block.
Simply stated the Helper circuit's output is in parallel with that of the normal detector. When the Helper circuit is activated by a train covering one of the phototransistors at either end of the block, the Helper's output holds the block as occupied even though there is no current flowing through the sensing circuit of the normal detector and has therefore turned OFF.
The explanations for the circuits on these pages cannot hope to cover every situation on every layout. For this reason be prepared to do some experimenting to get the results you want. This is especially true of circuits such as the "Across Track Infrared Detection" circuits and any other circuit that relies on other than direct electronic inputs, such as switches.
If you use any of these circuit ideas, ask your parts supplier for a copy of the manufacturers data sheets for any components that you have not used before. These sheets contain a wealth of data and circuit design information that no electronic or print article could approach and will save time and perhaps damage to the components themselves. These data sheets can often be found on the web site of the device manufacturers.
Although the circuits are functional the pages are not meant to be full descriptions of each circuit but rather as guides for adapting them for use by others. If you have any questions or comments please send them to the email address on the Circuit Index page.