Abstract: An adaptable string detector for currency validators has an infrared emitting diode on one side of the note path and an infrared phototransistor on the other. The phototransistor is connected to a window comparator and to an EEPOT. The EEPOT establishes a quiescent value for the output of the phototransistor to lie within the window of the window comparator. If a string or other retrieving implement is attached to a piece of currency traveling along the note path, the output of the phototransistor falls outside of the window, signaling that the note is to be rejected and no goods, services, or change is exchanged therefor.