The search functions in the universal remote controls are not really automatic to the degree that you may think. Communication between the remote control and the target equipment is strictly a one-way path.

A common way that a universal remote control does its search function is as follows.

1) The user is instructed to turn off the target device such as the VCR.

2) Next the universal remote control is aimed at the VCR and the search function is started. This is usually initiated via a special button press sequence on the remote control or via a special slide switch in the battery compartment.

3) The user then presses some particular key on the remote control which acts as a NEXT type key. As each "next" key is pressed the user is to observe if the VCR has turned on.

4) After the VCR has turned on the remote control is then set back out of the search mode so that the current device selection is locked in and remembered.

What is happening inside the universal remote control is that the microcontroller has a device code table for all of the target type devices that it knows about. Within each code table there is the definition of the IR transmit sequence for POWER ON for that device. Stepping in sequence with the special button press process the microcontroller sends the POWER ON command for each device out the IR transmitter. Obviously when the VCR has turned ON it is a good possibility that the device code table corresponding to that POWER ON sequence is likely to work for the other other defined VCR control codes such as OFF, CH+, CH-, VOL+, FAST FW etc.

(Note that sometimes when performing a search function with a universal remote control you may find a POWER ON function that works for the VCR but other functions will not work. It is not uncommon for there to be device code tables in the universal remote control that have similar POWER ON sequences for several different models of VCRs. If you locate an improper device it is necessary to start a redo of the search and skip the found entry in the search and try to locate a subsequent entry. For this to work correctly it is necessary to have the native control for the VCR in hand so that you can turn the VCR back off so that a search on the universal remote control can be resumed. It can also be handy to count the number of presses of the "NEXT" button so you can easily get back to the currently selected entry and skip it.)

Please note that this discussion is intended to be a general conceptual presentation of the process and that most universal remote controls have quite varied ways of functioning. However this should give you an idea of how you could implement such a function in your own microcontroller.

One other thing to keep in mind is that an IR search function should be thought of as a SETUP type function that is done one time. It is not particulary practical to anticipate a search mode that begins every time the remote control is picked up for use. Also keep in mind that it would be necessary to have some type of non-volatile storge to save the found device code table entry so that the selected device type can be recalled after the remote control batteries are changed out.

Michael Karas