Is a toy’s remote controller interchangeable

How hard is it to replace the circuitry - Not hard at all.

Here's a circuit for the sender unit capable of 16 unique single key presses.

The uC has three basic jobs to do.

(1) Scan/read the keyboard and determine which key is pressed.

(2) Convert the 'keypress' to a particular code value (see standard codes e.g. RC5) with various start and stop bits

(3) Send out a stream of bits with the 38kHz carrier (internally generated) to the output pin.

You'll find the necessary software in most libraries or other uC projects. From a power saving point of view you should use the key press to wake up the processor.

IR Remote controls are generally designed to be tight to their functional requirement in order to keep their manufacturing cost extremely low. This generally means that the chip in the remote is a dedicated chip designed to produce a certain IR protocol that is specific to the manufacturer's device family.

Each key on the remote will cause a different IR modulated sequence to be sent to the target device. All of the key codes for that target device will be within the same IR protocol family. A protocol family will support a certain number of key codes within the protocol whether that be 32, 64 or 128 codes (or some other number). I mention this because some remotes can support multiple target devices for example TV, DVD etc. Each target device may be a different protocol family. There are handful of different protocol families in use and are generally lined up with major equipment manufacturers such as Sony, NEC, Philips and others.

It is highly unlikely that the remote that you have in hand will be easily convertible for use with your alternate target device. You would have to first know what IR protocol family was required for your target device and what the required individual key codes are. Unless you have the actual remote in hand this information is almost impossible to reverse engineer. The best you could hope for is that the family protocol and key codes may be documented off on some web site.

If you did manage to find the target device family protocol and key codes then the process of converting the existing device would only be successful if it natively supported the same IR family protocol. It is usually possible to reverse engineer the IR protocol and key codes for an in-hand IR remote control. Two general methods are used for this, both of which require the use of an oscilloscope. With one scheme you probe the signal in the remote itself that drives the transmit IR emitter LED. In the second scheme an IR receiver component can be rigged up on a proto-board and then used to receive the signal from the remote control which is then probed with the oscilloscope. The latter scheme is somewhat easier to see the IR signalling envelope because the IR receiver removes the carrier frequency from the detected IR stream. At the transmit side you would see this carrier as part of the waveform and is typically 36KHz, 38KHz or 40KHz although other carrier frequencies may be used by some IR protocol families.

In the long run you would probably be best off to look into finding a replacement remote control for your target device or look at universal replacement type controls that can be programmed to emulate the IR family protocols and key code sets for several thousand different target devices. Even if the universal remote does not directly support your target device it is possible to sometimes experiment with selecting alternate similar type device selections and find one that will allow partial control of your device.