vk6kro said:
That relay needs 50 mA coil current. How would you get that much current through a phototransistor lit by an infra red diode held at any reasonable distance away?
Indeed. 50mA is definitely not unobtainable, but if that is still an issue, here is a relay that operates at 11mA:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=255-1899-ND
This is only after a very brief search, and I'm certain you could find one with even lower current requirements (or you could use another BJT as a current amplifier...)
vk6kro said:
Infra red receiver chips have a very nice tuned amplifier built in and these would extend the range of the system by a factor of 10.
Yes, I agree, but they also increase the complexity/cost of the circuit by an order of magnitude by introducing an lm555 etc, and considering other factors such as debugging time, experience, etc, it is probably most wise to use an equally functional but much simpler solution that provides on/off functionality.
vk6kro said:
All this was explained to the poster and he wants to get good coverage by using a more complex circuit. He made the right decision, especially as he wants to learn these techniques.
I was certainly not insulting your suggestion. In fact, I think it would work quite well, and I definitely recommend the OP build a circuit like this if he is interested. I was only suggesting that all things considered, it might be easiest to build a more simple circuit for his nephew and fool around with more complicated things on his own.
vk6kro said:
Even the 38 KHz system would not be able to supply enough current to drive a relay directly, so an extra amplifier is still required.
The current needed to switch on a transistor is a LOT less than the current needed to drive the relay directly.
I disagree with this. Even considering a relay that requires 200mA (or some much larger current,) it would still be easier to use a darlington pair (or another basic amplifier configuration) than several IC's. Regardless, it is quite easy to find latching relays with tiny coil current requirements that would eliminate a lot of complexity from this project.
Though range is undoubtedly an issue, it could easily be improved by using a couple (very affordable) diodes in parallel/series, or by using a single high power diode, without increasing the circuit complexity at all (or really reducing the battery life, since the remote is just used briefly).