RC Circuit – Determining the Value of C

For old/obsolete ICs like this, you can sometimes find them on eBay.

Here is one listing from China for the UM66, and another one from France.

Note that there are some unscrupulous semiconductor vendors on eBay so be careful, I would always buy from somewhere like Digikey, Mouser, Farnell, etc if you can (I had a quick look but couldn't see anything particularly suitable)

You could quite easily make a similar circuit using a small micro, a 555 timer or two, using discrete transistors, etc. It depends on your level of knowledge and whether you actually want to design a circuit or simply build an existing one. There are many doorbell circuits/kits out there that do have parts you can find - Velleman make lots of kits, here is a doorbell/tone generator kit that may be of interest.

Atmospheric pressure, or the lack thereof, does affect electronic components. Components in low to near-zero pressure tend to outgas, and while ICs are relatively simple to condition for this, parts like electrolytic capacitors will fail. Hence, components specifically designated for zero-pressure are used instead.

Radiation affects ICs in two ways: Firstly, semiconductor behavior changes significantly under increased ionizing radiation, such as exists outside the earth's protective atmosphere, and in the highly ionized belts of the stratosphere. Hence, radiation-hardened parts are manufactured specifically for such purposes, and are used in space electronics.

Secondly, under normal operation (on the ground) the thermal output of any IC gets removed from the package by a combination of radiation and by being carried away passively by moving air... In low pressure or vacuum, only radiation of heat works, not passive air-borne cooling, thus changing thermal dissipation calculations for any component.

Thus non-traditional cooling mechanisms and considerably greater distribution of conductive cooling paths are required.

Regarding gas-related precautions for space electronics: Manned space vehicles have sometimes used oxygen enriched environments. This leads to a necessary rethinking of such circuit design elements as PCB spark gaps, which could lead to catastrophe.

Also, non-design sparking, such as due to motor / coil field collapse, metal contacts of switches, or just a loose connection, need to be eliminated entirely - much more critical than for normal earth atmosphere. Silicone-filled contact casings, like the classic oil-filled switches, are worth considering. Similarly, space-safe epoxy potting of practically all exposed metal including PCB traces, is a way to go.

Further, there is the whole thermal operation range to be considered, especially for unmanned craft: From very hot (due to solar exposure without atmospheric protection), to very cold (due to no "atmospheric" heat when facing away from the sun).

This cyclic heating and cooling causes potential metal fatigue, junction stress and fracture such as at solder joints, and loose contacts due to uneven mechanical expansion and contraction between different materials.

Finally, not all semiconductor components are specified for extremely low temperatures. While heat might be an obvious concern, cold is an equally big issue. Some parts are specifically manufactured, and tested, for extreme low temperature operation. For other parts, the component behavior changes need to be taken into design consideration. For instance, the simplest PTC resettable fuse is no longer a trivial circuit element in space electronics.

I hope this has given an insight into just some of the factors around your question. For the rest, a search engine is your best bet.