The following circuit is very similar to most other RC switches. The only difference is, this one has a latching output. You can switch devices that are powered from the same battery on your receiver (circuit 1), or higher voltage devices from a separate battery (circuit 2).
Circuit 1 will only switch loads up to a maximum of 4.8 volts or 6 volts (the Rx battery), so it's ideal for switching LEDs and small motors, etc.

I've used circuit 2 with no problems on a 8.4 volt ni-cad battery, powering a pump motor. Since I only needed less than one amp, and in order to keep the board small, I removed the heatsink tab from the TIP122. The '122 is rated at 5 amps or so, so it runs cool with my small current load. But you will have to leave the tab in place if you intend running it at maximum rating.

'Twas very tempting to design this with a microprocessor at the heart. But where is the fun in that? It's obvious that British modellers are a clever lot. Judging from the comments that I've read on various forums, a lot of you would prefer to build, rather than buy. And since this design is of the KISS principal, (Keep It Simple, Stupid), here is a circuit you can build with components right off the shelf.
To think, just a few moons ago, we were building this stuff with valves. These days we have micro's and suchlike. But a lot of today's stuff ain't got no soul.

This circuit is based around the trusty ol' 4013 flip-flop IC. Two '4013s and a few other components were stuffed on a piece of stripboard measuring 25 holes x 9 strips. Fig.1 shows the bare board, seen here enlarged for clarity.

Before you start soldering, the board needs to be prepared first. Turn it over, then cut 21 breaks in the copper tracks (Fig.2). I found that a 3mm or 4mm drill bit usually make the neatest cuts.

Next, solder 15 wire links (Fig.3). In order to keep the board reasonably small, four of these links are soldered under the chips -- two under the first chip, and two under the second.
At first glance it appears there are just 14 links. But if you look closely around E12 - G12, you'll notice that two links are sharing the same hole at F12. If you use single-strand wire, you should have no problem getting them in. Just remember to fit them before you solder them.

Solder these links at the following locations:-

Make sure that the four links under the chips are not in contact with each other, then align pin #1 of the first chip at location C6, and pin #14 at C9.
Align pin #1 of the second chip at C13, and pin #14 at C16.

The 4013 is a pretty hardy chip. I've soldered / unsoldered several and still not popped one yet. For this reason I opted to omit the 14-pin DIL sockets. (In other words, I didn't have any in the junkbox).

Next, add the 100K trimpot and the two resistors (Fig.5). Solder the 470-ohm resistor at locations E19 and I19. Solder the 2K2 resistor at locations D20 and G20. The trimpot is a mini-type trimmer. Be sure that its wiper pin is connected at F3. Solder its other two legs at locations D2 and F1.

I robbed my trimpot from an old VCR, but they're just as easy to get from Maplin, ESR, etc. The leg spacings are 0.1", so you'll have to use a bit of jiggling to get them in diagonally.
Although this trimmer is shown as 100K in the schematic, I never had one of that value in the junkbox. Instead, I shoved a 220K in there. It works okay - just means the adjustment is a little more course.

Fig.6 shows the location of an on-board LED. When the circuit receives a signal from your Tx, the LED turns on. It then remains on (latches) until it receives further input from your Tx. I stuck it there as a visual aid to adjusting the trimpot when it comes to setting up.
Most LED cathodes are usually denoted by the shortest leg, and / or a 'flat' on the body itself. Solder the cathode at E22 and the anode at D22.

The 22nF capacitor is the timing cap. It's value is pretty critical. Solder it at locations F4 and H4. The 1nF capacitor is a supply decoupling capacitor. It isn't shown in the original circuit, but I stuck it there to counteract any sporadic glitching. It's value isn't critical, so anything between 1nF and 100nF should do the trick. Solder it at locations G3 and I3.

The output transistor is shown in Fig.7. You need to bend each of its three legs 90-degrees down before soldering to the board. Be sure that the written side is up.
In order to keep the board compact, I hacked off the heatsink tab (included here for clarity). Since I'm only using light loads, it still runs cool.

Solder the '122 as follows:

A standard servo plug is used at the input. (Fig.8). Most servos use red and black as the supply connections. The signal wire is usually yellow or white. Connect these three wires as follows:

Fig.8b shows the output load being powered from a separate battery supply. The drawings are not to scale, but that's supposed to be a Speed 600 motor and 8.4v nicad battery there. Even at full tilt and without the heatsink, the '122 still runs without smoking.
If you're using a separate battery, then take these connections from locations H25 and I25.