Versatile relays that can work fine for most functions in your motorcycle, such as switching lamps, running horns, etc., is the Bosch (now Tyco) 330-073, rated at 30/40 amperes and 12 volts, SPDT, 5 pin, with tab for screw (tab area can be removed); or the Blazer DF005 or DF005W which also has a tab/screw mounting.
This article was written because someone asked about potential problems with the left-hand switch of BMW Airhead motorcycles if using higher powered headlight bulbs. He did not understand why the existing stock headlight relay did not automatically take care of the wear on the switchgear. The reason is that the stock headlight relay does not do what many think it does. Variances of this question have come up many times, often on the Airheads LIST, but also on various lists & forums for other BMW models.
It is important to know that the MAIN purpose of the stock headlight relay FROM 1978 is to turn off the headlight, leaving the dash lights and rear running lamp ON, during the time the Starter Motor is actually CRANKING the engine. The stock headlight relay DOES NOT simply switch the high or low beams on or off. The relay switches the headlight off during cranking.
The headlight relay does not defeat, nor is it directly involved, with the High Beam momentary flash button, depending on year, model, country shipped-to.
Using additional relays to control higher powered than stock headlamps is a must; although some have gotten away with it for years. Note that the left bars switch assembly is NOT CHEAP. It is NOT designed to handle higher-powered headlamps. Using relays will reduce wear on the left switchgear EVEN WITH the STOCK headlight; and usually will increase light output.
Sources for adding relays are any auto-parts store. HOWEVER, www.EasternBeaver.com sells complete plug and play kits for your motorcycle, and they may still have a version WITH a modulator if you wanted that.
Section 1:Here is how the headlamp relay and its associated circuitry works on the 1981-1984 R100RT, and similarly on others from 1979 in most ways. There are some minor details that are a bit different on the ST, G/S, R65 and R45; and also if one has the Euro switchgear, but the basics are very similar.