There are a number of bike indicators on the market these days. This one is from the UK, and looks as if it might work (for a change).
Not just an indicator, but a mirror too, and a front and rear light.
Check it out:
This bike bit comes from the USA and may be useful for mountain bikers as well as the tandemists amongst us who need some quite small inner cogs due to the sometimes lost chains that go down where they don't belong.
Even if it had to be posted here, this device would not break the bank, for a change: