What weak signal and what circuit? The answer may involve at least one transistor, but it would help to know what you've got.
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well, it would be a quite easy circuit. I just want to use a kitchen timer (which gives a really weak signal to the speaker) and let this start another circuit which runs on a 9v battery and lights a light.
You've got a switch in the timer, is it electronic or mechanical? But all you'll need is something for it to turn on. Simple transistor I think, so I'll guess a diagram: L
lemonie is right, we need a bit more info:
if it is a digital signal you will need a bipolar transistor in saturation mode or juste a plain mosfet.
if it is an analog signal and you need more power (higher voltage AND higher current) you will need one or more transistor in analog mode...
if you only need an higher voltage a simple Op amp (google for 741 op amp)
will do it.