Electronics > Beginners
Use transistors to switch between two signals
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Zero999:
I'm amazed most people, including myself, missed that. Yes, it won't work. R2 will limit the current too much for the relay to turn on.

It might still work with an emitter follower, just with R2 and R1 removed. The relay will get about 4.4V, which may still be high enough to activate it.
ogden:

--- Quote from: Hero999 on April 21, 2018, 12:35:09 pm ---I'm amazed most people, including myself, missed that.

--- End quote ---

That's because question was not about particular circuit. - I missed (to be honest - ignored) to look into that circuit as well :)
glarsson:

--- Quote from: Hero999 on April 21, 2018, 12:35:09 pm ---I'm amazed most people, including myself, missed that. Yes, it won't work. R2 will limit the current too much for the relay to turn on.

--- End quote ---
Didn't miss it. The circuit is weird, but it will work with a sensitive 2V relay. However, it wasn't relevant to the question.
boffin:
Just take HamsterNZ's circuit, replace the transistor with a FET (2N7000), remove the resistors and you're in business (you could keep the Rs if you really wanted, but if the source is guaranteed Hi/Lo you're fine).  Almost no voltage drop across the FET, the relay will turn on/off just fine, rather than being a bit iffy at 4.4V
tron9000:

--- Quote from: gbuilder on April 20, 2018, 10:07:43 pm ---tron9000 thank-you for the clear explanation. My confusion was that I was thinking of the signals as analogue. CLearly this is a solution suitable for digital logic signals.

--- End quote ---

Ahh, well then, your best bet is an analogue mulitplexer: DG409

glad you thought my explanation was "clear" - usually I'm not very good at explaining things!
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