Re: Liquid level indicator

Somebody on my brewing forum suggests this cct, which seems good.

http://www.uoguelph.ca/~antoon/circ/sensor3.htm

I am abit of a newbie, can anybody explain to me what's going on here?

>>From what I understand, a small a/c voltage is sent out, and taken back
on another probe, which is then rectified and 'compared' to a reference
level which energises the relay. Which bits of the cct do what
parts/how they do it, I'm not sure about.

I need to be able to alter this circuit by taking the logical state(0
or 1) of the sensor output, and combining it with a few other gates
before then driving the output relay.

I'm always bad at understanding transistors, and CMOS outputs. Why
does he use PNP rather than NPN here? Does it matter? Can I use say
BC107's (which I have at home) instead?

Regards,
Mark

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