Circuit to trigger an alarm on water level change

This is a little bit better, and it would be interesting to test, but it's still a crap minimalist type of thing: View in a fixed-width font such as Courier.

. . . . . V+ . | . .-----------------+--------------. . | | . | [4.7k] . | | . | +--------. . [R1] | | . | +|47u | ---- . | |\\ === [100k] | | . +--------|+\\ | |\\ | | | . | | >--+--[100k]------------|-\\ | | | . red --- .---|-/ | | | >--+--> | | . LED ~ \\ / | |/ [51] .----+--|+/ | -- -- . 1.5V --- | | | +| / | . | +----o o-+--[100k]----' === [100k] . | | || |47u | . | [1.5k] Rx +--------' . | | | . | | [4.7k] . | | | . '----+-------------+-------------' . | . --- . gnd . .

That part is obsolete.

By gawd they are or at least National is stopping making them. Obsolete really would mean that there is a better part to replace it. I don't know of one.

I think this ends up as a two stage design. We can just insert a buffer amplifier and use an LM211 in place of the LF211.

Yep, that's a standard 5% value.

Replace the 51 ohm resistor with a pot. The sensitivity in ohms decrease in Rx will be numerically equal to 1mA x Rpot in millivolts.

Hmmmm...that one is not as easy. It will require more components.

The simplest thing is to use the output of the above circuit to trigger a 555 monostable which in turn drives a transistor to activate the relay.

You still haven't stated what kind of water the organism lives in. There is a huge difference in conductivity between fresh and salt water and you will need to compensate for that. You will also need to compensate for the change in conductivity as the organism defecates in the tank.

A more detailed requirement is needed before anyone can design a good system for your homework project.

Maybe it's giant electric eels :P D from BC

Skjul sitert tekst -

Thanks Fred, I'm new to this. Can anyone draw up the 555 circuit in conjunction with the abowe - The 555 triggers on faling edge and this circuit gives high when triggered. Can anyone also draw up how to adjust the time sensitivity? Thanks

Kan this circuit on 555 be used?

V+ -+--------+---+--+-------------+-------+------ | | | 1N | | | | 4 |8 4004 -+- .--+--. [10K] .--+------+-----. / \\ | | RELAY | 7| R VCC | -+- '--+--' +-----+ DIS |5 '------+ IN | 2| +--- .---+-. >---- | ----+ TRIG NE555 |3 | C | | 6| Q+---[18K]--+B |BC548 +-----+ THR | | E | | | GND | '---+-' === '-------+-------' | |470U | | | | | GND --+-------------+----------------------+---------

The LED idea as a reference is a good one but I'm not sure we really need it here. Since the thing we are measuring is a resistance change, we can make everything a ratio so that the supply voltage drops out of the equations.

That's not the reason I decided to comment here however. I have a question the OP needs to consider:

Does this water already have conductive things in it by which there could be a path to ground? If so, we need to be careful not to put any large voltages on these new electrodes we are putting in. This may require that one of the two electrodes we use be grounded.

tekst -

I think the water is slightly salty. I measured the resistance in the working erea to be between 200 and 1000 Ohm, and I want to detect a drop of approx 50Ohm (adjustable) within a approx 10 seconds (adjustable) If possible I also want the constant current leak in the measurement circuit to be as low as possible, as I consider using a 9V battery as supply.

snipped-for-privacy@touchsoft.no

Why can't you use a pressure sensor? Frank

Skjul sitert tekst -

Invert the op-amp output.

It should give you about a 5 second pulse. Add a .01 uF cap from pin 5 to ground. The BC548 is too wimpy and the base resistor is too big. Use something like a 2N2222 for higher collector current, and lower the base resistor to 1K.

Ed