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7555 timer to measure water level?

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New Member
Hi all,

Doing a google search for a water level sensor, I came across A digital water level meter and the section "Digital Electronics Unit". It looks perfect to hook up to an Xbee wireless device to remotely montor water levels in pot plants in a home-automation setup.

However, my knowledge is a bit limited and I can't figure out how IO1, IO2, IO3 and IO4 are connected or the purpose of the raisers. Can anyone here help me out?

John Jore


Well-Known Member
Most Helpful Member
IO1 & 2 go to the home made capacitor assembly, IO3 & 4 is the Resistor specified in the text.

he purpose of the raisers/stand offs/pins are so the resistor can be easily changed and also its subject to the ambient air temperature.

The resistor should be a low temperature coefficient type eg. from the RC55 series from Farnell or equivalent. The value required can be determined by immersing the probe to the maximum level and setting the resistor value so that the 7555 frequency is about 5kHz. This will be the minimum frequency and the maximum period of oscillation. Typically, this is around 1nF.
Last edited:


New Member

aha, so the top part of the article describes the addon module to make it all work. Kinda missed that and focused on the diagrams below. *blush*

Thanks for responding, much appreciated.

I also emailed the author and got this extract back:
> Except my knowledge level in electronics is not good enough to figure out:
> what IO1, IO2, IO3, IO4 are connected to?

IO1 goes to the probe wire
IO2 goes to the probe frame (stainless steel or aluminium grounding
rod). Alternatively, you can put two insulated wires as probes to
avoid earth loops should this be a problem.

IO3 and IO4 are pcb stakes that you solder the precision low
temperature drift resistor, after you first try various resistors to
get a full scale deflection you want.

> what the purpose of pcb stakes / pcb raisers are?
Because soldering and desoldering through hole resistors is quite
tedious :)

John Jore
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