I also wanted to try the circuit in Model Boats magazine of August 2007. With some help I added a 555 timer-IC to the circuit so I could adjust how long the pump should run each time it started. After some further experimenting I discovered that the trigger in the 555-IC was sensible enough to detect current passing trough water without help of any op-amp. The circuit is very simple, I have attached a schematic. The pump will start when the probe loses contact with water and will continue to run for a certain time after there is contact again. The time is controlled by the product of the 22µF capacitor and the 1000K pot meter. In this case the time can be adjusted to 0-18 sec.
I have mounted a probe on a Maccsteam 3.5” horizontal boiler and connected the circuit. Last Sunday this system operated continuously on a bench on a model show for nearly 6 hours without problems. I use a 4.8V 2200 mAh battery as power source.
Please note I am not an expert in electronics.
HelgeBe
