Water level Alarm

This electronic circuit is an example of those used to monitor rising water levels such as on rivers, dams, spillways, etc. You can use it to prevent overflow when filling up your bathtub, sink, swimming pool or any other container when it's important that it not overflow and you are unable to watch it constantly.

This project is a radio/transmitter water level warning device. An RF (radio frequency) oscillator is controlled by a second Transistor which is controlled by contact probes placed over the water.

The transmitted signal is received on an ordinary AM receiver placed nearby. When the water contact plates or probes are out of the water, no RF output is sent out. When the contact probes both touch the water, RF output is sent out indicating that the water level is up to the probes.

The 2SC Transistor is in the RF oscillator circuit. The emitter is effectively connected to the ferrite coil center tap through the 10uF Capacitor, which acts as a short circuit at these frequencies. Feedback to the base is through the 100pF Capacitor.

The 470K Resistor sup plies the base-bias current to turn the Transistor ON.

Notice that Battery current must flow through the 2SA Transistor to get to the oscillator circuit and back. With the probes insulated from each other, the only current that can flow is the leakage current 1 CEO (current from the collector to emitter with the base open).

This weak current cannot drive the RF oscillator. When the probes both contact the water, some current can flow through the water to supply base current to the 2SA. This base current turns the 2SA ON so that oscillator current can flow between C-E of the 2SA with little resistance.

We're using the 47K Resistor as a current limiting resistance. Without this resistance the 2SA could be burned out by excessive cur rent, especially if the probes are accidently shorted. With the 2SA ON, the RF oscillator produces an RF signal, indicating the water level has reached the probes. These probes may be made of almost any insulated conductors, but large surface areas provide the most abrupt turn ON characteristic.

Place an AM radio receiver nearby and tune it to a weak station. Then adjust the oscillation frequency with the Tuning Capacitor to a point where the signal is heard through the radio.