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Simple Water operated relay

A easy way to find out if the Overhead Water Tank or Water Sump is full. It uses the mild conductivity of water to bias a Darlington Trio.

This was done in my early days, i have upgraded it, it ought to work, reduce the number of transistors to make it less sensitive, also a lower value in place of 10M will reduce its sensitivity, use clamping diodes to protect.

Relays and Contactors

BD139 is used to drive the relay as it has good Ic. So you can even use a low ohm relay. If a Relay resistance is high its quality is higher, its power consumption is less and it needs thinner wire SWG-AWG. T2 and T3 form a darlington pair which drives T1. LED1 shows that the water level has reached the top of tank and also that the Relay is energised. D1 a freewheeling diode. R3 10M ensures that the high gain input does not float, yet the low leakage current thru the water is not drawn away by the 10M. R2 limits base current in case water is saline.

                        Water operated relay

The Source file is here

R2 prevents noise pickup from causing relay chattering. A small cap could be added across R2 but this will slow the response time. Inputs of High gain configurations can be very sensitive to noise, emi and static.

RS232 with Opto-Isolation

Made up of only Transistors and Diodes, No Chip at all. I had to once interface an high voltage circuit to PC, The uC had to communicate thru RS232--Comm port--Serial Port.

Fluid or Water Level with Reed Relays

This design has a Chip but can measure the water level in stages. It can be used for any fluid unlike above circuit.
What is a Relay - Jim Asks


A Relay is an electrically operated switch. Many relays use an electromagnet to mechanically operate a switch, but other operating principles are also used, such as solid-state relays"

There is a control signal and an output for relay, if you are using just a buzzer for alarm, no relay needed. A buzzer, LED or Lamp can be driven by Transistors.

If you want to operate a big hooter or 230V lamp a relay is needed- or even a motor or solenoid to move some thing. Even a Solid State Relay can be used.