Automatic Water Pump ON and OFF (2005)

Automatic Water Pump ON and OFF (2005)

This interesting project dated back in 2005 where I was still searching for 2nd job after finish 1 year Internship/On-The-Job training from Atambua City (NTT). The reason I searched for other job was because I believed I deserved something better, because I am always different than other :).

Ok, let’s check what’s so special with this interesting project…
I designed this for my Teacher/Mother house. A small dedication :). This electronic circuit will automatically turn on “electric water pump” after the “lower sensor” is triggered (means the water tank is [almost] empty). After some time the water pump is on and filling up the tank, the “upper sensor” later will be triggered when the water tank is [almost] full. Once “upper sensor” is triggered, the circuit will turn the pump off. It will stay off until the “lower sensor” is triggered again (back to first state in the cycle).

Here we go with the finish view: 

Assembly guide:

• Attach the “auto water pump On and Off” (let’s shorten it as “device”) to nearby water tank.
• Put the water empty sensor (lower sensor – contains of two electrodes) to lowest water level you want it to be detected as empty.
• Put the water full sensor (upper sensor) to the highest water level you want it to be detected as full.
• Attach this device to input voltage 220V and the water pump you want to automate.

Let’s review the schematic I designed:

• This device is powered by 12V DC adaptor as you can see above. It has a 12Vdc relay, 2 NPN universal transistors and sets of electrode. The electrodes I use made of enameled diameter 1mm wire. I stripped the end as a conductive sensor.
• Water pump is ON by default, because of relay NC position (NC means Normally Close). It is not energised by the transistors.
• When the water is filling up the tank and touching upper sensor, there will be resistive connections from +12V electrode to upper sensor and lower sensor by the water (Java water resistance is around 10K Ohms). The water resistance will give current around 11mA to base of both transistors. 

Ib = (12V-0.6V)/10K = 11.4mA
Ib is base current
0.6V is Vbe

• Both transistors is then turned on by the Ib (Ic = hfe*Ib = (hfe around 300)*11.4mA = 3.4A, note that the actual Ic current will depend on actual relay usage and transistor maximum Ic rating which is only around 100mA for BC547). This cause negative pin of Relay is LOW/connected to ground. The relay is energised.
• Energised relay pull its’ switches to NO (Normally Open). Note that left switch bypass upper transistor and let the relay always energised provided lower transistor is still on (=lower sensor is still immersed in water). Energised relay switches the water pump off.
• When the water is used by user, the tank will run empty. Lower sensor will no longer be immersed. Lower transistor doesn’t receive base current, means collector current is also 0. This transistor is off, relay is disconnected from ground; relay is not energised.
• Un-energised relay turn the water pump on… and repeat the first state of this cycle.

Above circuit was designed when I was just graduated from my technology high school. I didn’t understand so much about circuit durability and safety. Now I understand, let us see improvements I drafted in below schematic.

1. Added 1.5K Ohms resistors to electrodes – this avoids shorting +12V power supply to ground by transistor base-emitter (which is only 0.6V) in case the electrodes were accidentally shorted. Otherwise, the adaptor or transistor will be blown off. This case can be happened e.g. when we clean out the tank and accidentally short the electrodes. Independent resistors ensure each base may have different needed bias voltage. 
2. Added diode in parallel with Relay – to swap off spike voltage generated by relay’s inductor when it is “suddenly” turned on. The shunt voltage can damage adaptor and transistors.
3. Replaced stripped enamel electrodes with rust-proof electrodes. I remember the stripped enamel was “electrolised” or oxidised (collecting water’s oxygen). When operating. In long term, it will be rusty. Tin(Sn)/Zn/Ag/Au/Cr/Ni coating on a “built in terminal” will be useful to prevent the rust. Simple and cheap example is AC plug bought from hardware store.

That’s all. Feel free to write a comment or any question. And most importantly, I hope this invention is useful to a lot of people :).