Water Pump Hysteresis Circuit

A simple circuit to prevent a pump being damaged by being turned on and off rapidly (multiswitching)

In our articles Float Switch Water Level Measurement (1) and Automatic Pump Shut Off Circuit (2) we looked at simple ways of controlling a pump automatically according to the level of water in a tank.

We mentioned in the second of those articles that a pump can be destroyed when it's run dry - i.e. when there is no water to pump. Another way to reduce the life of a pump (and also to waste power) is to switch it on and off rapidly (multiswitching). For the first few seconds after a pump is started, a lot of power (up to three times the power used when it is up and running) is necessary.

When using a Float Switch (3) or Liquid sensor (4) there is always the risk that turbulence in the water (caused by the container being moved, or by water being pumped into it) will cause the sensor to turn on and off rapidly as the water sloshes about. If the sensor is used to control a pump, then the pump will also be turned on and off rapidly. In this article we will look at a circuit which provides a time delay between the water level being reached and the pump being activated.

Time Delay Pump Controller

This circuit is designed to operate very simply. When the water level reaches the point necessary to trigger the float switch or liquid sensor, a timer is started. If during the following 10 seconds the switch/sensor does not switch off, a relay (5) will be triggered which will in turn feed power to the pump turning it on.

Whenever the switch/sensor turns off (i.e. whenever the water level falls below the preset value even for a split second) the 10 second delay timer will start again from zero. Therefore, it will be impossible for the pump to toggle on and off rapidly.

To achieve the desired result an NE555 integrated circuit (IC) can be used as a timer. This IC (click here for full NE555 Specifications (6)) can output up to 200mA of current - easily enough to energise the coil of a typical 12 Volt relay. The technical desciption of its use in this application is monostable operation delay on - i.e. after a delay, the IC will output a continuous positive current until its power supply is cut.

Monostable operation delay on NE555 circuit used to prevent rapid on/off switching of a pump

The length of the time delay is set by adjusting the values of the resistor (7) and capacitor (8) according to the following formula:

Time Delay (secs) = 1.1 x R (kOhms) x C/1000 (uF)

Therefore, for our target 10 second delay, we can use a 100 kOhm resistor with a 100 uF capacitor. For a shorter delay use a lower value resistor, and for a longer delay use a higher value resistor - for example, a 47 kOhm for a 5 second delay, and a 220 kOhm for a 20 second delay.

Connecting everything up for the final system is also very simple. The 12V supply voltage passes through the float switch or liquid sensor and into our circuit. The output (from pin 3 of the NE555 IC) connects to one end of the relay coil * with the other end connected to the ground (labelled 0V).
* Note in the diagram below the diode running parallel to the relay coil to protect the NE555 from high voltage spikes when the coil is de-energised..

Connecting the time delay circuit to a relay

The relay can be fitted onto the circuit board with the NE555 chip, resistor, and capacitors making a simple plug-and-play integrated unit as shown below.

Buying a Pump Time Delay Circuit

If you would like to purchase a complete soldered circuit pre-set with a 10 second (or any other) time delay, a fitted relay and an LED water-detected indicator for Ł9.99, contact neil@reuk.co.uk (9). We sell a range of complete circuits in the REUK Shop (10), as well as liquid sensors and float switches.

Web Link References

(1) http://www.reuk.co.uk/Float-Switch-Water-Level-Measurement.htm
(2) http://www.reuk.co.uk/Automatic-Pump-Shut-Off-Circuit.htm
(3) http://www.reuk.co.uk/buy-FLOAT-SWITCH.htm
(4) http://www.reuk.co.uk/buy-LIQUID-SENSOR.htm
(5) http://www.reuk.co.uk/Relays-and-Renewable-Energy.htm
(6) http://www.fairchildsemi.com/ds/NE/NE555.pdf
(7) http://www.reuk.co.uk/Resistor-Colour-Codes.htm
(8) http://www.reuk.co.uk/Smoothing-Capacitors.htm
(9) Email address: neil@reuk.co.uk
(10) http://www.reuk.co.uk/shop-COMPLETE-CIRCUITS.htm

Article from REUK.co.uk:
http://www.reuk.co.uk/Water-Pump-Hysteresis-Circuit.htm
Published: 19th November 2007
Š REUK 2009