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Circuit Notebook

Interesting circuit ideas which we have checked but not built and tested. Contributions will be paid for at standard rates. All submissions should include full name, address & phone number.

Adjustable float-switch triggered timer

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This circuit is triggered by a float switch when water rises past a certain level and it then turns on a solenoid for a predefined period to open a valve, draining the water. It could also turn on a pump or other 12V DC load up to about 10A. The on-period for the load is variable over a range of 10-35 minutes.

IC2 is a 4060B binary counter with in-built oscillator. It’s driven by NOR gate IC1d which operates as a gated oscillator. When output O13 of IC2 is low, the oscillator runs and its timing is set by the combination of fixed 33kΩ and 330kΩ resistors, potentiometer VR1 and the 1µF cap­acitor from the pin 9 output of IC2.

Pin 9 of IC2 outputs a buffered digital signal which follows input pin 11. This is fed back into one of IC1d’s inputs. Since IC1d is an inverter, this makes the circuit astable and thus it oscillates. Varying the setting of VR1 changes the RC time constant of the delay circuit from IC1d’s output to pin 11 of IC2, varying the frequency.

IC2’s O7 output drives NPN transistor Q2 which in turn drives LED1 via a 1kΩ current-limiting resistor. LED1 flashes at 1/256th (1/28) of the frequency at IC2’s pin 11 input.

While IC2’s O13 output is low, the gate of Mosfet Q1 is driven high by IC1a & IC1b which are configured as inverters and wired in parallel. This turns on the solenoid and also red LED2. After 8192 (213) oscillator pulses, output O13 of IC2 goes high, turning off the Mosfet and the solenoid. Diode D2 absorbs any back-EMF from its inductive windings.

The circuit is activated by the float-switch wired across the emitter and collector of PNP transistor Q3. When the switch contacts close, 12V power flows to 9V regulator REG1 via the switch contacts and reverse polarity protection diode D1.

The 9V supply powers both ICs and when it is first applied, inverter IC1c resets IC2, initialising the counter to zero. Once the 100nF capacitor at the inputs of IC1c charges, its output goes low and the timer runs, turning on the solenoid for the set period.

As long as the solenoid is on, the base of PNP transistor Q3 is pulled low via an 8.2kΩ resistor, allowing current to pass to the 9V regulator even when the float switch contacts open. Q3 turns off when the solenoid does and so the circuit powers down, ready to be activated again by the float switch.

To set the timer, you activate the circuit by closing the float switch contacts and then measure the period that green LED1 is on when it flashes. This will be 1/64th the load on-time. Adjust VR1 for the appropriate flash period. For example, if you want the load to run for 15 minutes, adjust VR1 until LED1 flashes on for 15 x 60 ÷ 64 = 14 seconds each time.

Len Cox,

Forest Hill, Vic. ($50)

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