Liquid level indication circuit design based on LM1830

The M1830 is a monolithic integrated circuit that can be used in liquid level indicator/control systems. The LM 1830 , manufactured by National Semiconductor, can detect the presence or absence of polar fluids. The circuit based on this IC requires a minimum number of external components and the AC signal is passed through the sensing probe immersed in the fluid. The use of an AC detection signal prevents electrolysis, which makes the probe durable. The IC is capable of driving an LED, a high-impedance tweeter or a low-power relay at its output.

Low level indicator light (LED)

The circuit with LED low level indicator is shown in the picture above. Capacitor CT sets the frequency of the internal oscillator . With a given value of C1 the frequency will be around 6kHz. Capacitor Cb couples the output of the probe oscillator and ensures that no DC signal is applied to the probe. This circuit detects the liquid level by comparing the resistance of the probe to ground R with the internal reference resistor R.

When the ground resistance probe is above R the output of the oscillator is coupled to the internal output transistor , causing it to conduct the base. Connect the LED collector (between pin 12 and Vcc) to the driver. Since an oscillator is used, the base of the transistor drives the transistor to be switched at the oscillator's output frequency, with a 50% duty cycle. There is no problem during driving, the LED uses the AC signal when using the speaker as an indicator, this method is very useful. Speakers can be driven only by AC signals and DC signals that do not produce any speaker sound anymore. The circuit schematic of a liquid level indicator using a speaker at its output is shown below. The circuit is similar to the first circuit, except that the LED is replaced by a speaker and the load current limiting resistor is changed from 1.2K to 1.5K.

Low level warning (audio)

Low level indicator with external reference resistor

There may be situations when the fluid resistance (probe ground) does not match the 13K internal reference resistor. In this case, there are options to avoid the internal reference resistor and an external reference resistor of your choice (in the receiving circuit) can be employed. The output of the oscillator is directly on pin 5 and RX is connected to the oscillator output probe via a DC blocking capacitor . The circuit diagram for the above scheme is shown below.

Grade Specifications Using External Reference Resistors

High level switch activated

One method of activating a relay when the fluid level exceeds a predetermined level is shown here. DC voltage is used to drive the relay, AC voltage cannot be used here as we are required in the case of LEDs and speakers. Pin 9 of the IC can be used to solve this problem. Connecting a capacitor from this pin to ground will keep the internal output transistor steady whenever the probe's resistance becomes higher than the reference resistance. External transistor Q1 is connected to the collector of the internal transistor. It is the load of the relay connected to the collector of Q1. When the probe is not in contact with water, it equals the open circuit situation, and the probe resistance will definitely have a lot of M ohms, which is larger than R REF (13K). The internal transistor will be turned on, and the base of the first quarter that is connected to the collector of the internal transistor will keep the relay off in the inactive state. When a twist situation occurs (liquid level touches the probe) the internal transistor is turned off, which in turn causes activation of transistor Q1. Drive pumps, lights, alarms, solenoid valves or any load connection via relays. Resistor R3 limits the collector current of the internal transistor , while resistor R4 provides protection to the IC from transients.

Activate high level switch

Probe: The probe used here can be a metal rod of any size and shape you choose. The tank must be made of metal and should be properly grounded. For non-metallic tanks, a small metal contact is fixed to its bottom layer, and to the ground. The probe must be placed at the level you want to monitor.