This circuit basically is a fluid level alarm indicator intended for alerting visually impaired individuals regarding the liquid level in a container such as cup, glass or bowl.
The unit can be simply hooked up across the rim of any container with the probes such that it detects the full level liquid in it while the container is being filled, and triggers ON the audible indication.
The container could be a cup, glass, bowl, bottle etc.
The circuit will detect the filling of the cup with any liquid that conducts electricity, such as beer, tap water, tea, milk.
This also means that the circuit will not be able to detect distilled or deionised water.
The circuit is extremely straightforwad to build and understand.
The entire circuit is built around 4 NAND gates using IC 4011.
N1 and N2 are configured as logic inverter/buffer.
With no liquid around the probes the input of N1 is held at a low logic due to the ground 1M resistor. As soon as the probes come in contact with a liquid in the cup or glass the N1 input is instantly pulled high. This causes N1 output to go low, which turns N2 output HIGH, enabling the operation of astable multivibrator N3/N4.
The astable switches T1 and T2 ON/OFF giving rise to an audible warning tone from the speaker.
The frequency of the astable built using N3/N4 gates can be chnaged by tweaking the values of the resistors 820 k, 390 k or the capacitor 1.2 nF. This will in turn help to adjust the tone on the speaker. It can adjusted from a sharp ear piercing tone to a milder buzzing sound, as per the user preference.
- 1M = 1
- 390k = 1
- 820k = 1
- 27 k= 1
- 100 ohm = 1
- Capacitor 1.2 nF Disc ceramic
- Transistor BC108 or BC547 = 1
- Transistor 2N2222 = 1
- IC 4011 = 1
- Battery 9 V PP3 = 1
- Small 8 ohm speaker 0.2 watts = 1
Using IC 555
A tiny, cup full indicator circuit could be also created using the IC 555, as shown in the above diagram.
The IC 555 is rigged as an astable multivibrator. As long as the probes are not touching the liquid inside the cup, the astable generates a low frequency 15 Hz clicking sound on the speaker.
When the cup is full, and the probes come in contact with the liquid, the frequency of the astable suddenly increases to 500 Hz sharp tone, which alerts the user regarding the cup full condition.
The speaker could be replaced with the piezo buzzer transducer also, to make the design very compact.