In my previous article, you learned how to build the “The 555 Test Circuit.” We are going to modify the test circuit to create a hot liquid level indicator for the vision impaired to help them fill cups with hot liquids such as tea or coffee. To fill a cup with cold or room temperature liquid, the vision impaired person simply puts his or her index finger inside the cup to monitor when to stop pouring the liquid when it hits his or her finger. When pouring hot liquids from, say, a boiling teakettle, vision impaired persons would not want to monitor the liquid level with a finger since they may scald themselves. The 555 test circuit can be used to generate a tone when the liquid in the cup reaches the desired level.

Parts Needed:

1 555 Timer IC (I used a KIA555p, but the NE555 will do just fine)
2 Clothespins
1 Coffee mug

Snap Circuits Parts:

1 Base Grid (11” x 7.7”) # 6SC BG
1 Eight-Pin IC Socket # 6SC ?U8
0.02uF Capacitor # 6SC C1
1 Variable Resistor #6SC RV
1 Whistle Chip # 6SC WC
1 4.5 Volt Battery Holder # 6SC B3
1 Slide Switch # 6SC S1
Jumper Wire 18" (Black) # 6SC J1
Jumper Wire 18" (Red) # 6SC J2
1 Single Snap Conductor # 6SC 01
5 Conductor with 2-snaps # 6SC 02
2 Conductor with 3-snaps # 6SC 03
2 Conductor with 4-snaps # 6SC 04
1 Conductor with 5-snaps # 6SC 05
2 Conductor with 6-snaps # 6SC 05

Build the circuit shown:

Once the circuit is built, use a clothespin to fasten the end of the red Jumper Wire (that’s not connected to the 555 test circuit) to one side of the cup with the snap against the side of the cup 2 to 3 centimeters inside the cup. Use the second clothespin to fasten the end of the black Jumper Wire (that’s not connected to the 555 test circuit) to opposite side of the cup with the snap against the side of the cup 2 to 3 centimeters inside the cup.

Switch the Slide Switch (S1) on. No sound will be heard on the piezoelectric speaker WC (Whistle Chip). Slowly pour tap water (or coffee) into the cup until the liquid level reaches the red and black snaps of the Jumper Wires inside the cup. This will close the circuit of the 555 test circuit and you will hear a tone on the speaker (WC).

Click here to see a video demonstration if the video does not display in the web page.