I add a water/essential oil mixture to this diffuser and light the candle inside and the LED plus Buzzer will turn on when the water/oil are gone.UPDATE: I put in a new 9v battery and found that I needed to change the resistance. I replaced R4 and R5 with a 10k potentiometer. I use the middle pin and one other on the meter. Now i can adjust the resistance for when the battery gets weaker and it acts as a volume control for the loud buzzer. NOTE: The bottom transistor is not needed. It can be unplugged while the LED and Buzzer are on and you can hear and see the circuit lose amplification.
1. Diffuser or container that frequently needs to be filled with water. 2. I recommend buying the Velleman kit "EDU01". It has the breadboard, transistor, resistors, buzzer, 9v battery connector and led. Or search them from Velleman and buy the parts. 3. The other transistor is 2N3906 came out of a "osoyoo electronic component kit" I got from amazon or search and buy it single. 4. The 10k potentiometer(SKU 2118791) and switch(SKU 2258831) came from "Jameco" out of the "Arduino RGB LED Infinity Mirror kit" or search Jameco and buy the parts. Also the black wire is 22 AWG hookup wire(SKU 2152876) from Jameco. 5. Water and my favorite "Peppermint and Sweet Orange essential oil". 6. Finally a 9v battery.
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Step 1: Assemble Circuit.
Plug in the parts as shown.
Step 2: Route Wires
Cut and route the wire as needed for your container or as shown. I wrapped 30 AWG copper wire around the ends to aide in current flow / sensitivity in the circuit. May not be needed now after the Update with the Potentiometer I put in.
Step 3: Test
Test out the circuit. This can be improved by cutting the wires to the parts shorter and covering it all up(except the LED, Buzzer and switch) with a container or electrical tape. Or get professional and put it on a PCB or some other board and solder it and put it in a container. Update: Version 2. This one doesn't look like it came out of "Sanford and Son" junkyard.