Today I want to show you a project really very simple but effective that can be used for various applications.
This is a high resistance continuty sensor (current gain approximately 30000 times), based on the interesting Darlington NPN BC517, and very low cost.
The basic design published here simply allows you to turn on an LED by touching or immersing in water two metallic elements (cables, plates or whatever), but “cloning” the circuit and putting more transistors and more LEDs, we can get a gauge with 2, 3, 4 or 30 sectors.
It goes without saying nothing forbids to replace the LED with a relay in order to drive loads the most important.
But let’s see in detail the circuit:
The operation is quite trivial: by touching the two contacts of the sensor “SENS” with hands for example, the weak current flowing through the body is sufficient to excite the transistor via the resistor R1 which serves as protection if we join directly the two contacts. The BC517 is able to amplify this current approximately 30,000 times, for which a current of 10 uA, is able to slide up to 300 mA through the transistor. The resistor R2 serves to prevent accidental starting by grounding the base of the transistor, while R3 is simply the resistor to limit the current flowing through the LED.
Although simple, I believe that this project will be a starting point for other applications.
Also sometimes the simple things are the most functional and a little ‘analog electronics’ sometimes … is not bad …
Greetings and see you soon!
Update – 03/10/2012
Schema corrected: obviously one of the two leads did not go to mass but on the positive … I apologize for the mistake and thanx who rightly corrected me with his comments.