really dumb question

hey guys i'm new to electronics and I am messing around a bit with a bread board, led's and resistors.

I am trying to come up with a soil moisture sensor so I fastened 2 wires very close together and wired it up to a 9volt bat.
i placed the sensor in water and the led was very dim. so.. I upped the power.
I attached a laptop power pack and to be safe, I checked the volts. it was 19.5 volts. I then tested the volts after a couple of resistors(1/2watt, 100ohm/ each) and i thought that would lower the volts but it didnt. i placed my sensor in water again and it was a bit brighter than before.

so why didn't my voltage drop?
will this burn out my led (5v, 20mA, 4.0mcd, blinking led)?

figuring the answer will have to do with resistors lowering amps not volts


Volts, Amps and Ohms are all related. Read about "Ohm's Law".

In the case you tested, the voltage across a part of a circuit depends on the relative resistance of that part of the circuit. The meter has a VERY high resistance (a.k.a. impedance), therefore almost ALL of the voltage in your circuit is going through the meter.

You should try Google (or Bing) to find moisture sensor circuits that you can emulate.

thanks so much for your help.

Here's a good example:

A moisture sensor in the site use the "voltage divider" principle. A voltage diver is a potentiometer, the middle connector is the voltage you to measure, as long the other connector have a + V ( 5 V ) and the other end connector is connect to ground.

So the moisture sensor is the same idea. It only have a fix resistor in serie with the a metal probe and the other probe connect to gnd. So when the resistance between the metal probe change, so the voltage is changing.

led (5v, 20mA, 4.0mcd, blinking led)?

I seriously doubt you have a five volt LED. I suspect this is the forward voltage. You need to check the datasheet to get this correct or your calculations to find the correct resistor will be wrong!

And by the way I like the really dumb questions. They are the only ones I have a hope of contributing to.


led (5v, 20mA, 4.0mcd, blinking led)?

I seriously doubt you have a five volt LED. I suspect this is the forward voltage.

LED's with built-in blinking circuits will typically also have the current limiting built in and can off 5V.

These, for example, can run at 3V to 14V. The flash rate varies for 2.5Hz to 1.5 Hz over that range.

Hi there. What is that you're trying to achieve? If you want a device that turns on led after you put 2 wires in water, just use a single transistor, like bc547 and appropriate resistors for transistor base and led. Connect emmitor to ground, positive side of led to +, appropriate resistor in series with led, and other wire of this resistor to transistors collector. Than put one "probe wire" directly from + and connect second to base of transistor (you may want to place a small resistor before base of it to prevent damage when short circuiting.) This is known as single transistor amplifier.