1. Make a Voltage Controlled Resistor
2.Use it to make a Voltage Controlled Oscillator
3.Use it to feed a signal into a laptop's microphone input.
The voltage controlled resistor is a very useful element in electronics. The one in this instructable takes a minute to make and does not require too much electronic knowledge to operate. It is made out of an LED and a Cadmium Sulfide (CdS) photocell. The LED changes light intensity with change of voltage/current. The photocell is a resistor that changes resistance with change in the intensity of light striking it.
To build a voltage controlled resistor, you need to glue the LED to the photocell then paint the whole assembly black to shut out ambient light. That's it!
The voltage controlled resistor can also be used to:
.Control devices like the volume of an amplifier
.Control gain in automatic gain control circuitry
.Feedback signals to control motors and robotics.
.Amplitude Modulate (AM) a signal.
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Step 1: Application: Voltage Controlled Oscillator
VCOs are used in FM modulation, mainly. In this case, it used to make a two tone siren which you can see in operation in the video below
(Note: the flashing LED is part of the voltage controlled resistor assembly, I did not paint it black to show how it operates. My hand is changing the pitch of VCO because it is shutting out the ambient light that affects the exposed photocell)
Step 2: VCO Schematic
The two oscillators are both based on a single CD4069 Hex inverter chip. CD4069 is a very versatile chip that can be used for a myriad of digital and analogue applications. For example, Two inverter of an CD4069 can make a simple cheap and rugged square wave generator /digital clock (as in this example). One inverter of the CD4069 can be used as a buffer and to drive a heavy load (in this case it drives a computer microphone input decoupler covered in the next step).
The two oscillator are RC type of oscillator, meaning the resistance (R) and capacitance (C) set the frequency of oscillation.
the frequency is = 1/(2*pi*R*C)
A high resistance or capacitance produces a low frequency and vice versa.
Step 3: A Signal Decoupler
In the previous example, I used a voltage controlled resistor to decouple and feed the tone from the VCO into my laptop's microphone input.
This setup reduces the noise that my laptop's microphone input can pick up from the circuit and protects my laptop from the circuit (you can burn the microphone input if you put too much voltage into it) .
I will show two configurations to decouple a laptop microphone input. The two configurations are based on the type of laptop microphone input. There are some laptop microphone inputs that provide power to the microphone and some that don't.
Step 4: Some Technical Details
However you can add a voltage to current converter to drive the LED. The simplest voltage to current converter device is a resistor.
A resistor converts voltage to current by means of OHMs law, so that the output current I=V(across R)/R.
The voltage across a resistor in series with an LED is Vinput-VLED (in a red LED it's Vinput-1.2V).
It is always a good idea to put a resistor in series with an LED because it protects the LED from over current which burns out the LED.
Another detail, the output light from an LED is not linearly proportional to the input current but changes exponentially.
I plotted the input current vs. the output resistance of the voltage controlled resistor in this instructable below. It clearly fits an exponential trend line.