Make a Voltage Controlled Resistor and Use It




Introduction: Make a Voltage Controlled Resistor and Use It

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This is a three-in-one Instructable:
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

A voltage controlled resistor can be used to change the frequency of an oscillator. When a voltage controls the frequency of an oscillator, it is called a voltage controlled oscillator or VCO for short.

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 schematic below shows the two oscillators from the previous video. The first oscillator controls the second oscillator which is the VCO. The first oscillator is a fixed frequency oscillator and produces a low frequency square wave which the VCO translates into an alternating sequence of low and high pitched sound.

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

A signal decoupler electrically isolates two devices while passing a signal between them. This is very important in cutting out interference noise and for safety.

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

An LED is a current device, meaning it responds to current change change not voltage change. The Voltage across an LED is always fixed (red LED have a voltage of 1.2V, white LEDs have higher voltages). So technically this device covered in this instructable is a CURRENT CONTROLLED RESISTOR.

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.



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    45 Discussions

    It must be me.

    Correct me if I'm wrong, but with this design, as the LED light increases the photocell will decrease resistance. Simply put, bright light shining on the photocell will allow current to flow from the AC source to whatever you plug into this adapter.

    It won't work the way you'd expect it to. Say you were trying to use this as a trigger for a light.

    Led's require little current so you could take a speaker wire pair, connect it to your control wires on this box, and get an incandescent light to flash in time with the music.

    Wouldn't work. No LED light = Outlet is live. LED light = Outlet is off.

    That would have to be reversed.

    PLEASE correct me if I'm wrong or mssed something. I really want to be wrong about this.

    2 replies

    You're wrong.  : )  Consider a residential street light:  as dusk approaches, less light shines on the photocell until the street light comes on (light is low = resistance is LOW).  As dawn approaches, more light shines on the photocell until finally the street light turns off (light is high = resistance is HIGH).  Another example:  You walk into a store and a buzzer rings.  What has happened is that your leg has momentarily blocked a beam of light shining across the doorway onto a photocell on the opposite side:  the resistance of the cell has gone from HIGH (light shining across doorway) to LOW (light blocked by leg), allowing current to flow to the buzzer, which sounds.  Simply put, the resistance of the photocell INCREASES as more light shines on it and DECREASES as less light shines on it. 

    wait... i thought resistance decreases with more light, thus letting more power though it. so in your case, the lamp would be on during sunlight and off at night. you need to invert that signal and change it to a either ON or OFF (binary) signal with a chip, and have the signal amplified with a relay to turn on or off the 400W street light.

    for more efficiency, use a reflective paint for shielding.  like metallic silver or metallic gold paint;  or plain white paint. or even siple piece of tin (aluminum) foil or aluminum tape.

    It is slightly misleading to call this a voltage controlled resistor. A more proper term would be opto-isolator. While it will work well in a variety of applications, a faster and more accurate alternative might be to use a digital potentiometer. They are available in various configurations, maxim has quite a few app notes on them.

    4 replies

    It would be more accurate to call it a -current- controlled resistor, in that the current vs light output from an LED is fairly linear over a wide range. While it is true that the resistance vs light of a CdS photocell is not linear, for audio applications you rarely want linearity.

    It is not misleading!! You are right in that it is an opto-isolator but it is a voltage controlled resistor. Think about it: 1 change the voltage applied 2. the resistance of the photo cell changes What is your criteria for a V.C.R.?? This soluton has been used even before LED's with incandecet bulbs. It was invented by Alexander Graham Bell. And is very cost effective, and simple. How does much does a digi-pot cost and the supporting parts to develop the digit input??

    My apologies. I did not intend to imply that it would not work, merely that it may not be optimal for certain applications. Particularly where a quick response time or linear response curve are desired.

    Thanks. Every application has its own options and optimal solutions. I need to check my spelling- Incandescent- not incandecet . also to many does's.

    The red LEDs where the only ones that I had that were rectangular. Rounded LEDs are harder to glue. Changing the type of LED changes the transfer function. The cadmium cell best respond to green light.

    I've done this for years, you just sand the end of the LED flat. You aren't even worried about it being smooth since it is glued onto the photocell.

    A number of things, anywhere I need a variable resistor and it is easier if it is isolated (versus using a FET as a variable resistor). I've used it as part of an AGC circuit. Used several to modify a Radio Shack digital echo box into a nice Effects box that would vary the echo depth, timing, volume, etc. so it would act as a multi-effects guitar pedal. Made VCO with a 555 timer. Remote wired volume/balance/tone controls. Etc. CdS photocells are most sensitive to green/yellow light, so I usually use a yellow LED (less forward voltage than a green LED).

    they are perfectly suited for AGS in the audio range and they are less of a headache to design than jfet in the feedback loop of an op-amp. They can be used as digitally controlled variable resistors with a voltage hold circuit. I even linearized them by using a reference CdS in a feedback loop matched with the working CdS ( I glued the CdSs side by side and used one LED for both).

    And they have a faster attack time (ie, change to low resistance with light) than decay (ie, change to high resistance with less light) that is well suited to AGC. I like your idea for linearization.

    i agree that they are harder to glue, but you get a lower resistance as a final product. also, what wattage are those photocells?

    be careful with cadmium it is both toxic and recognised as calcium