Introduction: Varicap Controlled 555 Oscillator and Varicap Diode Tutorial

About: Geophysicist, Paraglider, explorer, comes from the desert and loves the world and all science.

I ordered some Varicap Diodes for radio experiments but decided to learn how to use them by making a Varicap LED 555 Flasher. You should think of getting Varicaps they are really fascinating.

My Hackerspace got an awesome package of really awesome Electronics Components, among them several 555 which I know can be rigged into making an Astable Multivibrator that uses Capacitors and Resistors to make a timer. Usually you can use a variable resistor to change the frequency of the square waves. You can also utilize a variable capacitor instead of a single value capacitor to change the frequency as well.

As you may or may not know, variable capacitors are hard to come by, so a solid state device has been invented to deal with this.

A varicap diode, varactor diode, variable capacitance diode, variable reactance diode or tuning diode is a type of diode whose capacitance varies as a function of the voltage applied across its terminals.

So lets get going

Step 1: What You Will Need

Bill of Materials

1 Breadboard
1 9V Battery
1 1MΩ Potentiometer
2 1KΩ 1/4 Watt Resistor
1 4.7KΩ 1/4 Watt Resistor
1 555 IC
Various Polarized Capacitors
2 Varicap Diodes of the same value

You can buy Varicap Diodes on Jameco this is an example:

Step 2: Insert the 555 IC

Insert the 555 IC into the breadboard

Step 3: Pins 2 and 6

Connect pins 2 and 6 of the 555 IC with a jumper cable as shown.

Step 4: 555

Connect pins 4 and 8 of the 555 IC

Step 5: Resistors

Connect a 1K Ohm resistor between pin 7 and the +9V rail of your Breadboard

Step 6: Resistors

Connect pins 2 and 7 with a 4.7K Ohm resistor 

Step 7: Capacitors

Connect your fixed value capacitor of any value in this case 10µF between pin 2 of the 555 and GND or the negative rail of your Breadboard. Mind the polarity of your capacitor if it is electrolytic. 

Here is some information about Electrolytic Capacitors

Step 8: Resistor

Connect your other 1KΩ resistor to pin 3 and any other hole in your breadboard. This is the output of the 555 that goes to an LED.

Step 9: LED

Connect your LED's Anode to the rail on your breadboard that comes out of the output resistor.

Step 10: LED

Connect the Cathode of your LED to the GND rail of your breadboard.

Step 11: 555

Connect pin 1 of the 555 to GND and pin 4 of the 555 to the Positive voltage rail of your breadboard.

Step 12: Varicap Diodes

We are going to make a Voltage Divider to control the Capacitance of the Varicaps, and we can do that with a 1MΩ Potentiometer.

Connect two jumpers in such a way that the edge pins of your potentiometer are hooked up to +9V and GND as shown in the picture.

Step 13: Voltage Divider

Connect a jumper that can bring the controlled voltage output of your potentiometer to a rail of your breadboard as shown in the picture.

Step 14: Varicaps

Look for a notch located at the top of your Varicap diode, this constitutes the Cathode of it, connect the two Varicap in the way shown in the schematic and the picture.

On the schematic disregard the Inductor! 

Step 15: Varactor

Connect one of the Anodes of the two Varicaps and connect it to pin 2 of the 555.

Connect the other Anode of the Varicaps to GND.

Step 16: Finished

You are now finished, connect the battery and see the LED go! Adjust the potentiometer to change the capacitance and therefore, change the Flashing rate of your oscillator.

I will try to add some more schematics soon for you all!

Check out the video below for a demonstration.

Weekend Projects Contest

Participated in the
Weekend Projects Contest