How to Use a 555 Timer As a Voltage-to-Frequency Converter

You can use the 555 timer as a voltage-controlled oscillator

The 555 timer integrated circuit (IC) is a versatile oscillator and general-purpose pulse source. Its dedicated control voltage input lets you use the 555 as a voltage-to-frequency converter. In this application, you configure the 555 as an oscillator (astable mode) and vary a voltage at the control input. This will change the frequency by +/- 25 percent around a center frequency ranging from minutes per cycle to 500 kilohertz. The 555 has good linearity of frequency control in this range.

Things You'll Need

• 555 timer IC
• 555 IC data sheet
• Assorted resistors
• Assorted capacitors
• Breadboard
• Small pieces of 22-gauge wire
• 10 V DC power supply
• Variable DC supply
• Frequency counter

Instructions

1. Examine the 555's data sheet. In particular, look for application circuits that use the 555 as a free-running oscillator.

2. Determine the starting or nominal frequency at which you want the 555 to run. This is the frequency the 555 will produce with no control voltage. Frequency is determined by a timing network of two resistors and a capacitor.

   You can calculate the frequency by the following formula:
   f = 1.44 / (Ra + 2Rb) * C
   Where f is the frequency in cycles per second, Ra and Rb are the ohm values of the two resistors in the timing network, and C is the timing capacitor value in farads.

3. Build a 555-based oscillator on the breadboard using the timing network calculated in step 2 and powered by the 10-volt DC supply. Use the application notes as a guide for connecting external parts and power to the 555. Connect a frequency counter's signal ground to the 555's power supply ground and the counter's signal input to the 555's output at pin 3. With the 555 circuit powered on, the counter should display a steady frequency.

4. Connect the variable DC supply's ground to the 10V supply ground. Set the variable supply to 5 volts and connect it to pin 5 of the 555. On the frequency counter, observe the effect varying the voltage between 5 and 9 volts has on the 555's frequency.

Tips & Warnings

• For normal astable operation, changing the 555's frequency with a voltage at pin 5 will also change the duty cycle of its pulses.

• Keeping the control voltage to between 45 percent and 90 percent of the 555's supply voltage will ensure the most linear control response.

References

• Williamson Labs: Philips 555 Timer Data Sheet
• Doctronics: 555 Control Voltage
• National Semiconductor: 555 Timer Data Sheet