Summary of NE555 timer sparks low-cost voltage-to-frequency converter
### Summary This article describes a low-cost voltage-to-frequency converter circuit built around the NE555 timer and TL071 op-amp. Operating with an input range of 0 to –10V, the device generates an output frequency between 0 and 1000 Hz. The design utilizes a Miller integrator where capacitor C1 charges based on the input voltage until it reaches specific thresholds set by the 555's internal comparators, creating a frequency proportional to the input voltage.
Parts used in the Voltage-to-Frequency Converter:
- NE555 timer
- TL071 operational amplifier
- Capacitor C1
- Potentiometer P1
- Resistor R1
In 1971, Signetics—later Philips—introduced the NE555 timer, and manufacturers are still producing more than 1 billion of them a year. By adding a few components to the NE555, you can build a simple voltage-to-frequency converter for less than 50 cents. The circuit contains a Miller integrator based on a TL071 along with an NE555 timer (Figure 1). The input voltage in this application ranges from 0 to –10V, yielding an output-frequency range of 0 to 1000 Hz. The current of C1 is the function of input voltage: IC=–VIN/(P1+R1).
As the voltage on C1 reaches two-thirds of VCC, the 555’s internal discharge transistor opens, and the voltage on C1 returns to one-third the voltage of VCC, the lower comparator threshold. At one-third this voltage, the discharge transistor switches off, and C1 again starts charging. The NE555’s output is high while C1 is charging and low while C1 is discharging. The product of the input voltage and the charging time of C1 is constant. Because the discharge time is shorter than the charging time, the following equation results for the output frequency: fOUT~VIN/(P1+R1)×C1×1/3VCC.
P1 calibrates the relationship between the output frequency and the input voltage. Because the discharge interval is approximately 30 µsec, the accuracy of the voltage-to-frequency conversion decreases as the frequency increases.
For More Details: NE555 timer sparks low-cost voltage-to-frequency converter
- What is the cost to build this circuit?
The circuit can be built for less than 50 cents. - How does the input voltage affect the output?
An input voltage ranging from 0 to –10V yields an output frequency range of 0 to 1000 Hz. - Can I use this project if I need high accuracy at high frequencies?
No, because the discharge interval is approximately 30 µsec, the accuracy decreases as the frequency increases. - What component calibrates the relationship between output frequency and input voltage?
Potentiometer P1 calibrates the relationship between the output frequency and the input voltage. - When does the 555s internal discharge transistor open?
The discharge transistor opens when the voltage on C1 reaches two-thirds of VCC. - What happens to the NE555 output while C1 is charging?
The NE555 output is high while C1 is charging. - What is the formula for the output frequency?
The output frequency is calculated as fOUT~VIN/(P1+R1)×C1×1/3VCC. - Why is the product of input voltage and charging time constant?
This occurs because the voltage on C1 returns to one-third the voltage of VCC before C1 starts charging again.
