Summary of NE555 timer sparks low-cost voltage-to-frequency converter
Summary (under 100 words): A low-cost voltage-to-frequency converter uses an NE555 timer and a TL071-based Miller integrator to convert 0 to –10 V input into 0–1000 Hz output. C1 charging current equals –VIN/(P1+R1); the NE555 toggles when C1 reaches two-thirds and one-third VCC. Output is high while charging and low while discharging. P1 calibrates frequency vs. voltage. Discharge time (~30 µs) limits accuracy at higher frequencies, producing conversion errors of about 0.3% to 3% for an assignment of 100 Hz = –1 V and 1000 Hz = –10 V.
Parts used in the Voltage-to-Frequency Converter:
- NE555 timer IC
- TL071 op amp (used as Miller integrator)
- Capacitor C1
- Resistor R1
- Potentiometer P1 (for calibration)
- Supply VCC (power source)
- Connections/wiring and PCB or breadboard
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. If you assign 100 Hz to –1V and 1000 Hz to –10V, the error of conversion ranges from 0.3 to 3%. If you use P1 to calibrate the output frequency in the middle of the input-voltage
For more detail:
NE555 timer sparks low-cost voltage-to-frequency converter
- What input-voltage range does the converter accept?
The input voltage in this application ranges from 0 to –10 V. - What output-frequency range does the circuit produce?
The circuit yields an output-frequency range of 0 to 1000 Hz. - Which components form the integrator and timing elements?
The Miller integrator is built around a TL071, and timing is provided by an NE555 timer with capacitor C1. - How is the charging current of C1 determined?
The current of C1 is IC = –VIN / (P1 + R1). - How does the NE555 switch during a cycle?
When C1 reaches two-thirds of VCC the 555’s discharge transistor opens and C1 falls to one-third VCC; at one-third the discharge transistor switches off and C1 charges again. - What role does P1 play in the circuit?
P1 calibrates the relationship between the output frequency and the input voltage. - Why does accuracy decrease at higher frequencies?
Because the discharge interval is approximately 30 µs, accuracy of the voltage-to-frequency conversion decreases as the frequency increases. - What conversion error can be expected for 100 Hz to 1000 Hz mapping?
If 100 Hz is assigned to –1 V and 1000 Hz to –10 V, the conversion error ranges from 0.3% to 3%.
